Questions about the principles of flight from A Kermode’s “Flight without formulae” b
Questions about the principles of flight from A Kermode’s “Flight without formulae” b
I don’t know if this is the right place to post this question.
I have been reading Kermode’s book for years. It contains a specific series of questions but no answers! The question is:
“Suppose one is flying an aeroplane well out. To sea when the engine fails; there is a good airfield just on the coast and it is touch and go whether one can reach it; (The picture shows the airfield on top of a cliff) you have disposable load on board, luggage, bombs, fuel passengers! or whatever it may be; should you jettison your load, and if so, when, and what should your tactics be to reach the airfield?
a) first consider that there is no wind
b)When your engine fails, you will probably be flying at a speed higher than that of your speed for flattest glide - what can you do with this speed?
c)What about drag? Can you get rid of any?
d) What about flaps? Should you use them or not? Yes, it all depends - but depends on what?
e)At what speed should you fly?
f)Should you jettison any load? If so, when?
Then repeat the above with a following wind.”
My thinking is:
b) Easy, I think! Trade speed for height.
The one that baffles me is f. I think that jettisoning load is throwing away energy, so load should only be jettisoned after all the potential energy has been extracted from it so this would mean just close to landing. Any ideas gratefully received.
Thanks
Paul
I have been reading Kermode’s book for years. It contains a specific series of questions but no answers! The question is:
“Suppose one is flying an aeroplane well out. To sea when the engine fails; there is a good airfield just on the coast and it is touch and go whether one can reach it; (The picture shows the airfield on top of a cliff) you have disposable load on board, luggage, bombs, fuel passengers! or whatever it may be; should you jettison your load, and if so, when, and what should your tactics be to reach the airfield?
a) first consider that there is no wind
b)When your engine fails, you will probably be flying at a speed higher than that of your speed for flattest glide - what can you do with this speed?
c)What about drag? Can you get rid of any?
d) What about flaps? Should you use them or not? Yes, it all depends - but depends on what?
e)At what speed should you fly?
f)Should you jettison any load? If so, when?
Then repeat the above with a following wind.”
My thinking is:
b) Easy, I think! Trade speed for height.
The one that baffles me is f. I think that jettisoning load is throwing away energy, so load should only be jettisoned after all the potential energy has been extracted from it so this would mean just close to landing. Any ideas gratefully received.
Thanks
Paul
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Jettisoning load isn't really throwing away energy, because the load will also consume energy in a proprtional sense.
Consider your answer for b - trade speed for height. Speed gives kinetic energy, which you are proposing to convert to potential energy. Which gives height.
Kinetic energy is 1/2 * mass * velocity squared.
potential energy is mass * gravity constant (g) * height
So the height you gain for a given speed change is given by m*g([delta h] = 1/2 * m * ( [start speed^2] - [end speed]^2)
"m" is on both sides of that equation. All of the kinetic energy for any incremental mass goes into providing the potential energy for the same mass. Mass is, for this exchange, a wash. There's no benefit, in energy terms, in retaining mass.
Consider your answer for b - trade speed for height. Speed gives kinetic energy, which you are proposing to convert to potential energy. Which gives height.
Kinetic energy is 1/2 * mass * velocity squared.
potential energy is mass * gravity constant (g) * height
So the height you gain for a given speed change is given by m*g([delta h] = 1/2 * m * ( [start speed^2] - [end speed]^2)
"m" is on both sides of that equation. All of the kinetic energy for any incremental mass goes into providing the potential energy for the same mass. Mass is, for this exchange, a wash. There's no benefit, in energy terms, in retaining mass.
Best glide speed varies with weight/mass. Best glide angle (L/D max) does not vary with weight but the speed to obtain that best L/D does vary.
The greater the mass, the higher the speed to obtain best L/D.
So, in still air, the same glide angle will be obtained at any mass, provided that the speed is adjusted to obtain best L/D
however; in tailwind conditions you would want to fly as slowly as possible, to take maximum advantage of the tailwind (i.e. you want to stay in the tailwind for as long as possible) therefore you should jettison mass in order to obtain best L/D at a slower speed.
On the other hand, with a headwind you want to fly as fast as possible in order to stay in the headwind for a short time, so keep the mass and get your best L/D at a higher speed.
It’s why gliders carry water ballast.
The greater the mass, the higher the speed to obtain best L/D.
So, in still air, the same glide angle will be obtained at any mass, provided that the speed is adjusted to obtain best L/D
however; in tailwind conditions you would want to fly as slowly as possible, to take maximum advantage of the tailwind (i.e. you want to stay in the tailwind for as long as possible) therefore you should jettison mass in order to obtain best L/D at a slower speed.
On the other hand, with a headwind you want to fly as fast as possible in order to stay in the headwind for a short time, so keep the mass and get your best L/D at a higher speed.
It’s why gliders carry water ballast.
