Roo engine thrust
 

Ok..now my roo gears are sorted out...what abt the engine thrust angle? A friend of mine has put some shims at the BACK of his p120, effectively pointing the engine downwards (tail cone UP, so effectively giving the plane a down thrust). He says this setup makes the plane fly well. He hold full up elevator from the beginning and the roo rotates comfortably.

I remember that there was a discussion on RCU some time ago...where someone suggested to shim the engine from the FRONT...so as to point it slightly upwards (giving it an upthrust). The argument there, I believe was, to compesate for the higher engine thrust line vis a vis the roo's itself.

Are there different setups for different flight characteristics? Advanced vs. novice? I am confused.

Thanks in advance.
Anurag


PS: Enigne is a P120

Re: Roo engine thrust
 

Anurag,

I have no idea why someone would suggest shimming the front of the engine UP relative to the back. That would make the problems of a high engine thrust line with the engine in back WORSE. With the engine in back, shimming the FRONT of the engine UP is in fact, DOWNTRUST - since it tends to force the nose DOWN. What you want is the engine thrust line to point as close as possible to the line going through the aircraft's center of pressure. Since the engine on the Roo is elevated and in back, pointing the engine DOWN in the front makes it point closer to the center of pressure, which is probably somewhere near the CG line at about mid-fuse in height.

I shimmed my engine UP in BACK as suggested by the instructions to start. I have since removed most of the washers and I notice that it takes a bit more takeoff run to rotate, but that is all I can tell as far as a difference. I have no doubt though that if you shim the engine UP in FRONT, you will make it into a somewhat hard to control RC turbine powered truck...

Bob

Roo engine thrust
 

hmm...

Roo engine thrust
 

Imagine hanging your Roo in a thread at the CG point.

Then imagine shimming the front of the engine. This would lift the rear end of the plane, and make it want to nose down. This would be bad. I tried this with an EDF plane with the engine in the rear. When I aborted a landing close to the ground, I slamed the throttle open, and the engine just pushed my plane straight down into the ground.

Imagine leaving the engine neutral on the roo engine mount with no shims. If the plane was hanging in a thread in the CG, and you pushed on the rear of the engine, the nose will still pitch down.

But if you shimm the rear of the engine so the nozzle points up a bit, then it will push down on the rear of the plane, and the nose will pitch up.

Thats how it works anyway......

You just have to find the best balance.


Hope you get the point of this, Im not the best at explaining :-)
SJ.

Roo engine thrust
 

Ragz,

While I don't have any shim on mine, I would suggest you use one for first flights. The shim will align the motor with the correct thrust line to the fuse. In other words, the airplane will not have any pitch change from low power settings to high.
Without the shim on my plane I get a slight pitch down when full power is applied from low speed. The only time I notice it is if I have to abort a landing, full power will dip the nose but some up elevator compensates for it. For first flights, take a 1/16 x 1" ply shim and place it between the motor and the mount at the very rear. Also program your radio so the speedbrake retracts at an power setting above 65%, this will make an aborted landing much easier. Once you get comfortable with the plane test the thrust line by slowing the plane down to just above stall speed and then apply full power. If the nose dips still add more shim, if the nose rises with power take a little shim away (1/32 instead of 1/16).

Hope this helps
Todd

Roo engine thrust
 

good info...i have made notes of these..and will use them when the plane is ready.

thanks a whole big bunch.

anurag