mawingho
Topic Author
Posts: 41
Joined: Sun Jul 22, 2012 4:23 pm

### Drag, Glide Speed, Spiral Dive And Flap

I am now reading another book, I have some questions regarding drag, glide speed, spiral dive and flap.

1. The indicated airspeed for maximum L/D ratio is the speed for minimum drag and is very important because it is also the best glide speed (i.e., the speed for maximum gliding distance)

If best glide speed is the max L/D ratio, is this only happen on gliding but not cruising?

2. How the spiral dive can cause yawing descend?
I just can imagine it's performing rolling... maybe both yawing and rolling because Yawing can cause Rolling, and Rolling can cause Yawing....

3. "If the trust acts below the drag, then the increased thrust force will also tend to pitch the nose up"
I have no idea, hope someone can explain a bit..

4. Why lowering the flaps can result in changing of the aeroplane balance?
if Mean Camber Line is the average camber of the top and and btm surface of the aerofoil, then lowering the flag could increase the upper's surface's camber (arc)?

Is it because of this, the Centre of Lift move beyond the original one?

Hope someone can help! Thanks in advance.

Starlionblue
Posts: 17212
Joined: Fri Feb 27, 2004 9:54 pm

### RE: Drag, Glide Speed, Spiral Dive And Flap

 Quoting mawingho (Thread starter):1. The indicated airspeed for maximum L/D ratio is the speed for minimum drag and is very important because it is also the best glide speed (i.e., the speed for maximum gliding distance) If best glide speed is the max L/D ratio, is this only happen on gliding but not cruising?

Best glide tends to be quite a bit below best cruise speed. If you're at cruise the L/D ratio is not as high.

 Quoting mawingho (Thread starter):3. "If the trust acts below the drag, then the increased thrust force will also tend to pitch the nose up" I have no idea, hope someone can explain a bit..

If the engines are below the wing (think of a 737), increasing thrust will push underneath the area of main drag (the wings). This will pivot the aircraft so that the nose pitches up.

 Quoting mawingho (Thread starter):4. Why lowering the flaps can result in changing of the aeroplane balance? if Mean Camber Line is the average camber of the top and and btm surface of the aerofoil, then lowering the flag could increase the upper's surface's camber (arc)?

It changes balance because if you extend the flaps you increase the lift, but not uniformly across the wing. So the center of lift will shift back (or forward as the case may be). Also flaps have drag which will typically cause pitch down.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo

rcair1
Crew
Posts: 1143
Joined: Wed Oct 28, 2009 8:39 pm

### RE: Drag, Glide Speed, Spiral Dive And Flap

 Quoting mawingho (Thread starter): 1. The indicated airspeed for maximum L/D ratio is the speed for minimum drag and is very important because it is also the best glide speed (i.e., the speed for maximum gliding distance) If best glide speed is the max L/D ratio, is this only happen on gliding but not cruising?

Best glide speed means you get the longest distance for descent - it will also give best climb rate (climbing over obstical) IIRC. It is important if you are trying to extend your glide (engine out). In cruise - you are not climbing or descending - so it really does not apply. You will not be utilizing all the lift the aircraft can create - only what is required to maintain altitude. Really comparing apples to oranges.

 Quoting mawingho (Thread starter):2. How the spiral dive can cause yawing descend? I just can imagine it's performing rolling... maybe both yawing and rolling because Yawing can cause Rolling, and Rolling can cause Yawing....

Need a little more context here - not sure what a Yawing descent - seems like a non-coordinated turn.

 Quoting mawingho (Thread starter):3. "If the trust acts below the drag, then the increased thrust force will also tend to pitch the nose up" I have no idea, hope someone can explain a bit..

Drag is a force acting 'backward' on the aircraft (retarding or slowing) - and there will be a center of drag. This is an equivalent line where the force acts on.
Thrust is an acting forward on the aircraft - accelerating or at least maintaining speed. There is a center of thrust.
If the center of drag is above the center of thrust - then you are 'pulling back' above where you are 'pushing forward' so the aircraft will rotate up (nose up). Under wing engines to this. This is one reason you cannot not just firewall the engines in a stalled underwing jet - you also have to push down (or maintain angle of attack).

Increase the thrust - the nose goes up.

 Quoting mawingho (Thread starter):4. Why lowering the flaps can result in changing of the aeroplane balance? if Mean Camber Line is the average camber of the top and and btm surface of the aerofoil, then lowering the flag could increase the upper's surface's camber (arc)? Is it because of this, the Centre of Lift move beyond the original one?

I'll defer to an airfoil expert on this one
rcair1

wingscrubber
Posts: 806
Joined: Fri Sep 07, 2001 1:38 am

### RE: Drag, Glide Speed, Spiral Dive And Flap

1. The indicated airspeed for maximum L/D ratio is the speed for minimum drag and is very important because it is also the best glide speed (i.e., the speed for maximum gliding distance)

If best glide speed is the max L/D ratio, is this only happen on gliding but not cruising?

http://www.nordian.net/pdf/jaa_principles_of_flight_demo.pdf
http://en.wikipedia.org/wiki/Lift-to-drag_ratio
Depends on the wing design. An aircraft can be designed to have highest L/D at its optimum cruising speed, but typically best glide speed is below cruise speed. Maybe someone else can give a better explanation...

2. How the spiral dive can cause yawing descend?
I just can imagine it's performing rolling... maybe both yawing and rolling because Yawing can cause Rolling, and Rolling can cause Yawing....
Are you talking about a spin? Or dutch roll? Or making co-ordinated turns?
http://www.apstraining.com/upset-rec...ery-training/stall-spin-awareness/
http://ocw.mit.edu/courses/aeronauti...g-2003/lecture-notes/lecture18.pdf

3. "If the trust acts below the drag, then the increased thrust force will also tend to pitch the nose up"
I have no idea, hope someone can explain a bit..
If thrust is acting below drag, then the thurst force creates a moment arm around the centre of drag, which points the nose up. Likewise if the thrust is above the drag, then the nose might be pitched down.

4. Why lowering the flaps can result in changing of the aeroplane balance?
if Mean Camber Line is the average camber of the top and and btm surface of the aerofoil, then lowering the flag could increase the upper's surface's camber (arc)?

Is it because of this, the Centre of Lift move beyond the original one?

Depending on the type, flap extension can change both wing camber and wing chord, and permit flying at steeper angles of attack, this changes the centre of pressure and centre of lift, and usually gives a pitching-up moment.
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vikkyvik
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Joined: Thu Jul 31, 2003 1:58 pm

### RE: Drag, Glide Speed, Spiral Dive And Flap

 Quoting rcair1 (Reply 2):Best glide speed means you get the longest distance for descent - it will also give best climb rate (climbing over obstical) IIRC

Best climb RATE (highest vertical speed) and best climb ANGLE (climb over highest obstacle) are two different speeds.
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zeke
Posts: 10107
Joined: Thu Dec 14, 2006 1:42 pm

### RE: Drag, Glide Speed, Spiral Dive And Flap

 Quoting mawingho (Thread starter): 4. Why lowering the flaps can result in changing of the aeroplane balance? if Mean Camber Line is the average camber of the top and and btm surface of the aerofoil, then lowering the flag could increase the upper's surface's camber (arc)?

On large aircraft, flaps move by a considerable amount to increase the surface area as well as to change camber. When the flaps move aft on their flap tracks, one is physically moving mass away from the CG.

On the A330, the slats weigh around 500 kg, and the flaps around 650 kg each side. So moving the flaps aft, you are moving 1.3t away from the CG. Extending the flaps results in a moment of around +2000 kg.m, for the slats it is about a -700 kg.m, all up around a 500 kg.m moment generated in the landing configuration.

Moving the landing gear also often results in a change in CG as well, normally the nose gear is retracted along the longitudinal axis, and by doing so, the CG is also moved.
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rcair1
Crew
Posts: 1143
Joined: Wed Oct 28, 2009 8:39 pm

### RE: Drag, Glide Speed, Spiral Dive And Flap

 Quoting vikkyvik (Reply 4):Best climb RATE (highest vertical speed) and best climb ANGLE (climb over highest obstacle) are two different speeds.

Yes - and neither is perfectly related to best glide (most distance traveled forward for altitude lost). But best glide will be closer to best rate (Vy).
rcair1

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