What affect if any would be to raising the engine mount flightly on the firewall? Not by a lot , perhaps a half inch.
It would place it slightly above the original thrust centerline.
Would the plane fly nose heavy or tail heavy? Anything else?

It wouldn't have any effect on CG.....as CG being off will either make your airplane very sensitve especailly in pitch if you're tail heavy, or hard to slow down or not flairring if nose heavy...those tendencys remain constant.....
If the thrust line is off then the airplane will change in the pitch axis with power changes....

What airplane ?

High wing .40 size trainer.
So you think it would cause the plane to want to pitch down under power if it was set higher? Much like increasing the down thrust.

Raising the thrust line vertically by a half inch while not a hugh change will reduce the tendency for the nose to pitch up during throttle up application and therefore will require slightly less down thrust (angled downthrust).

John

Slap that engine anywhere and it will work .

on a trainer, shouldn't make much difference....

but I guess the question is why do you feel you need to move it?

It will have little effect on the thrust line, but you also need to consider the tank placement. Many trainers have the tank mounted too low to begin with.

Iam just trying to diagnose some issues with my trainer. Ihad to change the motor mount and it placed the engine slightly higher but according to yous it shouldnt have been a big deal. We have tried everything , it eventually suffered a crash on takeoff and is in the hospital at the moment.
The engine performance has never been any problem.

Besides the possibility of creating the fuel problem explained above, relocating the engine higher will reduce a little the pitch up related to full throttle.

The balance will not be affected.

If you want to keep exactly the same pitch reaction with full throttle of the original design, you could add a 1/20" shim to the bottom of the engine mount.
A couple of 1/16" thick washers will do it.

Check the schematic and adjust the dimensions to your trainer, if they are very different.

That is the problem, with the original thrust angle the plane was ballooning badly but flying with a tail down attitude despite proper GC balance.
When the engine was adjusted with enough donw thrust it flew better under power and didnt balloon too much but still flew tail heavy.
The control surfaces were checked and nothing else was out of the ordnary.
It was suggested to take the down thrust out and add nose weight but that ended in disaster.
When Iget it bolted back together I will give it another shot.

Flying with a tail low deck angle of the fuselage in level flight is a result of the stabilizer to wing decaledge i.e. the associated incidence of each and downthrust will not correct this.

Any given wing will require a given angle of attack to maintain flight at any given speed/weight and the stab will require a given incidence to provide the trimmed constant angle of attack to the wing. The fuselage is mearly the structure to tie the stab and wing together, if the mounting angles of the wing and or the stab are a little off then the fuselage may fly in level flight with the tail low, high or even level.

So that begs the question of exactly what trainer do you have and have you made any changes to the incidence of either the wing or stab?

Also you said the airplane is at the correct CG, so what is that CG that you have choosen?


John

The trainer is the Tower40 MKII. I made no changes , it is an ARF. Ibelieve the instructions call for 3.25" CG but Idont have the build manual in front of me right now. It has been suggested that the wing incidence may correct it but Iam unsure why the incidence would be off in the first place.

It seems to me that your wing needs additional angle of incidence.

Read these threads:

http://www.rcuniverse.com/forum/m_92...tm.htm#9281709

http://www.rcuniverse.com/forum/m_99...tm.htm#9961798

You could try shimming under the leading edge, in very small increments and testing.

The wing will adopt the same AOA to sustain level flight in balance, but the tail will rise up and the downthrust will increase.

That is what Iwas thinking. But why my particular plane has a wing incidence problem is strange. It is just an ordnary out of the box Tower 40 trainer.
Iwill add some shim to the leading edge when Iget it back together.

You know its really hard to try to understand because it seems as though you are using contradictory descriptions of whats going on in each sentence. Not trying to be argumenitive but just trying to help and that last sentance seems almost like a CG to far aft but just what does "flew tail heavy" mean. Does that mean it was unstable and difficult to trim out for steady level flight or does that mean it was stable but just flying with the fuselage hanging low in level flight?

John

Sorry, Imeant to say it flew with its tail down in level normal cruise.

OK, well that would indicate that the decaledge or incidence of either the stab or the wing may be slightly off and some subtle changes suggested by Lnewqban in post #14 could bennefit.

John