I have a Spitfire with a Thunder Tiger 120 Pro. The engine is broken in with rich needle setting on 15% fuel with suggested 13 by 8 prop. At full power it ran for 10 seconds and then started losing rpms. After break-in I put an oversize prop (15 by 8) and it ran great but a little down on RPMs (I thought because the prop was to big). The manual says the engine should have a 14 by 8 prop with 75% methanol / 25% oil. That is why I bought the larger prop because the 15% fuel I am using. After seeing marginal thrust compared to the 13 by 8 break-in prop, I decided to trim the ends of the prop 1/4 inch on each blade. Now the engine loses power after ten seconds just like It did with the break-in prop. Must I use an oversize prop to keep the engine running smoothly? I still have not flown the plane and I am worried I will lose power if I go to full speed. Is it the fuel? Should I switch to 5% or 0% fuel? It seems the engine does not like high rpms, although it is rated to 15,000. Should all engines be able to run at full power at the test stand without losing power or do planes need airspeed to keep engiones going smooth? This is my first nitro plane. I am scarred I am going to lose power on my first flight!

15,000 RPM was probably attained with a 10 x 4 prop. Thunder Tigers are good engines - I really like the Pro-46 version - but that is the maximum RPM achieved in the lab under non-field like conditions. Like the maximum highway mileage used by car manufacturers. Your mileage may vary . . . a lot. If you get 14,000 RPM with a 13 x 8 you're doing well. Isn't the 15 x 8 the normal prop for that engine, anyway?

Engines should run well on a test stand and similar to on the airframe. The exception would be if it was a cowled engine that was overheating - so unless you are using a cowl while on the stand that's not it. Engines do tend to run a little leaner in the air; hence the practice of richening them a few clicks from maximum while on the ground.

I don't believe switching nitro percentages will help you here. It would improve idle or ultimate RPM, but losing power after 10 seconds would indicate something else is at fault. How old is your fuel? Once opened it may only keep a few months (depending on climate & conditions).

25% oil is 5% more than most engines I am familiar with would recommend, and I don't know of a fuel supplier that offers it "over the counter". Are you mixing that up special?

If you get 10 seconds and then it drops off it could be several things. Overly rich, overly lean an air leak in the fuel line or a fuel delivery problem. If you had lots of vibration the tank would foam and that would kill the engine after a few seconds. The test stand would run better than in the model if that was the case. Did you balance that chopped prop? Does it just die or does it burble or rev up slightly before konking out? My guess would be towards overly rich because it runs but slower. Over lean would just stop, usually suddenly.

Pinch the fuel line and if it stops after eight or ten seconds with a little increase just before dying the mix is good. If it goes 20 seconds you are too rich. (If it dies in a few seconds without increasing you are lean.)

I noticed the words " first nitro plane" and "Spitfire". What is your flight experence? Just curious

i would definitely NOT put a 13" prop on any 1.20 sized engines.. Use a 15x6 16x8 and you'llbe happy I run a 16x8 or 17x6 on my OS ax120, and 16x8 on my webra 120 speed, and a 16x8 on my saito 120



Dont bother paying attention to the "practical RPM range" try to stay in the 9-11k range for your 1.20 2 stroke and you'll be fine



Edit: Just noticed i hit 2000

Losing power after ten seconds sounds like overheating, likely caused by a lean condition if it's on the test stand. As stated above, double check your tuning and make sure you aren't getting any fuel foaming from vibration.

Thanks for all the info. My plane has a fiberglass cowling that allows the engine to pultrude about an inch. I think overheating may be the problem. I will make vents that allow more air to exit the cowling. A larger prop sounds good and an easy fix. I will try a 17 by 5. I'm going to fly it tommorow. About experience, I have over a hundred hours flying gliders. Everyone I talked to says this plane is not a good trainer. I'm a bit nervous. Flying and landing are what gliders do all the time; it's taking off that has me woried. I have no idea how fast to go before I rotate.

You are in seriously over your head. If you try to fly it The result will not be good. I can only urge you to seek help from a quailified mentor/instructor.

And yes I doubt you did not provide any lower outlet cutouts allowing the cowl to become a bake oven so overheating is highly likely.


John

Here is what happened; the cowling now has plenty of cooling for the engine.  I bought a 16 by 10 prop.  The engine now runs good at high RPMs, but it is now stalling at low RPMs.  We had it running good on the table at my club, someone was carying it to the runway and it stalled just before he set it down.   I took the engine apart; the piston is silver on the side but brown on top.  Have I done permanent overheating damage?  If the lower part of the engine was buried in the cowling, shouldn't the sides or lower part be overheated?  And not the cylinder head?
I have so much time invested in this plane I'm thinking of buying a new evolution 120 and put this engine in somthing simpler and cheaper, maybe a used plane.  Do aluminum pistons normally turn brown on top?

that is normal it is carbon from the fuel being burned.