Occasionally I read that the angular momentum of the turbine shaft and wheels will cause gyroscopic forces making 3-D flying of a turbine powered model helicopter impossible.

My Jetcopter weighs in at 20 lb fully fueled, and I can only get -4 to +11 or so on the collective, so it's not going to 3D like a 10 lb machine.

Still, I thought, I could try something. I can't tell whether anything happens with the pitch axis coupling to the yaw axis since the gyro would hide it. However the yaw axis has only the tail rotor to affect it. The turbine will couple yaw to pitch, and the tail rotor will couple yaw to roll.

I cranked up the rudder ATV to get a decent yaw rate (a bit more than 1/s) and observed the result....

I couldn't see anything, but my spotter thought he may have seen the tail move out of line a bit in the expected direction, but that could have been me bumping the cyclic a bit.

Has anyone else tried this?

Morning Phil

Will this test be with the 110mm or the 134mm t/r blades?

thanks
Don Dow

I'm assuming your talking about the gyroscopic tendencies of the turbine? In other words gyroscopic precession. The gyro (the spinning turbine in this case) would act in the direction of rotation and approximately 90 degrees ahead of the point where force is applied to the gyro. Any force. In the case of a helicopter this could cause gyroscopic precession along the lateral and vetical axis'. So if looking at the rear of the heli and the turbine turns clockwise you'll get the following control interactions.

Pitch forward will cause a left yaw.
Pitch back will cause a right yaw.
Both of these interactions would be hidden by the gyro.

Yaw left will cause a pitch up.
Yaw right will will cause a pitch down.
These interactions may be observed. However you need to take the weight of the compressor blades into account. The shafts weight can almost be thrown out all together since its at the center of rotation. Its directly proportional to the amount of observed interaction. It may be so slight you wouldnt consciously register it. Matte of fact, the main rotor may mask these interactions all together due to its own gyroscopic precession. Also keep in mind that these forces would be applied at the inertia point (CG of the turbine) and would dissipate further out since the weight of the heli would dampen the motion.

Moving along, since the turbine is mounted longitudinally, you wont have any coupling intereaction to your longitudinal or roll axis from the turbine.

The tail rotor wont affect roll through gyroscopic precession. First the force applied to the gyro ( the spinning tail in this case not turbine) is equal on all points with a commanded left or right yaw. However a roll left will cause a pitch up and a roll right will cause a pitch down. However these forces are so light, and applied so far from the CG they wouldnt or shouldnt cause a noticeable interaction.

Torgue tendencies of the turbine may cause the aircraft to want roll in the spinning direction of the turbine though. However this intereaction would really only be evident at low airspeeds and high rpm, and again the main rotor's gyroscopic precession would help null this out. Or should if the aircraft mechanics were properly designed.

When the rubber meets the road from a physics standpoint, the turbine's affect on aircraft stability is negligible or non-existant. They should in no way affect the aircrafts ability to perform 3D.

Don,

I'm using the 134 mm blades. GY501 gyro and a 9253 rudder servo. It feels pretty much like my X-cells on 105 mm blades.

I note that v-east has big tail blades available, and also RC Market in Belgium.

I have not tried 110 mm blades, and don't really want to. I've tried blades that are too small before and it look ugly with the tail wagging all over the place.

Eric,

The tail rotor will precess while it's yawing the helicopter. It has torque applied in the vertical axis, resulting in a roll-axis torque. If there was no vertical axis torque applied to the tail rotor, the helicopter would translate sideways without yawing.

I didn't consider the affects of the moment of intertia of the tubine shaft since the HP5 runs at more or less constant RPM.

Actually torque tendencies from the tail rotor would translate to the lateral axis, not the vertical axis, affecting the pitch, not yaw.

The yawing you observe while translating sideways comes because as the angle of attack on the tail rotor decreases from 180 degrees (in relationship to gravity) in a right or left turn with no increase in the pitch of the tail blades, the tail will lose "grip" in the air and yaw.

I'm lost Eric...

"Actually torque tendencies from the tail rotor would translate to the lateral axis, not the vertical axis, affecting the pitch, not yaw"

My line of thinking went like this:

The tail rotor's angular velocity has its axis parallel to the pitch axis.

When the helicopter is yawing, the whole thing (including the tail rotor) has an angular velocity around the yaw axis.

The precession torque is simplified as

T = I W x w or T = L x w

where

W and w are the angular velocity vectors of the tail rotor spinning and the yaw rate.

L is the angular momentum of the tail tail rotor (= IW).

I is the moment of inertia of the tail rotor

T is the torque vector due to precession.

x is the vector cross-product symbol.

The precession torque axis is at right angles to the two angular velocities' axes, i.e. the roll axis.

My comment re translation was meant to show that the tail rotor must experience a vertical angular velocity, otherwise it would not be yawing, not that I was seeing things happen while flying sideways.

I agree that there may be forces due to airflow not hitting the tail rotor exactly parallel to its axis, and that I didn't consider them. These forces may dominate any gyroscopic precession at the tail rotor.

You could also expect some roll effects if the tail rotor thrust is not directly in line with the center of mass.

Completely agree, but the torque tendency of the tail couples to the lateral axis affecting pitch much the same way the torque tendency of themain rotor couples to the vertical axis causing yaw, which is countered by the angular velocity of the tail rotor.

Phil, I think were both saying the same thing but were using different language to describe it. I'm getting confused when you use the term precession torque, and in my mind thats those are two different forces. Torque Tendency (like the main rotor applying force to the heli causing it to yaw about the vertical axis in the direction of travel) and Gyroscopic Precession where a force applied to any spinning object will cause an interaction 90degrees in advance of the direction of travel, and parallel to the original forces vector.

Assuming were on the same page (I think!), I still dont see how a turbine could act with sufficient force to prevent the full mobility of the heli.

OK, I understand now. You're talking about the torque reaction of the tail rotor. I'm talking about the precession torque caused when the tail rotor is yawing. And you're right: I didn't consider changes in tail rotor torque reaction. This will occur since the tail rotor pitch did get a step input to induce the yaw.

In my case, I gave a right rudder input, which would reduce the tail rotor torque, which would have caused a nose-up pitch. This happens to be the same direction as the turbine precession pitch moment caused by precession. So I don't really know what I was seeing - except that it didn't appear to be a whole lot different to my piston powered 60s.

Quote:

Phil, I think were both saying the same thing but were using different language to describe it. I'm getting confused when you use the term precession torque, and in my mind thats those are two different forces. Torque Tendency (like the main rotor applying force to the heli causing it to yaw about the vertical axis in the direction of travel) and Gyroscopic Precession where a force applied to any spinning object will cause an interaction 90degrees in advance of the direction of travel, and parallel to the original forces vector.

Assuming were on the same page (I think!), I still dont see how a turbine could act with sufficient force to prevent the full mobility of the heli.

I didn't think the turbine would have enough angular momentum to cause problems either. However if you look through the forums, you'll see conjectures that it would.

If I knew the mass and dimensions of the rotating parts in an HP5 I could estimate the moment of inertia and know for sure that precession from the turbine is not significant. A least I could see how it compares with the torque reaction from the tail rotor.