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Flap-Drag At Landing

Sat Dec 08, 2007 3:08 am

I know propeller planes even with full flaps and power chopped off would glide okay for a bit...

I know certain aircraft, such as the B-727 are known to produce large amounts of drag with their flaps down requiring substantial amounts of power during the approach to hold the glide-path...

What I don't know is how much drag the B-707 and DC-8 produced with their flaps fully down, and how much power was needed to hold the glidepath, from what I read it wasn't as bad as the B-727, but still enough that it wouldn't just glide easily with power chopped.

Also, does anybody know how the Comet and Caravelle handled with their flaps fully down -- the Comet because of it's very high flap deflection, and the Caravelle because of it's flaps adding large amounts of area in addition to the deflection?

Andrea Kent
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RE: Flap-Drag At Landing

Sat Dec 08, 2007 6:38 pm

Quoting Blackbird (Thread starter):
I know propeller planes even with full flaps and power chopped off would glide okay for a bit...

Even a house brick glides OK for a bit....  Wink Any aircraft will still glide with full flap, but at a steeper angle.

It's nothing to do with how the aircraft is powered, purely to do with flap design. Leading edge flaps (uncommon on prop designs) will affect gliding characteristics. An aircraft with constant speed props will drop like a stone if power is set to idle on approach because the blade pitch will go fully fine adding lots of drag.

As the whole purpose of extreme flap angles on aircraft like the Comet is to add drag on approach this clearly will ruin any glide performance. The 727 had drag problems at full flap, so some had the flap lever physically limited to 30 degrees to reduce the problem.
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RE: Flap-Drag At Landing

Sat Dec 08, 2007 9:48 pm

Remember that the primary purpose of landing flaps is to reduce the stall speed, and thus, the touchdown speed and runway required to land to a minimum. The fact that many early generation jet airliners required substantial thrust to stabilize the approach speed with landing flaps was deemed desirable.

Prior to the jet age, many piston engine pilots were accustomed to making a constant deceleration to landing with final flap settings made very close to touchdown. This made for very low power settings throughout the approach. An experienced pilot could keep the speed up to the last minute, reduce power nearly to idle, extend gear and flaps as the speed bled off, and then touchdown at the proper spot on the runway and at the proper speed.

Many of these pilots attempted this same technique when they first transitioned to jet airliners with less than satisfactory results. The early jet turbine engines were slow to accelerate from idle to arrest the speed decay or execute a go around. Having the landing flaps set earlier in the approach and the approach speed stabilized with thrust ensured that power would be available at the end of the approach, should a go around be necessary, and that the approach path and speed could be consistently achieved and assured early in the approach.
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RE: Flap-Drag At Landing

Sun Dec 09, 2007 4:04 am

In the CRJ-200, at 210kts, we typically do idle descents at 1400fpm with flaps 0.. going to flaps 20 increases it to almost 2500fpm.

Going from flaps 0 to flaps 45 in the CRJ doubles our drag.

Gear is actually a lot less drag than flaps....

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