orientation of helicopter exhaust

[tejas]

please clarify why in some light helicopters that to single engine the orientation of exhaust with reference to longitudinal axis of the helicopter will be there.
why not exhaust orientation be parallel to longitudinal axis. is there any theory behind this.

[WaspAir]

I don't think there's very much thrust to be had from the exhaust, most of the energy having already been extracted by the turbine to drive the rotors, but I've never seen a measurement that I can recall. (However, I understand that Westland's speed record setting helicopter used every little advantage they could get.) I suspect that there may also an issue of what the heat will do to structures exposed to the exhaust gases and that might influence the designer, too.

[tejas]

thanks for your reply, but any stability issues will be there especially during side slip at max level speed

[tejas]

thanks for your reply, for example like aerospatiale SE 3160 ALOUETTE III helicopter which was designed during 1970's had the exhaust orientation may be 5 to 10 deg with respect to longitudinal axes. Is there any stability issues associated during side slip at max level speed.

[StanFoster]

Tejas- My Helicycles turbine exhaust points to the right side. This keeps the exhaust out of the tail rotor. It also helps to offset a lot of the translating tendency. I was told the exhaust has 30-35 pounds of thrust. On the ground, you can feel a definite side thrust when you rev it up. Stan

[jebthereb]

stan when you are in the hover how much left stick do you have to put in?

[StanFoster]

Jeb- Hardly any left cyclic all. The turbine exhaust is offsetting some of the left cyclic that would have been needed.The R22 took more. Hover autos in the Helicycle don't need much right cyclic when the power is rolled off.

[jebthereb]

cool thanks!! As soon as you said that about the amount of thrust I went to thinking about control inputs!!!!!

[StanFoster]

Jeb- I have 100 horsepower available. Let's say all this horsepower is being consumed to hover on a heavily loaded - high density altitude day. Assuming roughly 90 per cent going to the main rotor and 10 percent going to the tail rotor, my basic math comes up with this. 90 horsepower being consumed by the main rotor is 49500 foot pounds/second being applied at a one foot radius. My rotor turns 620 rpm, or 10.33 revolutions per second. The force being applied at a one foot radius is moving at 64.92 feet/second. 49500 divided by 64.92 equals 762.48 foot pounds of torque............................................ ..................The tail rotors center has a 12 foot arm-so its horizontal force to oppose the torque going to the main rotor is 1/12 of 762.48- ending up with 63.54 pounds of force pushing counter clockwise......................................... .........The turbine exhaust is pointed to the right with a reported 30 pounds of thrust. The elbow on the exhaust is short, so to be more precise, the actual exhaust force is not exiting at a 90 degree angle like it would if the 90 elbow were longer, but instead exits slightly less than 90 degrees. Just my guess but I would say it exits at 80 degrees. Using 30 pounds of thrust at 80 degrees only drops that 30 pounds to 29.54 pounds. This exhaust force is close to a 4 foot arm, so this actually will cause the tail rotor to have to push with an additional force of 1/3 of that since it has a 12 foot arm. This causes an additional load of 9.84 pounds of sideforce to counter the turbine exhaust force. .................................................S o now the tail rotor has to push 73.38 pounds to counter the main rotor torque and the turbine exhaust force. But still, the 30 pounds of exhaust force at 80 degree exit angle causes 29.54 pounds of sideforce to counter the transalating force of 73.38 pounds that the tail rotor is pushing. That is reducing 40 per cent of the translating force, and is why the Helicycle translates less to the right as it flies, and why hover autos don't need as much cyclic correction. ........................of course if the actual turbine exhaust force were say 20 pounds instead of the reported 30 pounds, the beneficial effect is still there, but at 2/3 the amount. Stan

[akoschier]

Hot gas ingestion into turbomachinery can cause compressor stall (See A10 - gun fire caused engine stalls)
The Helicycle tail rotor system is adequate but not excessive. Under some flying conditions which require large amounts of TR thrust (large blade pitch) hot exhaust ingested will cause the TR stall earlier than normal. Its critical for high altitude and hot day operation. A stalled tailrotor is not what you want to experience. To provide the most margin BJ insisted that the engine exhaust be kept out of the tail rotor inflow.
avk