rclement

My Feedback: (1)

I have a plane that I suspect doesn't have enough down thrust on the firewall. How thick of a shim ( under the engine mount ) can I add to change it? Do I use washers or can I use some .010 shim stock?

FBodyexpert

Your downthrust from the pilots view should actually be down-right and is usually 2 deg in each direction. Sometime a little more for a plane that has a higher power to weight ratio. Start by using the thinnest washers and work from there. Yes you can use shim stock to make your own washers. Really the best way to check for the right downthrust is to fly your plane. When in strait level flight, open up the throttle. If the plane rolls left, not enough right thrust. If it pitches up, not enough downthrust.

alan0899

G'day Mate,
Your statement about right & down thrust, are not quite correct.
Depending on the plane, it may need right thrust, but no down thrust, & if the plane needs right thrust, it won't roll, if it needs more, it will yaw, left.
To test for needed right thrust, fly full power, trim for straight & level, hands off.
Pull up into a vertical climb, & watch the nose, trim for straight up climb with rudder. you may need to do this multiple times.
Land & measure rudder angle, on the ground, with radio on.
The amount of right thrust required, is 1/2 the rudder angle, measured.
The amount of down thrust is tested by, flying the plane at full power, straight & level, trim for hands off, & chop the throttle, & watch what the plane does, if it continues on, without any pitch change, thrust is OK, if it pitches up, a lot, you have too much down thrust, if it pitches down, more down thrust needed.

aerowoof

alan0899 is correct on how to check for proper thrust,as far as shimming goes ernst used to make thrust wedges that allowed you to make 1 degree incremental changes by mounting them behind the mound.

MinnFlyer

My Feedback: (4)

How much? The only way to tell is to try it.

Put a washer behind the top two mount holes and the firewall (Or under the two rear engine tabs) - If it's not enough, add another washer.

Repeat as necessary

LDM

My Feedback: (15)

all the talk about engine down thrust , ever think its the wing incidence ???
I had a blue max , flys like a baby , but in the beginning it was such a hand full thay just wanted to keep going up and up , all the engine down thrust in the world was not fixing it , one simple wedge under the trailing edge was all I needed

rclement

My Feedback: (1)

Thanks for the info guys. [8D] I can't change the incidence because the wing (s) is pinned into the fuse when it's mounted each time. I'll give the shimming a try tomorrow when I go fly. I just didn't know where to start as far as thickness and I didn't want to be adding too much and loose control of the plane.

aerowoof

I hope you will do the testing before anding any shims to see if it is even needed.

rclement

My Feedback: (1)

Yeah, I'll test fly it again tonight and bring the shims along to experiment with.

Ernie Misner

Hi Rob, keep us posted. From being at the field with Rob, his problem seems to be that when rolling inverted, it is taking a lot of elevator to hold it inverted, although the balance is already moved way back from the recommended point. So...... to add a bit of downthrust as an experiment, or to keep pushing the balance further and further back???

Thanks,

Ernie

MinnFlyer

My Feedback: (4)

What plane is it?

Ernie Misner

Hi MinnFlyer, it's the GP Reactor with Saito 82. It's set up the same as most in the Reactor thread with the battery moved back in the fuse and even has 3 sticky weights added to the tail also. Shouldn't still be nose heavy, seems to fly great except that it still takes more elevator inverted than it seems like it should have. Others haven't complained of this so it has us wondering.

Thanks,

Ernie and Rob, the local Reactor pilots.

alan0899

G'day Ernie,
You have a balance point problem, keep moving the balance point back, until it flies like you want. Also remember as you move your balance point back, your elevators will be more efficient, & you will need to reduce the throws accordingly.
Remember the balance point shown on the instructions is only a guide, on some planes, this is very nose heavy.
If you keep adding down thrust, you will also need to add, up elevator, when straight & level, & this will make it even worse, when inverted, as the up elevator will be pushing the nose DOWN, when inverted.

MinnFlyer

My Feedback: (4)

My favorite way to check if a plane is nose heavy is to do a spin.

If the plane spins easily, it's right.

If it just spirals down, it's nose heavy.

rclement

My Feedback: (1)

Thanks for your input Ernie (and Minn and Alan). We'll be able to experiment more on Monday. Yippie!!

rclement

My Feedback: (1)

I tested it on Monday. It seemed a little better than before (I added about .030 thousandths shim behind the top of the engine mount. It helped some but it still takes too much to hold it level when the plane goes inverted. Any more suggestions?

Ernie Misner

Hi Rob, methinks you gotta move the CG back a little more then. At least that is easy to try. You could temporarily put on some more sticky weights, or take off the TT spinner for a couple of flights to see if you are going in the right direction still.

If it were too tailheavy already the elevator would probably be too sensitive, which it doesn't seem to be....?

Ernie

rclement

My Feedback: (1)

I wasn't sure if it was too tail heavy or not. I could add more though. I shimmed between the engine / engine mount rails and that brought it down but I'm not sure if it's too much or not.

rclement

My Feedback: (1)

I just re-read this part so maybe I've added too much down thrust.

Ernie Misner

Also, if you have too much downthrust when you fly along and then chop the throttle, the plane will want to balloon up. Your's didn't though.

Ernie