palgia

Quick question: Why don't most airlines have automatic flap retraction after touchdown at the same time the spoilers are deployed.

Thanks for the answers,

palgia

Mad (Flt) Scientist

One possibility - landing flap is usually pretty draggy, and the spoilers/lift dumpers are usually destroying most of the lift incrememnt due to the flaps (since they are usually aligned). So retracting flaps might have only a marginal benefit in terms of weight-on-wheels and braking, and actually degrade the aerodynamic drag. i.e. less drag with flaps retracted, which is bad for stopping of course

The complexity of another automated system to certify probably isn't worth it.

Hay Ewe

Because when the flaps are in the landing position there is heaps of drag and air brake effect.

When flaps retract they go through a setting that actaully increases lift, you dont want that on landing.

and any way, give something for the pilots to do

Also, why would you want auto retract flaps

Tankengine

Sorry to be cute but the "airlines" have no say in the matter.
The aircraft "manufacturers" have not added "Auto flap retraction" to their list of modes available!

QNH1013

Rightfully so I think. I wouldn't want a malfunction of the 'auto flap retraction' which starts retracting the flaps uncommanded when on final approach at landing flap setting and low speed

pigboat

Dunno about the new ones, but the early Hawker 125's had a system that actually increased flap extension after landing. On touchdown, one seleted lift dump which extended the flaps from the normal landing position of 45 degrees to 80 degrees. Very effective.
An even worse scenario to QNH1013's auto flap retraction would be auto assymetric flap retraction. Ugh!

Mad (Flt) Scientist

Well, those failure cases aren't going to be much less survivable than uncommanded ground lift dump deployment, which is often automated. You'd use the same architecture for sensing "on ground" so it wouldn't necessarily be a particularly unsafe option. It just doesn't do much for you anyway.

SlowAndSilly

Not exactly for landing, but the Dash-7 had a auto-flap-retraction system for FLAP full (45?) to 25 when you added power above a specified power lever angle (PLA). Never flown the acft, but I've heard it could be quite interesting in gusty conditions when large and rapidly power lever adjustments had to be made.

rgds

slingsby

I used to fly weekly on LH B727, on landing they always used to retract the flaps immediately on landing.

TR4A

I was Flight Engineer on the 727 at Pan Am. We would retract the flaps to 25 after touchdown.

spannersatcx

The 747 has automatic Leading Edge Flap retraction on slection of reverse thrust. But not trailing edge flaps.

JJflyer

Yip... In the Old threeholer we used to retarct flaps to25 after landing:

a) To put more wight on the wheels and improve braking
b) To reduce FOD on the flaps from flying stuff from wheels. Those flaps hang really close to the rwy surface when in the 40 position. Flying from unimproved fields could really cause wear on flaps and associated equipment.

On the "Whale LE devices retract when reverse is selected mainly to reduce FOD to LE device drive mechanism and wing anti-ice ducting etc, as these are uncovered when LE's are extended.

JJ

Flight Detent

spannersactx,

Some B747s had the LE retraction feature.

Definitely not all of them!

Cheers

FD

palgia

Thans you everyone for your replies!


Very interesting points here.
I see what Mad scientist is saying, but I have a few questions:
Do the ground spoilers completely spoil the airflow? I thought there was a re-attachment point aft of the spoiler where some flow over the trailing edge still exists. Do you think the flaps in the approach setting might promote this re-attachment (by increasing chord and sfc area) and increase lift?

What about the lift produced by the lower half of the wing?
Is it significant at those speeds and so close to the ground? How do the flap settings affect it?

Haw Eye also has an interesting point. When the flaps retract they would be passing through all those settings that add more lift than drag (such as T/O flap settings).
For this reason I find it hard to explain why on the 727
you would want to retract the flaps only till 25degrees. Maybe only for a FOD-protection reason. And evidently even at 25 degrees, it was determined that they were creating more aerodynamic drag than the increase in rolling-friction-drag if they had been fully retracted.


Plus, correct me if i'm wrong, but increasing the weight-on-wheels only increases the MAXIMUM amount of braking action that is achievable (the point that can only be ridden by the anti-skid). For normal braking, it should not affect the performance significantly.
(I think its safe to say that the small percentage increase in rolling friction attributed to a weight increase is negligeble when compared to amount of energy dissipated by the brakes, or the amount of aerodynamic drag produced by the aircraft while shooting down the runway at over 100kts.)


Tankengine,

it was actually a typo, I meant to write "airliners".
Though I'm not sure I agree when you say "the airlines have no say in the matter". The cu$tomer is always first....



I think the main questions we have to answer now are: How much added lift do the flaps provide (compared to a clean wing) at those speeds and following ground spoiler deployment?
How much does this reduction in weight-on-wheels affect the deceleration rate and how does it compare to the amount of aerodynamic drag produced by the flaps?


Thanks again for your contributions, keep `em coming

palgia