OK here's what happened yesterday: This is a restored old trainer with a Magnum 40 GPA. The plane is a bit heavy at 6 lbs. My instructor tried twice to get it up, but we used the whole grass runway and it never had enough oomph airspeed to take off. The engine is used, but sounded good, actually kind of quiet and purrs at idle, tuned well, the fuel system seems sound. Foam wheels which didn't seem to be hanging up on the grass, grass it was pretty short. I'm sure at the other field I use, it would have gotten up off the asphalt runway, but not quite enough thrust on the grass. It is swinging a 11 X 6 prop. I took it apart last night, and there was a baffle in the muffler, which I read in a another post when removed could give more power. I'm using 10% nitro fuel.
So what would you do first to get more thrust? Rubber wheels (heavier)? Different prop? Take baffle out of muffler? 15% nitro? Something else I'm missing?

Oh I forgot to add the spinner is heavier thatn plastic, has an aluminum backplate and a big aluminum bolt, I assume it was for nose weight, there is qit a bit of lead glued to the back of the firewall for balance.

I also have a .46 Magnum GPA I could switch out to. Someone told me the .46 is not that much more powerful than the .40. I have 10 x6 and 10 x 7 props I could switch out to on the .40. Shouldn't the .40 get the plane in the air on a decent short grass runway?

Check your wing encidence you may be trying ti fly into the ground. If you got inverted you would have unlimited vertical

Did you check the balance point.

With that engine on the trainer you might try the 10x6 or an 11x4 for more pull. It's a bit over propped. Larger light wheels [2.75 or 3 in.] would help taking of from grass as well.

I am assuming a tricycle gear. How does the wheels turn, any binding? If you pull full up, will it lift the nose or isn't it getting up that amount of speed.

If you can swing that 11-6 at 10,000 rpm, you should have plenty of HP in the engine. I doubt that removing a baffle is going to make any difference.

like said before, check the balance. unless it is grossly nose heavy, it should fly.

If you can find a large piece of cardboard, ply wood, etc. try using it for the first part of your roll out. It will give you and 8 foot runway that might allow you to get up enough speed to get on top of the grass. Have a helper hold the tail and go to full throttle then release the plane .You should get up about 1/3 the speed needd for lift off by the end of the board, and have enoght speed that you are not trying to lift the plane out of the grass with the wheels.

Just to be sure it will fly though, I would take it to the field with the paved runway and see how it does there.

One last stupid question, but I have to ask, are your controls set up OK, IE up elevator is really up?

Don

Slightly larger foam wheels would help.
Try an 11x5 prop for a little better acceleration or 10x6 if you are cutting grass
Make sure the plane is sitting LEVEL when parked and NOT nose low.
Recheck that UP elevator is "UP" elevator
Lower both ailerons a few degrees(more lift), easy if you have flaperons hooked up but probably not
More nitro will have a negligible effect
Check CG
Keep nose steering movements to a minimum as this creates drag
Taking out the baffle may help a tad as will good tuning

Good luck.

Oh yeah, hold the tail and run it up to full power before releasing it for takeoff roll. Not allowed at some fields though.

I would try the 10 X 6 or 10 X 7 prop, I think the 11 X 6 is a bit much for that engine (it is more of a .46 prop). I would also look at taking the baffle out of the muffler if that is possible, I understand this could give you an extra 1000 RPM, but I do not really know for sure.

Other than that I would look at the options Bruce has suggested, try to make the plane a bit lighter, but even at 6 lbs that .40 should be able to pull it. I have a 6 lb tail dragger trainer that I just flew yesterday with an old OS .30 on it. There was not much power, but with a long take off role it got into the air (with a heck of a head wind - 20 mph).

I think you need the shorter prop and it will get off the ground.

BTW - the .46 should have about 15% more power than the .40 you are running, that may be just what you need to get it off the ground.

All the stuff above yet here are some more 2 cent suggestions.

11-6 on a sport 40 is a bit much. 11-5 may help, but I suggest an 11-4 or 10-5.

Grass can well slow a big airplane down. Grass can be a deceiver.

You say the machine weighs 6#. YOU DON'T SAY what the wing area is. [:-] If a 6# model has less than 600 sq.inches w-a, then you may have a low-power problem. If the wing area is 700 sq. inches, it should leap right off.

And as pointed out above, be sure the airframe is sitting level. Nose down provides negative lift to hold you down. Too much nose up will normally either drag you down or leap off before flight speed is attained. Then you will bump the nose gear first when you land because it's difficult to land with the nose so high. Bend that main gear down using both weels at once. Set it level. Almost every day at the field, I see those problems but the self-ordained experts don't want any advice from the old flart that never has such trouble. so I keep my mouth shut with a smile.

Good luck.

The balance point, the elevator throws, and all the other obvious factors were all checked out by me and my good instructor. I'm a beginner at actually flying, but I didn't fall off the turnip truck last night. I had all my i's dotted and t's crossed, such as I can.

You're running an 11X6 on a .40? I'd recommend that prop for the .46, but not for the .40. I think the .40 would be more suited to a 10x6.

If that doesn't get your plane off the ground, try the .46 with the 11X6. The .40 Magnums I've seen were less powerful than the .46's.

Good luck,
Dave Olson

Several people have suggested to use 10x6, 11x4, or 11x5 prop on the 40 engine. What size diameter wheels do you have on the plane? Larger wheels work better on grass.

Well it sounds like you are good to go.......

I agree, a 10x6 prop will increase the RPM a lot and give you much more thrust.

A guy at our field last week could not get his trainer in the air no matter how much runway he had. Come to find out he had his prop on backwards.

Now thats funny

I can see why it never got off the ground

Turnip truck

Ok where here's what I'm doing based on the consesnus:
1. Switch out the prop to a 10 X 6 from 11 X 6 since the 11 might be overpropped for a .40 on a grass field.
2. When compared to my other trainers, the wing incidence was absolutely flat, vs. my other trainers which had a definite 1 or 2 degrees up when sitting flat on the ground. For some reason the nose wheel was in fact smaller than the other wheels, so I can raise the nose either via a bigger front wheel, or adjusting down the nosegear.

3. Might as well switch out the .40 for the .46 that I have on a lighter trainer.

I'll write home to my 18 siblings in the turnip field to tell 'em how smart we all are.

If you are going to switch out the 40 for a 46, then stay with the 11X6 prop.

Right. The .40 gets the 10X6 and the .46 gets the 11X6. Think I'll leave the mufflere baffle in for now just to see where I'm at.

i know this may be absurd to ask (since you may waste a perfectly good day for flying), but after all the suggestions and all the changes... wouldn't you like to try it with the 40 to see if it flies first? I am curious to know if it will get off the ground with the changes suggested...

My other question is for bruce88123.. why would more nitro have a negligible effect?

The performance difference between 10% and 15% in a sport engine of that type is small. There would be some but I doubt he would notice it. That's the simple truth.

Yea, a 6lbs trainer with a .40 and a 11x6 prop is exactly what I have. The thing climbs like a tame sport plane. Leave the combo and find out what's wrong with the airframe. A bigger motor won't fix an airframe problem. (Well, within reason of course )

Gotcha! Thanks for the info.

well I already have both engines off so I thought I'd put the .46 on the 6lb. plane and the .40 on the 5.5 lb PT40, just for symmetry, I guess. They have the same engine mounts so no redrilling etc. either way. But true, it makes sense it should fly fine with the .40 with the prop and nosewheel/incidence fixes.

oops--sorry-- I hit quote instead of edit..