Blackbird
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What Do You Call This...

Fri Jul 27, 2007 3:46 pm

If you watch the DC-8 and B-52 taking off, the climb angle is increased to takeoff, but as it gets into the air (due to the high-incidence of the wing I guess) the fuselage angle lowers once it's actually in the air at first...

What do you call that effect?

Andrea Kent
 
BAE146QT
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RE: What Do You Call This...

Fri Jul 27, 2007 4:22 pm

"Avoiding a tail-strike"?
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tdscanuck
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RE: What Do You Call This...

Fri Jul 27, 2007 6:02 pm

Quoting Blackbird (Thread starter):
If you watch the DC-8 and B-52 taking off, the climb angle is increased to takeoff, but as it gets into the air (due to the high-incidence of the wing I guess) the fuselage angle lowers once it's actually in the air at first...

What do you call that effect?

Priority while rolling down the runway: get off the ground before you run out of runway.
Priority once airborne: establish best-rate-of-climb, which is typically a lower angle of attack and higher speed than immediately at takeoff.

Both the DC-8 and B-52 have low bypass engines so their acceleration is pretty lousy. They eat a lot of runway before they reach takeoff speed so I suspect they're taking off a the first possible opportunity and rather close to stall speed. First priority in that situation, once the wheels are off the ground, is to build airspeed. The means getting the nose lower.

Tom.
 
Starglider
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RE: What Do You Call This...

Fri Jul 27, 2007 6:20 pm

And at a height of approximately half a wingspan they are out of the ground effect which decreases initial lift at take-off slightly. In combination with low acceleration mentioned in the previous thread, ground effect is a factor to reckon with for these type of aircraft.


Starglider
 
Blackbird
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RE: What Do You Call This...

Wed Aug 01, 2007 8:40 am

Actually as I look at it... it would seem that once in the air, the lift from the wing naturally evens out at the correct AoA for speed. Well the high incidence would rock the nose down as the wing reaches the correct incidence right?

Why did the B-52, and the DC-8 have such high incidences (the wing), and later designs did not?

Andrea Kent
 
oly720man
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RE: What Do You Call This...

Wed Aug 01, 2007 9:44 am

Quoting Blackbird (Thread starter):
If you watch the DC-8 and B-52 taking off, the climb angle is increased to takeoff, but as it gets into the air (due to the high-incidence of the wing I guess) the fuselage angle lowers once it's actually in the air at first...



Quoting Blackbird (Reply 4):
Why did the B-52, and the DC-8 have such high incidences (the wing), and later designs did not?

The B52 can take off horizontally with all wheels lifting off the ground at the same time so there is little, if any, rotation of the fuselage. Because of all the undercarriage it wouldn't make a lot of sense to rotate the B52 that much, I'd have thought, because of the stresses imposed on the fuselage..

The wing incidence is set for cruise lift and drag since that's where the plane spends most of its life. I don't think the DC8 wing incidence is any different from its contemporary, the B707, or any more recent aircraft.

On take off, the stick is pulled back to set the aircraft at a particular attitude and thus increase wing lift. When off the ground the stick is pushed forward so that the angle of pitch is 15 degrees, or whatever, for climb but the incidence is closer to zero because the aircraft is climbing. Take off is at, say 160kts and climb is at 220kts (?), a ratio of 1.9 in velocity squared, proportional to lift.

Quoting Blackbird (Reply 4):
Well the high incidence would rock the nose down as the wing reaches the correct incidence right?

????

Are you assuming no pilot intervention in the attitude of the aircraft? The plane won't change attitude itself. If you pull back on the stick and hold position, then yes, the plane will climb for a short period, but it will slow down because of the higher drag and maintain steady horizontal flight at the set attitude unless it stalls.
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Blackbird
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RE: What Do You Call This...

Wed Aug 01, 2007 9:46 am

But... if the plane speeds up after lift off it's AoA will drop won't it?

Andrea Kent

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