See the below figure. Does the exhaust gas coming out through the nozzle have any force?
If the exhaust gas has force then how is it controlled by the helicopter?
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Welcome to the site. As your equation is stated now it is not clear to us what you are asking. Managed exhaust gas in what way? Which opposite forces does it not affect? – Koyovis Aug 17 '19 at 08:04
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Please edit your question so that it is clear for people who first read it, you'll find an "edit" button underneath the question. I still don't understand what you're asking about opposite forces. – Koyovis Aug 17 '19 at 08:21
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1Are you asking about how a turbine engine works (how does the gas know to turn the turbine instead of going back in the compressor) or whether a helicopter turboshaft also generates thrust? Or something else? – Sanchises Aug 17 '19 at 08:36
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1Do you mean the rotor will push the exhaust back down in to the engine? – Andreas Aug 17 '19 at 09:04
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related: Can a gas turbine be able to switch from torque to thrust? – Manu H Aug 18 '19 at 08:12
1 Answers
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Yes, the exhaust of any engine does provide some thrust (very little for piston engines).
The amount of jet thrust for turboprops is in the range of 4%-15% of propeller-produced thrust. For a helicopter, it's only 2%-4% of rotor lift, but it's directed backwards. The helicopter in your picture would get 100-150 lbf of thrust from the engine exhaust, compared to 3,000-5,000 lbf of lift provided by the rotor.
This thrust is not controlled in any way. It helps a little in forward flight, and is compensated for by cyclic pitch in hover or maneuvering.
Residual thrust can be calculated, but is rarely specified in most sources. You can find some analyses of residual thrust for common engines online, such as for the PW-100 series, or for the Allison T-56.
The T-56's residual thrust, per above analysis, is 750 lbf, with 3442 kW on the shaft. That allows the T-56 engine to produce up to 10,000 lbf of thrust when used as a turboprop with a propeller. If used in a helicopter, such an engine would be able to deliver 25,000-40,000 lbf of lift with a rotor.
Therac
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Awesome to know! While I certainly don't doubt your claims, it would be great if you could add some sources to back them up. – dalearn Aug 17 '19 at 20:20
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@dalearn Added a couple sources. Hard to find them online for helicopter engines specifically. – Therac Aug 18 '19 at 06:07
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@Therac That is because the residual thrust of a turboshaft is really inconsequential, comparable to that of piston engines. – Koyovis Jan 28 '20 at 13:34
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Indeed, for turboprops, the 'ideal' residual thrust for cruise is commensurate with the prop efficiency (for 85% prop, it would be about 15%), and the engines/nozzles are designed that way. For helicopter turboshafts, every bit of energy is better delivered to the shaft, and their nozzles are often designed with overexpansion (delivering lower than ambient pressure at the exhaust), which results in no significant thrust. With two engines, the nozzles are often directed sideways rather than backwards. – Zeus Jan 30 '20 at 02:26