Down Thrust/Up Thrust

[Hogster]

I have a typical trainer a/c which I have to trim with 2-3 degrees of down elevator for level flight at cruising speed. I have been told that I could counter that with some down thrust and I can achieve this by putting a washer on the bottom two bolts between the engine mount and firewall. This sounds odd to me because surely this would push the nose up meaning I would have to add more down on the elevator.

[steve 1]

Hi Hogster, if you put washers under the two bottom bolts of the engine mount then that will give you up thrust, so pulling the front of the aircraft up wards.  Are you sure that the c of g is correct, not to tail heavy. I set mine up so if on 1/4 throttel it has a very slow desent, at tick over it desends a little quicker. Hope that helps

Steve 

Edited By steve 1 on 17/11/2010 22:05:24

[Martin Harris - Moderator]

You'd need washers under the top as you've realised to give downthrust.

 

Thrustline adjustments primarily affect trim changes with power adjustments - the likely "proper" cure is to raise the wing trailing edge fractionally if the elevator trim isn't changing adversely with power changes. 

 

 With a typical trainer you should expect a gentle climb under power and settling into a slightly nose down descent when you throttle back with a neutral trim at cruising power.

Edited By Martin Harris on 17/11/2010 22:05:56

[Biggles' Elder Brother - Moderator]

Hi Hogster,

 

if you want to add down thrust you add the washers to the top bolts between the engine mounts and the firewall not the bottom.

 

The need for this happens, usually, because the thrust line from the engine is not properly aligned to the centre of lift, its passing below it too low. The result is a upward couple on the whole plane - hence it wants to climb and you have to put in down elevator trim. Tilting the engine downward brings the thrust line to match the centre of lift and removes the nose up couple. Its a common problem on high wing planes. You could also fix it by changing the incidence angle of either the wing (downward) or tailplane (upwards). But that is rather more complex and may have other undesirable effects. Best to stick with altering the engine thrust line for now.

 

Try it with just one washer at first - see how that goes. You should be able to remove a little of the down trim. Then decide if you need to add a second washer - and so on.

 

BEB 

[Biggles' Elder Brother - Moderator]

Re the wing incidence change - great minds think alike eh Martin!

 

BEB

[Stephen Grigg]

my first brushless powerfull model was an Easi Cub which has an adjustable motor mount.First flight took off straight up above my head in milliseconds,,now heading straight down having completed my first loop,convinced it was about to be destroyed  I watched in wonder almost a passenger by now as it missed the ground and headed straight for me,as I dived to safety it was by now heading back up towards power cables which i missed and eventually remebered I had a throttle so landed in the next field.Phoned the shop for help  and was told You need to add DOWNTHRUSTRe read the instructions added downthrust and had many happy hours with the Easi Cub.

[Hogster]

Thanks guys. That makes it a lot clearer. So following this logic to also counteract the right trim on the rudder, I should add right thrust by adding washers to the bolts on the left side of the engine mount. Is that correct?

[Biggles' Elder Brother - Moderator]

Spot on

 

BEB

[Hogster]

Thanks BEB. I thought it wasn`t as difficult as it appeared to be. I must have misunderstood what I think was told to me in the first place.  

[Biggles' Elder Brother - Moderator]

Very easily done - we've all get it back to front sometimes!

 

A lot of models need a bit of down and right thrust - its often even built into the firewall by the designer.

 

BEB