and Frequently Asked Questions about Model Airplane Engines
These are questions I have received from
visitors regarding model aircraft engines and propulsion systems.
Other Mail and FAQ Pages
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
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
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
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
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
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
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
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
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
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
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.
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.
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.
normally use Marvel Mystery Oil to protect the engine by adding it
through the carburetor when it is fully open.
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.
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
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
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
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
about 4 hours on low heat.
Discussing this with my friend, he brought to my attention
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
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
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
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.
My only experience with
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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