Please email Karl your questions by clicking "Contact Us". Please label your email "Karl: Question".

Some questions and answers will be posted in this section.

Page 1: Questions 49-72, click on links or browse page.
Q49 Larger engine in .60 sized Corsair ? Q61 Four cycle mufflers ?
Q50 Radial for Air World FW-190 ? Q62 More information from engine manufactures ?
Q51 91 octane for gassers ? Q63 Most realistic engine for P-51D ?
Q52 Radial for Royal Corsair ? Q64 Gear reduction design for Fuji 80 twin ?
Q53 Three blade prop for 3W 100B? Q65 Scale prop for P-47 ?
Q54 Yellow P-38 engine choices? Q66 Top Flite P-51 engine choices ?
Q55 Exhaust too long? Q67 Don Smith B-17 engine choices ?
Q56 Marvel Mystery Oil? Q68 Gas engine and radio interference ?
Q57 Saito 100 FA break in procedure? Q69 Multi engine servo and tank set up ?
Q58 Ziroli P-40 engine? Q70 Saito FA-72 three blade size ?
Q59 Radial for BalsaUSA Stearman? Q71 Zenoah GT-70 converted to GT-80 ?
Q60 Inverted glow engine and tank position ? Q72 Four blade for C-130 ?

Question 49: " I'm starting on my meister 100in corsair soon, but bought a 60 size d&b corsair kit with all extras was thinking on using a rvc four stroke which will fit inside its cowl bout same size as royal and top flite kits the 90 or the 120 rcv? thanks as for the meister its gonna have a quadra 100, or might see if a zenoah gt 80 twin would fit its cowl will be sheeted and glassed, thanks again "

Karl: "Thanks for the question, when researching information to answer you, I realized that your situation is not too unlike the same faced by racers and hotrodders--Power-To-Weight ratios. The lighter the vehicle, the less work needed to go fast or stop fast. Of course, if you really want alot of power for a light plane, then you will have to make some improvements to the airframe to compensate, you follow? I am not really sure how big the cowl is on your plane, but here is a good rule of thumb---" try to go with the largest allowable powerplant the kit manufacturer states". Now, if your RCV .90 is larger still, there might be some concerns about balance. For example, if you have bought this kit with the hopes of having good handling, the 1.20 size of any engine will likely be too big power-wise. You will have to add so much weight to balance it it may fly or.... Safe to say that if a .60 size engine will fit, possible so will a .90. Stay on the safe side here and go with the .90 for weight and size. You can only compensate so much with the propellor, and if you use an engine that really is questionable, your flight characteristics will suffer. Pretty sure you don't want to bring home your plane in more pieces than when you brought it to the field, eh? Thanks again, Karl"

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Question 50: "Hi Karl, Welcome aboard! I think that you might me just the guy to answer this one...I have a Stuart Mackay / Air World FW-190 ( I know, there are some discrepancies from scale in the outline...nose length, wing width, etc. but it's still a beautiful model )and I have the 215 cc 5 cyl radial to go in it. I was wondering if you know anything about this beast of an engine or have any experience with it? Thanks, Rob Bailey / Finish Advisor"

Karl: "Hi Rob, thanks for the greeting. I may have a few finishing questions for you later. I don't have any hands-on experience with this beaut nor do I happen to know of anyone that does. Been to some of the local aerodromes but haven't found one. What I can tell you from research and my gut is this engine should be great for you. It will swing a 32" wood prop, weighs 11 lbs so balancing the rest of the plane might take some doing since it appears to me that most FW190's are big in the nose. It could handle anything over 35 lbs (within reason...) It sounds awesome, is bigger and more powerful than the Robart and cheaper--go figure. Robart's put alot into engineering, though, and has two more cylinders. I hope this helps, Rob. When I was a kid 30 years ago I designed my own plane--it wasn't a warbird, but the plans got shelved for all these years. I found them a year ago and seeing them relit the fire. But it's burning in a different "field" so to speak, now. I have alot to learn and catch up to. Thanks for the question.Karl. "

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Question 51: "If you have a choice of gas to use what would you use for 2-stroke gassers? I have been using 91 octane non - oxygenated. Is it really worth the extra cost or is plain old 87 octane oxygenated the same thing?"

Karl: "Thanks for the question, Kris, it's a good one. One that warrants some context. When it comes to a 4-stroke engine of nearly any type, especially one that uses gas, the octane rating, or "grade" if you will, is less important because of the wider range of combustion chamber heat that it can tolerate. This means that, for example, you can put 91 octane in an Expedition just as well as 87, and you wouldn't hardly be able to tell the difference. the greatest thing you would realize is mileage difference. When you are talking about a 2-stroke gasser for an airplane, things get a little tighter. Since the fuel,and sometimes a lube additive in the fuel, is used for engine lubrication as well as combustion, then having the proper mix is crucial--at the risk of over-stating the obvious. Whether you're taking off or doing any maneuvers the mix needs to be right. The biggest reason Av-Gas is so high of an octane for real planes is it burns cooler, has better flame characteristics and is generally more reliable to keep the engine lit than a lower octane fuel. So, when it comes to yours, I recommend that you stay with the 91 so you save yourself the hassle of engine repairs ( and with planes, read PLANE repairs!) Yes, I would say the 91 is worth it. From all that I have learned about the different gassers out there, they all recommend premium fuel. Great question."

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Question 52: "Welcome aboard as the new engine advisor, Karl :) I'm just doing some homework/research about engines for a Royal/Marutaka Corsair Sr. and fell in love with Saito's FA-90R3D. Looking at this radial engine (I'm guessing that this baby can/will fit into an 8-inch diameter cowl?), I notice that there are three exhaust stacks.. Assuming that I would like to build scale exhausts that actually work, how do I deal with the third stack? Thanks in advance, Wil"

Karl: "Thanks for the greeting, Wil and the question. I asked Saito for some info on their multi-cylinder engines and radials and they did send them. Unfortunately, between the time I asked and they sent, my computer decided to go stupid on me and I have not been able to download them to see them, or figured out a way to get around the problem. Anyhow, I am sure the engine will fit the cowl, although after all the Corsair kits I have been able to find, yours is a new one so I don't have any info, sorry. As for the exhaust system, I would ask Saito if there is an exhaust ring available for the radials. Ask at the same time if you can have it with two exit tubes out the bottom. This is good for scale appearance and to keep glow fuel residue on the bottom, not spraying all over the aircraft. I know that RCS and Robart can customize the exhaust, ask Saito as well.Can't hurt. Besides, if you are in a position like I am for a prop design, the only other alternative is to find a guy who's good in metal fab to make you one. Happy Flying, pal. Karl"

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Question 53: "Hello Karl I am installing a 3W-100B in my 100" Corsair. The plane will weigh about 40-45 lbs. What two and three bladed props would you recommend for the best scale-like performance for this combination? Thanks, Lou"

Karl: "Thanks for the question, Lou. To begin with, remember that what ever increases you make in prop diameter, you have to give up pitch, that's why the larger the diameter (26", 28", 30"...) the lesser the corresponding pitch ( 12 or 14, 10, 8...). Even when using a three blade of the same diameter as a 2, because of the addition of potential lift of another blade, the pitch is lower. Now, as for a 100" Corsair, true scale diameter is 31.8". The plane I have been researching for a year is the same size, I have asked dozens of questions about scale props, and no one makes one that large, due to load on the engine and RPM capabilities. In the 2-blade range you might try 30x8 if you want a more scale diameter, but no Corsair ever came with a 2-blade. Of course there are smaller ones--28x10, 26x12. The three blade choices are somewhat narrower ranging from 24x12 to 28x8. If you are hard-set like I am on a scale, flying 3-blade ( no statics for this guy!...) prop, you will have to find someone to assist you in making a hub and blade custom for that engine and plane weight. Hope this will give some guidelines, pal. Happy flying, Karl."

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Question 54: "Hi Karl, I have a Yellow P-38 and am trying to decide on engines. So far I have my short list down to Yellow recommended G38's and ZDZ-40 (side or rear exhaust). I am looking for reliability above all other attributes but I would also prefer not cutting much into the sides of the nacelles to preserve the scale looks as much as possible. This plane will end up weighing in the 35 to 40lb range so I would also want to have enough power to deal with an engine out emergency. Which of the 3 engines would you use??? Thanks, Kevin"

Karl: "Thanks for the question, Mr. Richards, tis a good one. The engines you are considering, based on the airplane's maker, are ok and certainly safe choices. One other engine you might consider is the Saito 200Ti. This engine has several attributes that, I think, make it a better choice. First, 4-stroke power is easier on the engine since you don't have to wind up to the moon on rpm's to get to the power band. Second, it sounds cool. Third, the in-line design makes it definitely the superior choice since it was
designed to fit narrow-cowl fuses or nacelles. Fourth, the exhaust exits are on the same side so you don't have so much fitting to do with the pipes and so on. Fifth, it sounds cool. Sixth, the displacement vs. power will be more favorable, in my opinion because the Moki engine that Yellow suggests only displaces 1.8 cid. The Saito offering is bigger at 2.0 cid. Now, when you're needing power to help with landing, touch-and-go's, or to have reserve for powering out of a potential stall, then, like they say in hot-rodding, "You can't beat cubic inches". The only problem, however, is getting one. According to Saito, the item is out of stock. I guess they better get'em in if they want to sell'em, eh? Hope this helps, pal. Happy flying. Karl."

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Question 55: "Hi Karl, I am building a 82" Piper J3 Cub, I know its not really a war bird but I value and trust the tech info on this site so much I just had to ask a question. I am putting a TT F-54S 4 stroke engine in the kit and would like to hide the exhaust inside the cowl. My plan was to put a 90 degree elbow on the exhaust port to direct the exhaust pipe and muffler into the cowl, then add about two inches of pipe to the end of the muffler with appropriate bends to bring it outside on the lower right of the cowl (as viewed from the cockpit) as in the real airplane. My questions are: Will the extension of pipe damage the engine or make it run differently? Will it require any changes to tuning and if so what sort of things would be required? What is the best pipe material to use, I was thinking aircraft grade aluminium or something? and Is there a better way to do this, such as a Pitts type muffler or something? I hope you can help me with this, and to all of you at RCwarbirds, keep up the great work! thanks Mark Penno"

Karl: "Mr. Penno, thanks for the question. I'm not anwering because it's not a warbird---haha---just kidding. We welcome ALL RC questions even if they aren't warbirds. From my experience with engines, breathing is necessary---DUH. The more pipe you have, the harder the engine works to exhaust the gasses. This builds back-pressure in the system and lowers the power curve. 2-strokes are alot more finicky about exhaust than a 4-stroke because there is only 2 strokes to carry out a cycle. It is more reliant on cylinder pressure and velocity than a 4 stroke. Another thing to consider is the fact that when you have exit pipes that are unequal, it will affect idle quality, fuel distribution and so on and these are hugely important to any airplane. In my opinion, the best option would perhaps to be to go with the pitts style muffler so that the pipes are more equal for the engine. If you go with the real-style exhaust, keep the pipe diameter the same for the cylinder(s) on the left side until it enters the cowl, install an expansion chamber and an outlet that is slightly smaller than the inlet, since the gasses are less dense, but the velocity is crucial for proper scavenging of
the cylinder so it doesn't load up on nitro. Try to keep the pipes the same diameter as long as possible. As for pipe material, usually a light gauge steel will do. Aluminum, to my knowledge, sometimes has adverse reactions to the chemicals in the fuel and may corrode after time. Of course, it only takes time to fabricate. I hope this helps, and we appreciate having you as a user of this site. Thanks, Karl."

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Question 56: "Hi, I have 4 new engines, two OS .46-2 strokes and 2 Laser 80-4 strokes. I've read that some people use Marvel mystery oil as an "after run" to lub there engines and even submerge their engines in it over the winter months. What can you tell me about this oil? Also, is there a specific difference in running in 2 strokes compared to 4 stroke engines and is it ok to break in an engine in flight instead of on a bench? Thanks."

Karl: "Good question, Bob, thanks. For starters, the Marvel oil is excellent for breaking loose sticky air tools. Being a finicky guy for things that have to work right, I have used the oil many times with good results. As for storing your engine assembled or torn down, in the oil I don't think it should hurt. However, the nature of the oil that makes it a great anti-rust and corrosion product may also be corrosive to some of the engine parts. Try setting a beyond-repair part in it for a period and see what happens. Generally, though, all you should need to do is spray some WD-40 on the parts and keep it in a plastic bag if you don't plan on using it much during the winter. If you store the engine assembled, you run the risk of setting so long that the cylinder sleeve gets a memory of ring location and hence a groove can develop after you start it up again, then it's time for a rebuild. As for break-in, it is by far the best method to bench-test and adjust the engine so you don't have any tragedy's during flight if you sieze it or stall it. It is easier to make the adjustments on the bench, get it right and then fly. Hope this helps. Thanks, Karl."

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Question 57: " just bought a Saito 100FA 4 stroke for my Hanger 9 P-51. It's my first 4 stroke, so I'm not sure how to break it in. The manual isn't very clear. I ran the first tank with the high end needle 5 turns out and didn't exceed 4000rpm like it said. That was about 10min run time. The manual says to break it in for a total of 40min, which means 3 more tanks. It says to lean it out a little bit at a time each tank. Is this at full throttle? and how do I know what the normal position is for the high end needle on this engine? It doesn't say. I don't want to go to lean and mess up the engine. Any help would be appreciated. Thank you. "

Karl: "Thanks, Kerr, for visting our site. To be totally honest, I don't know if a 4-stroke needs more than 3 tanks to break-in than a 2 stroke. I know a guy that uses 2 strokes and he usually goes through 5 tanks. Now, the important thing here is that in all reality you will not be flying at wide-open-throttle (WOT). Usually working the throttle back and forth--which is more realistic as far as how the throttle acts as you fly it--is the preferred method. Since we can't make any mixture adjustments based on emissions as you would a car, we have to rely on head (engine) temp and the amount of smoke from the muffler. You want a haze that you can see coming from the exhaust at what you would consider good cruising speed, goose it as you would for climbing, hard turns etc., and if the smoke diminishes, it's probably lean. In an RC car, for example, if you stall from loading it up or cooking the engine, the car rolls to a stop. There are no emergency parking lanes for airplanes of any kind. It is best to err slightly on the rich side than lean, in my opinion. Use a laser pyrometer to check the engine temp as you break it in, usually keeping the temp around 180-220 degrees. Check it at the glow-plug so air doesn' t interfere. Idle quality after 3 or 4 tanks should be smooth and not alot of smoke. Steady RPM which does not dip or slow down, and fairly rapid throttle response are good signs that you are close to achieving good mix for flight. Hope it helps. I'd be interested in feedback as well, after you go through it. Let me know what you did and how it went. Thanks, Karl. "

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Question 58: " Karl - I am about to begin my first scale warbird. A ziroli P-40. I want to have all my parts on hand for proper fit. I was wondering if you could give some insight. Ziroli calls for a G--62, or Quadra 75. I was told by someone else that these are good engines but lack power compared to others. I was also told they probably suggest these, because of their weight. This plane happens to be a tad tail heavy, so a heavier engine probably helps out. Do you know the fact or fiction on this. I was going to go with a DA, but now I don't know what to get. Mike"

Karl: "Hi Mike, Sorry for the long time to respond. Been researching to help you better, I hope. The DA looks like a good choice for size, power output and weight. Regardless, you may still have to add weight to balance. The P-40, with the 85" w/s needs around 3.5-4.2 hp. The DA is about 3.5 and you can play with prop choices to finally achieve the best flight. Also look into Saito Engines. They have a shallow-vee twin 4-stroke that might just fit the bill. Probably sounds cooler, too. A good rule of thumb is to have slightly more power on hand than you think you will actuallly need, for two reasons 1) it's nice to know you have the power to avoid catastrophe when you need it; and 2) when you set it up that way, you can have more prop choices that won't put such a load on the engine and you can keep the rpm's down, which is going to save you money on repairs and make it more reliable. Hope this helps, pal. Happy flying. Karl."

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Question 59: "Karl, I am building the BUSA stearman. I have not yet decided on the engine, but am considering a 3-W 140 and , if I want to part with the money, the RCS 215 radial. What can you tell me about each of these? Should I consider sometning else? Thank you. Laurence"

Karl: "Hello, Lawrence and thank you for the question. This is my favorite area, radials. If you haven't seen a full-size radial up close or a cut-away, I would recommend it. The radial is an engineering marvel. Together with your excellent choice of airplane, the RCS 215 is perhaps the best choice. The people who have installed these engines in their planes all say that all it takes to fly is 1/3 to1/2 throttle! What an engine saver! The other radial to look into is the equally awesome Robart offering. 7 Cylinders of 4-stroke radial power. The Robart unit has 10 hp, RCS has 13.5 hp. Usually you can mess with props to adjust for good flight. Now, I know nothing of you and how much love you have for vintage aircraft, but since you are building a Balsa USA bipe, my guess is it's a pretty strong bond, eh? The Robart R780 is pricey at $4,000.00 plus a deposit, and smaller in diameter. The RCS one is still stout at over $2,400.00 and seems to fit the fuselage perfectly. The 3W140 series (B2 and B2F...) is much more affordable at just shy of $1,400.00, but they aren't radials and the plane didn't ever come with anything other than a radial. The R780 uses glow fuel, the others gas. I read about one guy who actually uses Coleman fuel in his because it doesn't smell as bad as gas or leave the slimy residue of glow fuel. Ask yourself this question, if I love this hobby and strive for authenticity, which engine will grace the front of my airplane? Since you asked I will give you this answer--use radial power. I love the radials and will encourage their use whenever an RC plane is built that models a full size one that had radial power from the factory. Make no mistake, sir, if you decide on that choice, you will not regret it. Hope to hear again from you soon and thanks for visiting our site. Karl."

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Question 60: "Hey Karl, I’m having the most difficult time getting my Thunder Tiger .61 engine to run inverted in my Top Flite Focke Wolf. Major hydraulic lock and when I start her upside down…its does start but then won’t run for very long when I flip her back over. I hear tank position is important but this is my first inverted engine attempt and I’m not completely sure about raising the tank, lowering it relative to the inverted engine. In a standard set up…the tank faces the same direction of the engine…up. When the engine is inverted, do you flip the tank also? Thanks for your help Karl for I’m a very frustrated fellow right about now, Pat"

Karl: "Good question, Pat, thank you. You are correct in realizing that tank position is important. Allow me to share what I have learned and if it makes sense, try it and get back to us. Since that plane is a tail-dragger, you want the tank at least level with the intake as it will have a tendency to load up at idle due to the gravity factor. This works much the same as older lawn mower engines and tractors with the tank above the engine to gravity-feed the engine. Some say to set the tank level with the intake when it is sitting on the ground. Some say to set it level according to level flight, as this will be the predominant attitude of the aircraft. I think it is best to set the tank level with the intake when setting on the ground, get the engine to idle and not load up so you can make adjustments, then if necessary install a Kline-pump to lightly pressurize the tank during flight. The thing to remember here is physics and gravity sucks for airplanes. Engines in these RC planes should run in any configuration. However, since it's inverted it is easier to get the fuel to the engine since it will "fall" into the carb throat rather than being sucked into it. Control the "fall" and you should be flying very soon after dialing in the engine. Hope this helps, pal. Karl. "

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Question 61: "Hi Karl... I have an O.S. 1.60 Gemini twin cylinder 4-stroke motor for my PICA WACO 1/5th scale biplane. I have not run it yet, but I am curious if there should be any mufflers on the headers coming out of the cylinders. I can't find any listing for mufflers for this engine, and the instructions don't mention it either. Is this motor going to be really loud with no mufflers in it? Sonny"

Karl: "Thanks for the question, Sonny. A 4-stroke without mufflers sounds really neat, kinda like a harley. Most engine manufacturers sell their units without a muffler, but should offer one for the application. If one is not spoken of or offered, I have two suggestions; 1) use a Pitts-style muffler and build the headers to fit to the inlets or 2) keep trying an aftermarket exhaust company to see if they can help. I forgot to mention possibly using a kerker-style baffle also, like in motorcycles and certain oval-track cars used in areas where there are noise ordinances. Of course, thinking of when the plane is airborne, does it really matter how quiet it is? I think it sounds pretty cool when one of these RC babies comes across for a half-throttle pass and the pilot runs it up a couple of notches and goes ballistic in a big loop. 4-stroke power at it's best, pal. Hope this helps. Thanks again, Karl. "

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Question 62: "Karl, Is there any reason that the engine manufacturers do not give us HP and Torque vs. RPM curves? I find this information necessary in order to make a better decision for the specific engine required for my project. Lewis"

Karl: "That is a good question, Mr. Brande, thanks. It is unfortunately true that, for example, O.S. does not list the horsepower figures for any of their multi-cylinder engines-including radials. I have a list of nearly all the offerings by O.S. and all the rest of the engines have the power listed. I do know, however, that Robart, Zenoah, TechnoPower, Fuji, and most of the other popular engines do list the power and suggested props. I suppose it's up to you to choose the prop that will best fit your project. Here are some guidelines that may help you. 1.5 - 2.5 hp engines are best suited for planes from just shy of 10 lbs to 20 lbs. 2.5 - 4.0 hp 18 lbs to 25 lbs. 4.0 to 10.0 hp can fly large birds 25 lbs to 35 lbs. If your plane is going to be larger than 35 lbs., you are going to find the choices are harder. If the engine has the right power for the plane, it may not fit the airframe well at all. It is easy to over-power the frame and torque rolls are bad. Too much power, no amount of feasible engineering will correct it. To find an engine that fits may not have the power you need. I believe the main reason that some engine manufacturers do not include this info is they may not see the need, based on the market they are selling to. I don't mean to sound condescending to anyone, but most guys and gals flying these birds don't have any background in aeronautical engineering. What they learn about power usually is at the field, testing. I agree completely with your observations, though. If a guy were to build a drag car, he starts with the engine. First he has to factor in the weight of the car and driver, fuel, etc. gear ratio, transmission. Testing is mandatory. The thing about airplanes is the prop produces the thrust, like gear ratio does in a car. The "taller" the gears, the better top-end. And vice-versa. I know that the people at RC Showcase have a phenomenal 215 cc gas 5-cylinder radial and it swung a 32" Menz prop in testing. I have seen several 2-blades that appear like the vintage prop from biplanes, trainers, etc. The torque is a product of the prop, and since the prop choice is purely subjective, I think the least they should do is supply test results for at least 3 different props. So, here are the things to consider: the manufacturers' suggested all-up weight, type of service, fuel and any additional scale features you will add to the airframe. I have been researching data for a 100" F4U Corsair project and have changed the engine choice 3 times, based on estimated weight due to features. I suppose you could build a 1/5 scale plane that weighed 100 lbs, and if it was balanced and had enough power, theoretically it should fly. I also found out, because I want a scale, flying prop, that no one makes one that large. So now I have to build one for myself. Oh well. This is long answer and I hope it helps. If there is anything else we at RC Warbirds can do to help, let us know. Thanks again. Karl "

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Question 63: "Dear Karl, I am in the process of building Top Flight's 1/5 scale P-51D. The goals that I have set for myself are to make this D as scale as possible. In terms of engine and prop I am determined to have working exhaust stacks and spin a four bladed prop of scale size. Everything must fit inside the cowl. The engine must have the most realistic sound possible. (4stroke?) I am a jeweler of 36 yrs by trade and I can fabricate almost anything, ie the exhaust stacks. Reliability, quality and power are important in that order. Cost is important but not a worry. I realize this may be a tall order, could you please help me determine the best powerplant/prop combo for my plane? Thank you for your help. Regards, Carver "

Karl: "HeHeHe, Mr. Tripp, have I got the lowdown for YOU! There is an outfit that has just the powerplant for you.I have been waiting for someone to ask about an airplane the right size so I can recommend it. First, if you wanted to up- the-ante- scale to 1/4, you have to check out this site: www.quarterscalemerlin.com. You will find the most impressive engine out there right now. I can't go into all the details here, the site does a good job of that. This has scale supercharger, water-cooled, 2-valve per cylinder (real ones had 4..), scale profile, you simply have got to see this engine. They are taking serious inquiries for the plans, castings, etc. It has all the stuff. Unfortunately, there is not yet a scale splined hub and 4-blade to go with it. I have only recently begun an undertaking to make such an item for my own kit, a 1/5 scale F4U. Still designing. Check it out. You will be impressed. Karl"

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Question 64: "Hi Karl, I just started working up the first draft of the gear reduction for my 1/3 scale Curtis Hawk P6e. Any thoughts would be greatly appreciated. As of right now I am going with the Fuji 80 twin and using 1/4" thick alumuninum for the gear reduction's "shaft bracing". As of right now I am going to use the engine mounting holes to go thru the braces. I am unsure if I should put a secondary brace midway down the shaft into the two bolt holes on the engine. I only started the drafting today, so it is a bit rough. Thanks for your input. Patrick"

Karl: "Wow, Mr. Laughlin, I thought I was hardcore when I realized I had to design my own prop and hub system for my plane! This gear reduction system is a good idea. In the 80's, this I learned from a friend at Techno-Power engines, Byron was selling more complex gear reduction units they called PSRU's--Propellor Speed Reduction Units. They were very expensive starting at $560.00 and going up. As for your design, after studying the drawings for a little while, I do not see a need for a mid-ship support system like you hinted at, simply because the reduction gear is mounted on the shaft at the front, and has a bearing in the rear. These will bear the weight and distribute load. Remember to design in a way to retain the shaft, as it will have a tendency to walk out the front of the bracket when under load of the prop. One way to do it is to have press-on sealed bearings--press on to the propellor shaft and retained in the bracket by set screws or retaining plate, or fit the shaft from back to front, with something like a bolt head on the aft end, slide into the bearings which are already in place, and retain the shaft at the rear mount plate. You may still have a better idea. Since it is early in the design stage, you can afford to make all the mistakes you want, eh? The drawings are cool. Let us know how it turns out. Thanks, Karl "

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Question 65: "Karl, I'm just getting back into the hobby after a couple of years away from it. I'm currently building the 1/8th TF Thunderbolt and while thinking about engine choice for this plane the question of prop blades and diameter came to mind. How is it that we can't use scale diameter and blade count props on our models and if we can what considerations do we have to keep in mind. I guess I'm asking for a small primer on flying with scale props. Thanks for your time, Scott"

Karl: "Thanks for the question, Mr. Dean. Coincidentally I am working on a 3 and 4-blade hub and blade system for 1/6 scale and larger warbirds, that has its origins in the need for my own project, a hybrid kit F4U Corsair, with a scale, flying 3 blade and possibly 4 blade. I am still in the design stages so I can't let out too much. The biggest reason we can't fly scale props mostly has to do with the engine load and type. A 2 stroke engine such as the popular Genoah offerings, ZDZ, 3W, etc., winds to a higher rpm than a 4-stroke. It's in the design, can't really change it. When considering the rpm of a real warbird, for example, the rpm's were not real high at cruise-2500 to maybe 3000, depending on the engine and altitude. When in combat, the engines did rev higher, for obvious reasons. As a scale modeler of warbirds, we have to think realistically about engines and props. We can't have an 18-cylinder alcohol-injected bird in 1/6 or larger because the engine likely would cost tens of thousands---for an RC plane, you ask? Yep. So, we take a different, purpose-built RC engine and adapt a smaller prop. The higher the rpm, the smaller the prop. A 2 stroker will rev around 8,000 to 12,000 rpm's. We can't run a scale 3-blade at 12k rpm. Get the picture? I ran across some info this summer thanks to a new friend at TechnoPower engines and he sent me info about some propellor speed reduc- tion units Byron made in the 80's, and these effectively slowed the prop, or around 6000. If you want to achieve a scale prop, look into some smaller radials, they are all 4--strokes, come in a range of power, from 2 or 3 hp up to 4 and 5 hp. Definitely find one that fits the cowl, and does not require cutting the cowl to fit the heads or whatever. They turn at slower speeds, and you can mess with prop pitch and blade count from there to see where you stand. Hope this helps and happy building. Karl."

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Question 66: "Hey Karl, I need an engine suggestion. I need an engine that I can use in both the new Top Flite Giant Scale P-51 84.5” ws and a Ziroli P-40 with a 94” ws. I am going to transfer the engine from the P-51 to the P-40 when construction on the latter is complete. Obviously, I would like something that would require the least amount of cowl cutting in both as possible. Thanks, Pat"

Karl: "Thanks for the question, Pat. There may be a few choices you have here, depending on the target weight of the plane. Based on my research, the ones that stand out as possibilities is the intruiging Saito 200Ti, which is a narrow, 4-stroke twin of 2 horsepower. The engine is intended for narrow-cowl or fused planes, to allow for muffler, etc., without having to cut. The other ones that looked good to me were the Zenoah G45 and Cactus 3W42i. The Saito engine is glow, the others gas. They range in prices from the mid-700's to high 800's in dollars. If the target weight is 20-30 lbs, you should be ok. Hope this helps and happy flying. Karl"

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Question 67: "Hey Karl, just wondering your choice of engines for a Don Smith B-17…138”. I would like to go with the scale look of the RCV 90’s but have heard many negative comments about them. Thanks, Pat"

Karl: "You are a busy guy, Patrick. Might I suggest the venerable Saito SAIE 170 R3 as a good choice? They are 4-stroke, 3 cylinder radials, fit into an 8 1/4" or larger cowl, and at 2 hp, they aren't too powerful. In a plane 138" w/s perhaps the collective 8 hp will be perfect, eh? Let me know what you chose. Hope this helps, Karl. "

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Question 68: "Hi Karl I have recently built and flown a scale, ten foot span, 50lb AD 5 Skyraider. It flew well but we developed radio failure (lack of range) and lost the airplane on its maiden flight. My question is: are you aware of any sort of radio interference from the ignition of these big engines ? Thank you."

Karl: "Good question, St. George. Any electronic ignition system gives off what's called EMF, or, Electromagnetic Frequency. In the automotive world where this is a concern, certain batches of wires in the harness are wrapped with a foil shield to shield the surrounding harness bunch from any interference. This is because the electrical system is running 12-14 volts and up to 3 amps. To my knowledge these conditions aren't present in even this large radial. For another thing, the ignition is solid state so I don't think it should be a problem. If you still aren't sure, I'm sure the folks at RC Showcase would be more than happy to answer your concerns. If you rebuild, try installing your own shielding around the ignition harness using foil, and if necessary, see if the compartments in the fuse need to be insulated as well, from each other. Hope this helps, and for what it's worth, I am sorry about the loss of the plane. The skyraider is a great warbird both in real life and for scale flying. Karl. "

Editors note: "Some interference can happen in any model engine. In the case of ignition engines you must keep your rx as far from the ignition box and wires as you can. Also avoid any metal going to the engine from the rx area such as a kill switch. Avoid metal throttle pushrods. Make sure all plug wire connections are covered and insulated (most engines come with some type of cover). Check for interference with a 50 foot ground check with engine running at different speeds. Use an FM rx to find the interference and after you have fixed it go to a PCM rx if possible."

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Question 69: "Dear Karl I am building my first multi engined bomber a Marutaka b17 and am just about to begin the wing, i have 2 questions the first is, should every engine have its own throttle servo or should i arrange some kind of linkage, the plan seems to indicate a flexible linkage system, i am sure this cant be the best when it comes to trying to syncranise the engines. Should each engine have its own small fuel tank or simply one large one. Thank you for your help Ed sumner."

Karl: "Thanks for the question, Ed. There are a couple schools of thought on this multi-servo situation. Some say that it is best to eliminate any delay in throttle response due to pushrod deflection by having each engine with its own servo, as you asked. Others say that multi-servos drain batteries too much and you have to have higher capacity batt's to get the signal out to the outboard engines so use one servo midships and use pushrods and belcranks. If it were me, I would opt for multi-servos, so if one goes out, I still have three engines I can control. Big deal about the batt's. As for fuel, I don't think you can find the space in the fuse for an adequate fuel cell, and if so you would have to deal with all the weight transfer due to sloshing. The best idea is to fit each engine with it's own tank. More stable, and if one runs out, you still have back-ups. When it comes to these big birds, redundancy can save money and your plane at times. Hope this helps. Karl. "

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Question 70: "I'm using two Saito FA-72s on my Wing Mfg B-25. The mfg recommends 13/6, 13/8, 14/6, 15/4 two blade probs for SCALE. What three blade prob should I use. I see that MASTERAIRSCREW has 11/8 three blade prop which I would select if appropriate. Thanks in advance. Dick "

Karl: "Thanks for the question Mr. Rovnyak. I'll try my best to answer. I would use a 14x9 with the FA72, or move up to the FA91S for only a few dollars more, get another 1/2 hp and use a 14x7 blade. These are all three-blades from Master Airscrew. Even though your projected finish weight is maybe 15 lbs., you still will find comfort in knowing you have the extra power if you need it. Karl"

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Question 71: "Hi Karl, After reading some of your answers to questions on design and building, i'm sure we could get into a lengthy "coffee sesion". We have the same interests. I have a chance to aquire a GT-74 that only has half-dozen flights on it. It needs only to have one exaust flange on each of the two cylinders welded in place, which is very do-able. Just wondering if I can convert to the GT-80 configuration? Cylinders, pistons, etc.etc. Thanks in advance for your reply....Jim"

Karl: "Hi Jim. As near as I can tell, the GT 80 is a spin-off of the GT 74. The main difference being the bore diameter. It appears that the stroke, crankshaft and many other parts/features are shared. This begs the question, however, about whether or not the rest of the engine is calibrated for the larger bore, such as intake, carb, etc. The Zenoah line is a great line of power plants. Here's where there are some options; 1) you can buy the "dedicated" GT 80 parts from Zenoah intake, carb, etc. since the theory is bigger bore, bigger breathing requirements or, 2) simply work on your own porting job to open up the intake side and let it take deeper breaths. I know from personal experience that go-kart racers are an excellent source of expertise where souping up 2-strokes is concerned! All in all it shouldn't be that difficult to change over. Cubic inches is a good thing! Thanks. Karl. "

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Question 72: "Dear, I'm a Germany RC model Pilot. My latest project is a German tranport Aircraft C130 Transall. Wingspan 4meter with two 30ccm engines. I'm looking for a 4-blades prop, but it is heavy to find in sizes of approx. 17x8" or 17x9". Does someone know from where I can get these. The blades should looks like original (warbird) You will make me happy if you can help. Mit freundlichen Grüssen / Kind regards Oliver"

Karl: "Gruus Gott, Herrprofessordoktor Tag! Wie Geht's? Yes, I spoke German in HS and college, but obviously I have no umlaut's on my computer, but we are glad to hear from you all the way across the ocean! There are a few choices for you for a three or four blade system. First, Iron Bay has a propulsion system of 3 or 4 blades, up to 23 1/2" in diameter. Master Airscrew also has several offerings of three-bladed units up to 17x10 pitch. If you are set on a 4-blade, you will have to hash it out with either of them as to how to get the prop you want. Check out these sites www.masterairscrew.com and http://www.ironbaymodels.com Both of these should be a good choice to look into. Hope it helps, Oliver. Danke Schon und freundlichen Gruusen. Karl."

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