Inverted Flight

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The air approaching a subsonic wing in basically incompressible...the air has no advance warning that the wing is approaching...and the air is able to maintain a laminar flow over the wing. A wing near sonic speeds now is able to begin compressing the air ahead of its path...giving the air "advance" warning that it is approaching.

[/ QUOTE ] The air is actually able to start moving out of the way when a subsonic aircraft is approaching. Due to air's incompressible nature at low speeds, a pressure wave travels slightly ahead of the aircraft. Think of this as the 'bow wave' of a boat.

When an aircraft is supersonic, then the aircraft 'sneaks up' on the air molecules it encounters and has to force its way through. (Remember, pressure waves are transmitted through air as sound waves. If you are faster than the speed of sound, no pressure wave can stay ahead of you.)

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Oops...that's what I meant to say! (regarding advance warning/comp/incomp). However...I did mean the part of sweep introducing the spanwise flow to reduce local velocities impacting the leading edge. I guess we'll see what Seagull has to say about this.
 
Also keep in mind that a swept wing cannot delay the onset of mach effects once the aircraft is traveling the speed of sound. Once the aircraft is going mach 1, then the air traveling over the wings is supersonic and will create shockwaves.

Wing sweep merely serves to keep the air traveling over the tops of the wings from creating shockwaves at a speed significantly slower than the speed of sound. This helps keep transonic drag rise and other unpleasant side effects from happening at a speed significantly slower than the speed of sound.
 
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While you're pondering that one, here is another one that is more simple. Why, in airline ops, does a heavy aircraft need more distance to descend than a lighter one?

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The angle of attack for economy descent speed is greater for the heavier airplane.

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Why would that be?

Here is a strong hint:
1. What is your company's programmed descent profile (does it vary with weight, talking about airspeed now)

2. What is speed is L/D max for your aircraft?
 
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Oops...that's what I meant to say! (regarding advance warning/comp/incomp). However...I did mean the part of sweep introducing the spanwise flow to reduce local velocities impacting the leading edge. I guess we'll see what Seagull has to say about this.

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Take a simpler view. That bow wave, does it move out radially from each point or just straight out in front? What do you suppose the bow wave at the root of the wing does to the air in front of a point farther towards the tip?
 
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While you're pondering that one, here is another one that is more simple. Why, in airline ops, does a heavy aircraft need more distance to descend than a lighter one?

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The angle of attack for economy descent speed is greater for the heavier airplane.

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Why would that be?

Here is a strong hint:
1. What is your company's programmed descent profile (does it vary with weight, talking about airspeed now)

2. What is speed is L/D max for your aircraft?

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Had to go fly a trip...so got sidetracked.

Let's see...heavier aircraft flies faster than a lighter one at econ speed. So...the faster groundspeed would require a an increased distance for the descent? Is that the correct answer?

I don't know L/D max speed....I know best angle is found in the FMS and approximates 230 kts. Driftdown is also an FMS calculated speed...and approximates 260 above 30,000 and 250 below. And best rate is approximated by Vref30+80+50 (or clean speed +50). Econ Crz is 270 IAS (475TAS). L/Dmax...I'm going to guess it's somewhere in the ballpark figure of 230 to 260.
 
What speeds do you use for descent? Is it a constant profile? We use cruise mach to 290kts.

Best rate of climb in a jet is your Vx speed. What speed is used for best angle (usually the same as your engine out climb speed, at least that's the way my company uses it).
 
Same...Crz Mach to 290.

Getting back to your original question...why does it take the heavier aircraft more distance to descend? Due to a higher Crz Mach?

Thx.
 
Your L/D max is almost certainly below 290 kts (I can say that because I know it is for the MD11 and the MD11 is the fastest out there that I am aware of). So, let's say L/Dmax is 230kts at max landing weight, and perhaps 200 kts at some lighter weight. With the constant speed profile, very simply, you are flying closer to L/Dmax at the heavier weight. Obviously, the closer you are to it, the better the glide ratio. Heavy airplane takes more distance to get down for this reason. You can also do it with vectors, but I think this is more intuitive.
 
Interesting. A couple of questions for you.

Is the AOA for the heavy and light airplane the same for L/Dmax? Does the airspeed vary to attain the same AOA for both airplanes? Would the heavy and light airplane, both flying at L/Dmax glide the same distance?

Thx
 
Yes, AoA for L/D max is a constant for a given airfoil, although compressibility may alter that a bit, not 100% sure on that.

The airspeed for L/Dmax is higher for a heavier aircraft. The glide ratio is the same regardless of weight if the aircraft is flown at L/Dmax.
 
Since AOA for L/Dmax is acheived at a higher airspeed for a heavier aircraft...I'm assuming this is why you burn more fuel flying a heavier aircraft. I.E., the aircraft must fly at a faster cruize speed. Are there are factors associated with burning more fuel with the heavier aircraft?
 
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Since AOA for L/Dmax is acheived at a higher airspeed for a heavier aircraft...I'm assuming this is why you burn more fuel flying a heavier aircraft. I.E., the aircraft must fly at a faster cruize speed. Are there are factors associated with burning more fuel with the heavier aircraft?

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Are there other factors....that is.....associated with burning more fuel with the heavier A/C?
 
Well, if youre at a constant speed, the heavier aircraft needs more lift to overcome the higher weight, which equates to more AoA and more power to overcome the increased drag. If you are just flying L/Dmax, you will fly faster at the heavier weights (look at the engine out driftdown speeds in the FMS to see what these speeds are). Faster speed means more parasite drag, so more power required.
 
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Yes, AoA for L/D max is a constant for a given airfoil, although compressibility may alter that a bit, not 100% sure on that.

The airspeed for L/Dmax is higher for a heavier aircraft. The glide ratio is the same regardless of weight if the aircraft is flown at L/Dmax.

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So at L/Dmax a heavy and light aircraft will glide the same distance....but in crz, at L/Dmax, the heavy aircraft will have a reduced range due to more power required to overcome higher drag.
 
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