Throttle bashing
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Join Date: May 2006
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Throttle bashing
Most in the flying game know that "throttle bashing" or changing power to chase airspeed or to maintain a formation position uses more fuel than if you used a constant power setting. My question is why.
Another example is using automatic throttle control, which in the aircraft I fly is said to be slightly less efficient than leaving the throttle at a constant position.
I was hoping someone with a good working knowledge of gas turbine engines might be able to answer this for me. Is it because some power is lost by having to accelerate and decelerate the compressor and turbine or something to do with the combustion efficiency with changing fuel flows?
A well informed answer would be much appreciated.
Another example is using automatic throttle control, which in the aircraft I fly is said to be slightly less efficient than leaving the throttle at a constant position.
I was hoping someone with a good working knowledge of gas turbine engines might be able to answer this for me. Is it because some power is lost by having to accelerate and decelerate the compressor and turbine or something to do with the combustion efficiency with changing fuel flows?
A well informed answer would be much appreciated.
Try it in your car and see what happens to the FC.
Join Date: Dec 2007
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Perhaps the cart might be getting ahead of the horse...
Efficiency, in the sense related here, is about the combination of aircraft and propulsion. Taking the formation case, the leader may fly at an efficient speed, the wingman's speed will vary around that, and some of his fuel burn at least will be a consequence of the time spent not flying at the leader's efficient speed.
Yes, acceleration and deceleration will mean less efficiency, but a given aircraft will use less fuel cruising at a given speed with a good autothrottle than cruising at a given thrust setting and allowing the speed to vary.
Efficiency, in the sense related here, is about the combination of aircraft and propulsion. Taking the formation case, the leader may fly at an efficient speed, the wingman's speed will vary around that, and some of his fuel burn at least will be a consequence of the time spent not flying at the leader's efficient speed.
Yes, acceleration and deceleration will mean less efficiency, but a given aircraft will use less fuel cruising at a given speed with a good autothrottle than cruising at a given thrust setting and allowing the speed to vary.
Join Date: Mar 2004
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From my experience working on the L1011, it used subtle attitude changes to correct small speed variations, as that was more efficient that constantly adjusting the throttle.
When you consider that the RB211 LP fan set weighs a tonne (or therabouts) plus the mass of the compressor and turbine, you are trying to accelerate a large mass for several seconds before the extra thrust takes effect, only to bleed that off again shortly afterwards. Now multiply that by three to get the whole aircraft effect...
Engines also have 'sweet spots' and work best for their intended cruising speed and altitude. They always suffer in the AAR role which is generally conducted below 25,000 feet since the fighters couldn't get much higher with long range tanks on...
When you consider that the RB211 LP fan set weighs a tonne (or therabouts) plus the mass of the compressor and turbine, you are trying to accelerate a large mass for several seconds before the extra thrust takes effect, only to bleed that off again shortly afterwards. Now multiply that by three to get the whole aircraft effect...
Engines also have 'sweet spots' and work best for their intended cruising speed and altitude. They always suffer in the AAR role which is generally conducted below 25,000 feet since the fighters couldn't get much higher with long range tanks on...
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If you looked at the ASI's of each of the Red Arrows in close formation you would see no discernable difference between any of them, but certainly noticeable throttle jockeying. The increased fuel usage of the wingmen is due entirely to the continuous accelerating (followed by decelerating) of the engines.
