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Model Aircraft Engines

March 01, 2016

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Airfield Models ( and Frequently Asked Questions about Model Airplane Engines

These are questions I have received from visitors regarding model aircraft engines and propulsion systems.

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I am wondering if the size of a spinner alters the recommended size of the prop.  I'm happy with all the "normal" spinners.  It's when you get to my project I'm busy with now, and have a 3.5" spinner and a recommended 10x6 prop, there is not much propeller left to do the work.

I only have 3.25" of prop showing on each side of the spinner.  This does not seem enough in my little brain to do the work.  My calculations tell me I have 2 x .40 motors turning 2 x 6.5x6 props.  It also looks kinda funny.  Help!

I have the same problem ó getting over how it looks like it wonít work.  But it does.  I think itís because the ends of the props are doing most of the work.  But yeah, it looks silly.  If it doesnít work youíd know right away when you try to accelerate for take-off.  Just go with what the plans says and you should be fine.



After the breaking in period of my Webra Speed 61F and flying for about 5 minutes each session my .61 T-28B Trojan kit built (by Dave Platt) I notice that new glow plugs are burnt and the exhaust fumes are black.  Is there any fix for this?  Fuel is 10%nitro WildCat 16% total oil.

Make sure youíre using the right amount of nitro.  New engines often have black exhaust ó itís the metal parts wearing together.  It usually stops after about 10 tanks of fuel.  If youíre still concerned about it contact the company or importer.



Your set-up and maintenance articles were just the thing I've been searching for.  I'm building an airboat (not your forte, I know) but I'm using an NS Bluebird engine that I've been given and don't know it's size or specification other than to say it looks just like the ones in your strip-down pictures.

I've been having trouble running it as it's new and needs running in, setting up, etc. (that should be no problem now that I've found your site though, lol).

The thing is I'm in need of an air-filter for it (preferably moisture proof) and I haven't the foggiest idea where to get one.  Can you recommend any?  Any help you might be able to offer would of course be very gratefully received indeed.

Unfortunately, model aircraft engines that turn aircraft propellers generally donít have room for a real air filter.  If you just want a basic air filter then Bru-Line makes one but I doubt it will do much to keep water out of the engine.

As far as setting it up, just take off the back plate and load it up with 2-cycle oil such as marvel mystery oil.  Leave the backplate off and turn over the engine a lot.  You can hold it in one hand with a rag over the open backplate and the carburetor closed.  That should get the oil well distributed as well as loosen up some crud that may have accumulated.  Rince often with more oil.  Once you have most of the crud out, put the engine back together and close the needles all the way.  Open them up a couple turns, mount the engine and see if you can get it to start.

With all that oil in it the engine will probably take longer than usual to start.  Once itís running let it warm up and then start throttling it up and down.  Adjust the needles to almost peak RPM but donít make the engine scream.

Also, find out what size engine it is for sure.  I have no idea as Iíve never heard of the brand.  Use a propeller one size smaller than recommended for running in.  For example, if the recommended prop is a 10 x 6 use a 9 x 6 instead.  Again, donít totally peak the engine.  You donít want to over-rev it and destroy it.  If the engine is not ABC (ringed or not) then that advice is good.  If the engine is an ABC then you can peak it a little more because ABCís donít like to run rich. They need to heat up to run properly.  If the engine is fairly old then you can safely assume itís not an ABC.

Hope you have good success with the engine.



There is a rule of thumb... which I am not currently able to put my 'thumb' on as to the relationship between the opening size in the front of an engine cowling that allows air to pass in and around the engine for cooling, and once having flowed past the engine, to exit the rear of the cowl near the firewall through openings of a different size.

My question is what is the relationship in square inches of the inlet aperture to that of the outlet aperture.  I recall it has something to do with venturi principle that sucks air through, but I can't find anything definitive, e.g., opening areas in front should be twice... or three, maybe four times the total of the open areas at the rear of the cowling... or vice versa.  For some reason I seem to remember that the openings in the front of the engine can be significantly smaller than those in the rear... but then again I may be wrong.

Youíre right ó the air outlet needs to be larger than the inlet.  The rule of thumb is 2:1 outlet to inlet.  More is better but that ratio has worked well.



Does the same carburetor mixture control adjustment method work for four strokers as you advocate for 2 strokers?  I have a pre-Surpass OS 90 4 stroke that seems to be "off tune" and I am having a time getting a rich high speed run and decent idle.

The principle is the same but you have to train our ear to hear the difference.  When a 4-stroke speeds up it isnít as noticeable as a 2-stroke.  Adjust the high speed needle to peak RPM while holding the plane vertical.  Then back it off a few clicks.  Once the high speed needle is adjusted set the plane down and use the pinch test to adjust the idle.

If the engine isnít broken in yet then get it as close as you can and fly the plane a few weekends.  Donít fly around at full bore.  Use the throttle and try to do most of your flying at no more than 1/2 throttle.  Use full throttle for short climbs.  You should be able to get the needles closer to ideal after every few flights until the engine is fully broken in.



I'm having trouble keeping an Irvine .46 2 cycle engine running in my model.  I can get it started but it only runs for a few seconds and then dies.  I have tried adjusting both the fast and slow mixture screws with no improvement but because it dies so quickly I can't adjust the mixture while it is running.  The RPM seems to increase before it dies.  I have both the fast and slow mixture screws set at about 2 turns.  This engine has been run before for a longer period but not for a while. I'd appreciate any ideas or advice.

There are a lot of reasons why your engine may be quitting.  I would turn both needles in all the way and then turn them each out 3 full turns.  That should be a sloppy rich setting.

My first guess is that there is fuel feed problem.  Can you mount the engine on a test stand and see if it runs that way?  If so you can bet that the problem is with the fuel system.  Try new fuel lines.  Also check to see if clunk line in the tank is ok.  It may have fallen off or torn.

Another thing to check is that all the bolts on the engine are tight.  A leak in the engine will cause all kinds of problems.

One other thing to check is the glow plug.  If the element looks good, it glows and itís not leaking around the stem then itís probably fine.

Let me know what you find out.



I'm switching from a 4-stroke to a 2-stoke on a 456MB Dragon.  The factory etched line in the firewall works perfect for a 4-stroke mounted upside down but using the same lines for a 2-stroke mount with head up seems to me to be too high.

I don't know if the weight hanging down versus the weight on top would have any effect but it looks funky being I'll have to cut a hole on top of the cowling.  Can I lower the engine down some and not have any effect on it flying?

If you move the engine the flight characteristics of your plane will change. The thing to look out for if you turn the engine over is the fuel-tank/carburetor relationship. If you build the model as designed with a recommended engine then the fuel-tank to carb relationship should be right. If you turn the engine over then that might change.

All you really need to do is make sure the centerline of the fuel tank is about 1/4Ē or less below the centerline of the carburetor and the engine should run fine.



I read with interest your segment on propellers and noticed no mention of reverse-pitch props.  I am a relative newcomer to the RC flying culture but I used to fly control-line a fair bit (when I was a teenager).  Now that I'm retired, I hope to get into scale RC and one of my longer-term goals is to build a P-38 Lightning (my personal favourite).

You are likely aware that this is a twin-engine WWII fighter-bomber with counter-rotating engines to counteract torque offsets.  To do the scale model justice, I would like to find a reverse-running engine (in the .47 two-stroke range, reasonably well matched to the corresponding regular-running engine, e.g. an OS .47 FXI) equipped, naturally, with a reverse-pitch prop.

Is any of this possible, in your experience?  I know that reverse-pitch props are available for those odd pusher applications but I haven't found a reverse-running engine yet.  I'd appreciate your insights.

Some engine manufacturers do make a crankshaft that allows an engine to run in reverse.  I believe Fox Manufacturing does this with some of their engines.  I'm not sure if OS does but you can contact Great Planes (the US distributor for OS Engines) to find out.

What I would do is find all engines that you would be willing to use in your application. Then contact those manufacturers and ask if they have conversion parts.

Probably the easiest way is to go electric.  An electric motor can turn in either direction and has the added benefit of reliability ó no flame-outs ó which can kill a twin fast!



We are wondering what is the best way to fix a cross-threaded glow plug? in his Saito 65 (heli-coil, retapping)?  Amazingly, the engine runs ok with the plug in crooked.  Thanks for your help.

I hesitate to give advice here because itís something Iíve never done and donít want to help you make the problem worse.  It would bother me, but if the engine runs fine with the cross-threaded plug then Iíd probably leave it alone until there is a problem.

What Iíve heard most guys do is use a heli-coil insert.  You should use a drill press to ensure the hole is square to the head.

One way to avoid cross-threading in general is to always start the item by turning it backward.  When you feel a click it means the threads are aligned correctly and then you can start threading the part in.



I'm working with a group on my Senior Design Project in Electrical Engineering at the University of Kentucky.  We're using a couple of AP Wasp .061 engines in our project.

Neither of us has had any experience with these engines before, and I was the one who decided to work with them.  I've been having problems getting it started. As far as I know everything's hooked up correctly.  I have the fuel tank set up ok with the vent and whatnot.  I have a glow plug starter that fits.

The engine is mounted on a stand for testing.  I've been following the break-in and regular starting instructions as correctly as possible.  The needle (2-1/2 turns out from closed) and throttle (1/4) are set as told.

I prime the engine, then hook up the glow plug starter and try to crank it.  Sometimes it sounds like it fires a few times, then it will quit after a few revolutions.  I can't get it to run more than a few cycles before it quits.

Do you have any thoughts as to what's happening, or any suggestions on how I could solve the problem?  Also, when the glow plug starter is attached I can hear the glow plug making noise.  Is this normal?  Any help would be appreciated.

You chose what can be a very tempermental engine to learn with.  Small engines are much more finicky than larger ones.

Be sure you have the right fuel.  Small engines have a larger surface area to volume ratio than larger engines and therefore tend to run too cool.  You need fuel that has lots of nitro in it ó preferably fuel specifically made for 1/2A engines.

Hand starting these engines can be very difficult.  I donít even attempt it any more.  I donít see any problem with the technique you describe, but I would open the throttle to about 1/2 when starting the engine.  This is for the same reason as mentioned before.  These engines usually wonít run at idle until theyíre hot so what youíre experiencing doesnít seem unusual to me.  The engine simply isnít hot enough to keep running at low throttle.

You might want to consider investing in an electric starter made for small engines.  I canít remember who makes them.  There used to be one made by GloBee but I donít know if theyíre still in business.

The sizzling noise you hear is normal.  It's fuel in the glow plug being burned.



I was wondering if you could help me with respect to fuel tanks and fuel usage.  I am not sure where I can find information on the fuel consumption of a .40 cu in engine, nor how much flight time a tank (such as 240 ml) would give me.

Fuel consumption depends on many variables.  Any given engineís fuel usage will vary considerably depending on what type of fuel is used and the propeller.  For example, a typical sport .40 turning 12K RPM will use less fuel than the same engine turning 16K RPM.

I donít pay much attention to fuel use as I tend to fly for less than 10 minutes and the tanks I use allow for longer flights than that.  I would say the typical sport .40 uses (give or take) 1 ounce of fuel per minute.

Iíve seen several engine reviews where the reviewer mentions the amount of fuel used by the engine, but again, I donít pay attention to that information as itís never been important to me.



I see your method of cleaning an engine is boiling it.  Can I boil the assembled engine?

I left it a year or two hanging in my Cub.  One day brought it down and the engine won't turn over so I have taken carb, backplate cover and head off.

I poured engine fuel on the piston hoping to undo it.  I ended forcing it to turn but was very hard.  I put oil on the bearing and still that didn't loosen it much.  I then put the oil on the shaft and that is where it was sticking.

The engine is a brown color inside.  I would like to make it look new again before I put it together.  Can I put all parts into boiling water?

What you've experienced is either the oil left in the engine gummed or the engine rusted or both.

I've had several engines gum up over the years, but it's not a big deal.  Normally I use a thin penetrating oil liberally through the engine while turning it over by hand to loosen things up.  If the bearings are rusted you'll know it because the engine will feel like there's a piece of sandpaper inside when you turn it over.

If that happens, then I disassemble the engine and purchase new bearings.  If there isn't rust inside the engine then I put it on my test stand after the engine has loosened and run it to clear out the junk.  Probably not the best way to do it but it's caused me no problems.

Another thing you can do is use a hair dryer or heat gun to loosen the engine.  Heat will soften gummed oil.

I would not put an entire engine in boiling water because you have to disassemble it afterward to get all the water out and prevent rust.  You may as well disassemble the engine before boiling it.



Is there an article on starting Nitro 4 strokes?  I have never started a large RC engine.  The little .049's were easy.  I have a heavy duty Sullivan electric starter and a Hangar 9 main panel with a 12 volt 7 amp sealed battery.  I have an OS FS 70 Surpass engine.

My experience is that a 4-Stroke engine are not any more difficult to start than a 2-stroke.  They like to be "wetter" though.

The way I hand start a 4-stroke is the grab the spinner between my thumb and forefinger and then give it a sharp flip backward so that it bounces off of compression.  If it doesnít start in one or two flips I pull out the electric starter which gets the engine running immediately.

O.S. makes excellent engines so I expect once you learn how to start yours youíll be very happy with it.



I bought a Tower Hobbies .46 engine that I haven't been able to take to the air.   The engine seems running good but I have this little problem every time I turn the engine/model upside down, it won't run for more than about 8 seconds.  It seems to go rich then dies.  It does it at idle to 50% throttle (I haven't try it a full blast).

I tried to check everything but still the same.  I made sure no fuel goes back through the muffler pressure line.  I disassembled the engine twice and there is not visible damage on parts.  I checked seals, adjusted the low speed needle...but no way.

In normal position it runs fine and throttles good.  So I have no idea what else should I do.  Is there maybe an air leak, fuel, or do Tower engines run poorly?

Mine is running Cool Power 10% fuel with 11x 6 prop.

Also I red in an R/C forum that WD-40 can also be used as after run oil on new engines or engines with no rust at all.  What do you think about this?

Do not put WD-40 in your engine. It can actually cause rust.  Itís bad stuff.  Use Marvel Mystery Oil or sewing machine oil.

If your engine quits with the plane upside down then itís probably a fuel feed problem, not an engine problem.  If your tank is too low when the plane is upright, then the engine will go rich when the plane is upside down.  If the tank is too high then the engine will go lean when the airplane is upside down.  You say your engine seems to richen, so I suspect the tank position is too low.

The center of the tank should be about 1/4Ē below the center of the carburetor spray bar (the tube going through the carburetor).

See if changing the tank position helps resolve the situation.  If not, let me know what happens.



I have found that if the aluminum is discolored it can be brighten up by boiling it in water with a little Cream of Tartar

This was in response to engines discoloring when cleaning them using automatic dishwashing detergent.  I haven't tried this yet, but if it works it is a great and welcome tip.  Thanks!



I do not have the instructions for a Royal .25 R/C ABC Schneurle Ported Glow Engine.  The engine is new. I would like to know where to set the high speed needle valve and the slow speed needle valve.

I think the slow speed setting is a small screw on the throttle arm centre.  I accidentally moved this from the factory setting.

Turn the high-speed needle all the way in until it just starts to become snug.  Donít force it or you may damage the spray bar.  Now open the needle about 2-1/2 turns.  Leave the low-speed needle alone for now.

When the engine is running, open the throttle all the way while standing behind the engine.  Lean the engine (turn the high speed needle in) until the engine is running at peak RPM.  Now back out the needle off until the engine slows down just a little, but is still lean.  This is an ABC engine so donít run it rich like you would with a non-ABC engine.

The engine has to be broken in before you can get a reliable idle setting.  Run a couple tanks of fuel through the engine by running it for a couple minutes and then shutting it down and letting it cool completely.

When the engine is running, move the throttle between low and high periodically and let it run at each setting for about 30 seconds.

Use the pinch test to check the idle setting. Again, the idle will not be reliable at first, but you can make coarse adjustments.

Thanks for your help - my engine is running perfectly now.  Do you recommend using after run oil?  If so, what brand of oil would I use and what is the best method of getting the oil into the engine.

I normally use Marvel Mystery Oil to protect the engine by adding it through the carburetor when it is fully open.

Marvel Mystery Oil in quart bottles is available with automotive oils from Wal-Mart.  It is not very expensive.  I have also used sewing machine oil.

The idea is to put enough in the engine to coat anything that can rust (bearings and crankshaft).  These oils will not protect your engine for long periods, however.

If you plan to store your engine for more than a couple months, you should remove the engine from the plane, take off the back plate and fill the engine with transmission fluid.  Put the backplate back on and put the engine in a Ziploc freezer bag.

Also see

In particular, read the part about running all the fuel out of the engine at the end of each flying day (in the after run oil section)



Where can I obtain a chart giving proper size or recommended propeller size for various size model engines, glow and gasoline fueled, two and four stroke engines?



What is the best way to convert/clean a two stroke engine and tank that has been run from new on castor based fuel to fully synthetic fuel?

Iíve never really thought that anything needed to be done.  Normally I open the needle a couple clicks just in case the new fuel causes the engine to run leaner than the old fuel.  After the engine is running I reset them using my standard procedure.  Other than that I donít do anything special.



I have acquired a second-hand model boat powered by an aeroplane engine (like an air boat).  Can you tell me the correct rotation the engine should run?  I.e. clockwise when looking at engine towards prop, or vice-versa?  I could be wrong but I think I have been able to start it in both directions and it still propels the boat forward, is this possible?  I have been trying to start it anti-clockwise as you would look towards the prop.  Engine type is MAGNUM GP and has 25 on the side.  Any help would be appreciated.

The engine runs clockwise as you look at it from the rear.  99.9% of all model engines run in that direction.  An engine can be modified to run in the opposite direction, but that means buying a new crankshaft.  I doubt your engine has that modification.

Any glow engine can run backwards at low throttle, but when you try to increase the throttle, the engine will quit if it is running backwards.

A propeller will not pull the boat forward if the engine is running backwards.  It will push it backwards.  If the engine is running in the correct direction you will feel a strong column of air pushed out behind the prop.



Hi, I am looking for a prop to test and set-up my race boat engine on the bench.  The engine is a 26cc petrol Quickdraw which revs to 15000 RPM under load and is 6.5 HP.  I have been told that a 16" wood prop cut to 14" is about right and it is cut to take the sharp tips off, but I was looking for may be a 4 blade to keep the cover as small as possible.

I strongly suggest you contact the manufacturer for their guidance.  I  have never run a boat engine and donít want to give you any advice that might ruin the engine.  Iím thinking you could run the engine on a stand, but as far as I know, boat engines donít have a hub that you can bolt a propeller to.  Check around with other boaters and perhaps visit some of the forums to see what the standard procedure is.



I have read two different articles in the magazines suggesting that a good way to clean an engine is to put it in a crock pot full of Prestone anti-freeze for about 4 hours on low heat.

Discussing this with my friend, he brought to my attention that Prestone attacks bearings in automobiles unfavorably.  Your comments on this cleaning method would be appreciated.

While the crock pot is not a bad idea, I would not use anti-freeze to clean my engines and have never even attempted it.  The problem is that engines are made from a variety of materials.  It will be hard to find any kind of solvent or cleaner that will not attack at least one material used in the engine.  I suggest you use the mildest cleaner you can get away with.  I use dishwashing detergent and have had no problems other than some harmless discoloring of a couple engine cases.



I like your discussion on the throttle cables and I agree the Z-Bend is the way to go.  I have never been able to get a really clean z-bend and wonder whether there is a special tool or trick that will make my work look more professional and make me feel better about the cables.  (My bends look more like a lazy S than a crisp Z).

Z-bend pliers are widely available through hobby shops and online/mail-order sources.  I use 1/32Ē music wire for throttle pushrods.  This size can be bent easily and cleanly using a regular set of needle-nose pliers if you do not have Z-bend pliers.  I used to use regular pliers to make Z-bends in 1/16Ē music wire but my results werenít much better than what you describe.  The Z-bend pliers make clean and consistent bends.  If you practice with them a bit you can place the bend very accurately to the point where you can put a Z-bend in both ends of a piece of wire and have the length almost exactly what you want.

Also, if you use a standard size servo the 1/32Ē music wire will be a sloppy fit in the servo arm.  You should drill a new hole in the arm somewhere that is a better fit.  My local hardware store sells wire bits by the piece, but you can get a set of them from MicroMark if you canít find them anywhere else.



I recently acquired a <insert engine here>.  The engine has no instructions with it and I wonder if you can give me the start/run needle settings or can tell me where to find it.

You donít really need the instructions.  Open the needle 2-1/2 turns to start the engine.  Once it is running adjust the needle to the setting you want.  This page concerning starting and break-in gives more detail.



I recently purchased a second hand AM 10 1c.c. diesel engine and found that the contra-piston was jammed solid in the cylinder.  I managed to free it but upon re-insertion it persists to jam even after cleaning.  Another point is that the piston itself will not go right through the inner cylinder bore and I am not sure whether it should or not.  If you could please give me some information on how to correct this matter I would be very grateful.  Thanks.

My only experience with Diesel engines was using a Davis Diesel conversion head on a Royal .40.  I did not follow the directions, put on too large of a prop, tried getting it started in the dead of winter, broke two connecting rods and gave up.  In other words, I am the last person you should ask about Diesel engines.  Try contacting the manufacturer or a recognized authority on Diesel engines such as Bob Davis.



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