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Glide range and weight

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Bosi72 created the topic: Glide range and weight

Hi,
Would this be correct answer if I don't have power to increase the speed?
I always assume gliding is with no power (or idling) in a powered aircraft and that's probably my mistake.
Thanks
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  • John.Heddles
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John.Heddles replied the topic: Glide range and weight

Playing with glide characteristics varying with power is not something I've looked at in testing.

However, think of things this way - for the typical engine failure case (on which the POH information is based) the aircraft will have what we call a "drag polar" (drag curve) which defines its drag and glide characteristics. In that situation, you are looking at varying your speed by using the elevator and trim tabs (rather than the power which you no longer have available) to position the aircraft at different points on the drag curve. This is what we are talking about when it comes to varying speed with weight to maintain an optimum glide situation as discussed in the bookwork.

Now, the overall glide characteristics will alter if you still have some power with which to play. The pilot will see an increase in power as reducing (flattening) the glide angle. Typically we see this as the difference between an engine failure and throttle closed cases. For a typical airline jet, for example, with throttles closed for the bulk of the descent, the aircraft typically descends with a nil wind glide angle of around 3 nm/1000 ft. Change this to a failure situation (of all engines) and the angle typically will increase to, say, 2 nm/1000 ft or even a bit more. It will be something similar with a training piston aircraft, albeit that the numbers will be a little different.

Normally, we don't worry about figuring the best speed with a part power situation as there are too many variables to make the numbers of use to the pilot and, in any case, we just use the throttles to achieve whatever target descent characteristics we might be seeking.

While I don't have any numbers to offer, I think it reasonable for you to think in terms of the "overall" drag curve's varying with a bit of thrust compared to the failure situation. That will vary the speed numbers a little but, in the absence of playing with the equations, I suspect not all that significantly. So, in response to your basic question, above, the exam questions will be looking at the power failure situation so we would constrain our thinking to the basic drag curve and varying speed (with weight) by elevator/trim rather than power adjustments.

Engineering specialist in aircraft performance and weight control.
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Bosi72 replied the topic: Glide range and weight

Thank you John for detailed response.

>So, in response to your basic question, above, the exam questions will be looking at the power failure situation so we would
> constrain our thinking to the basic drag curve and varying speed (with weight) by elevator/trim rather than power adjustments.

If the question pertains to elevator/trim (AoA) only, should my answer be correct then : Range reduces if angle of attack is maintained ?
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John.Heddles replied the topic: Glide range and weight

If you are on the optimum angle for a particular weight then, for a different weight at that previous angle, you will be off optimum - the glide angle will increase (steepen) and the range will decrease so, yes.

Engineering specialist in aircraft performance and weight control.
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