Full flap on a crosswind landing..........problems?

[Bear Cub]

Following the enthusiasm for my other posting "Slide slipping.....problems?" in this forum I am tempted ask another question of you aero buffs, aerodynamicists and technically knowledgable.

Can anybody explain the pro's and con's of the theory "Do not use full flap for a crosswind landing"?

If 30° or 40° (depending on type) is NOT good in a crosswind - then what is? And how can 30° full flap in one model of Cessna be worse than 30° not quite full flap in another model of Cessna...given the same crosswind landing conditions.

(Edit was for a typo)
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Hunting is bad!!
Support the right to arm Bears!!

[This message has been edited by Bear Cub (edited 24 January 2001).]

[411A]

Don't know about single engine Cessnas, but Boeing recommended on the B707 landing with flaps 40 (rather than 50) in very strond crosswinds. Vref was increased by 2 knots. Found it very useful at HKG on several occasions.

[Bear Cub]

But do they say WHY??? Aerodynamically.

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Hunting is bad!!
Support the right to arm Bears!!

[411A]

Ah....well no, not that I remember anyway. I mentioned this to SQ management at the time,and although it was not in their ops manual, the manual was changed (big surprise) to reflect the Boeing recommendation. Less possibility of downwind drift, I suppose. PanAmerican also used this technique and it was recommended during their contract training. As PanAm had the most experience with the 707, it may well have originated there. Increased the required runway distance by about 600 feet as I recall.

[chicken6]

Less flap = faster threshold speed

Faster TCS = more aileron effectiveness

More aileron = better drift control

That's how I was taught, mind you this is single engine Cessna material, not sure about things with wing-mounted engine pods and propellors and stuff to scrape on the ground.

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Confident, cocky, lazy, dead.

[Bear Cub]

Then - anticipation of further responses in this vein - can I ask what TCS you would personally use in a C152/C172 with 0°/10°/20°/30°(/40°) flap settings....and the PA28 guys could do the same for 10, 25 & 40.

All I'm wondering is DO people alter TCS for different flap settings - or just two speeds for with flap/no flap?

And when down in the 60 knot region at less than a ton...does it really matter?

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Hunting is bad!!
Support the right to arm Bears!!

[aerotowman]

We discussed this very issue on my instructors course. The recommendation was to use full flap for crosswind landings.

Imagine a scenario where you have to land at a short grass strip. The wind blowing is all xwind (no headwind component at all). Now if you approach with no flap and use a higher threshold speed you can see that you may very rapidly run out of runway.

By using full (or almost full flap) you also expose yourself to the xwind for the minimum amount of time reducing any drift problems.

[Bear Cub]

I have heard said before "the only airflow that affects the aircraft handling is that which is directly in contact with it" - so why do an approach with reduced flap which will give an "unusual" (as in different to normal) pitch attitude.

If the crosswind is so significant that a normal crab approach is uncomfortable then perhaps the decision to divert should be made rather than struggle to get it in?

(Just stirring things...whatever you agree with I'll paint the opposite picture!!!)

(Edit was for a typo)
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Hunting is bad!!
Support the right to arm Bears!!

[This message has been edited by Bear Cub (edited 24 January 2001).]

[G-OOFY]

Swept wing a/c have poor lift due to the sweep. Increasing Vth/VRef increases aileron effectiveness and the reduced flap/lift means that the a/c is more likely to stay on the ground. It definately work with Turboprops no experience with jets I'm afraid.

[willbav8r]

Surely the higher airspeed with 30 deg flaps as opposed to 40 gives better handling/manoevreability (lift?) in gusty x wind conditions?

For a lightie, the extra 5 knots or so cannot mean too much of a trade off for landing distance? Unless a short field ldg required?

[bookworm]

AFAIR the issue with full flap is that it reduces aileron effectiveness - it's not a speed issue, but the interference of the controls with each other.

[rolling circle]

willbav8r inadvertently touched on the nub of this argument whne he said:

"Surely the higher airspeed with 30 deg flaps as opposed to 40 gives better handling/manoevreability (lift?) in gusty x wind conditions?"

It is not the crosswind that should predicate a reduction in flap but the gusts. In a gustless crosswind, reduced flap will simply extend the float on round-out and is more likely to result in the aircraft leaving the downwind side of the runway after drift has been removed. However, in gusty conditions where windshear is more likely, the benefit of increased response with less flap outweighs the disadvantages after roundout.

We are talking here in terms of light aircraft, not heavy, swept-wing, metal (or swept-wing turboprops???).

[Luftwaffle]

In addition to the issue of not wanting to float over the runway when a crosswind is blowing you off it, for those who use the side slip method of crosswind landing, there's the whole Cessna "slips with flaps" issue.

Whether it's a forward slip or a side slip, the C172 elevator can lose authority in a slip with flaps. Later models are placarded that slip with flaps are not recommended. They are not prohibited.

Also, your rate of descent is higher in a sideslip than a straight descent, so for the same headwind component you'll have a steeper approach path in the sideslip. Adding full flaps steepens that approach even more.

[Unusual Attitude]

In response to your TCS question, in my C172 it depends where I'm landing and what conditions are like.

My aircraft is based at Insch and Aberdeen which are 2 very different places and require 2 very different types of approach.

At Insch which is a 550m grass strip with many obstructions on both the approach and the go around, I tend to come over the threshold with 30 flap at 55 knots. As long as this is nailed you can get in with a good bit to spare even with the downslope on runway 13.
I tend not to use 40 flap at Insch as the go-around from 13 has a set of power cables not too far from the end of the runway and C172's don't climb at all with 40 flap. Not only that but theres often some nasty rotors lurking in that area that tend to suck you downwards towards the cables.

At Aberdeen things are a lot more relaxed, I tend to keep the speed up along finals since it usually a busy place and then bring it back to 60 knots over the threshold with either 30 or 40 flap depending on what kind of mood I'm in.

As for crosswinds I must admit I don't actually reduce my flap setting at all.
At Insch, if I tried to get in with 20 flap at 60+ knots I'd end up planted on top of a stone dyke at the end of the runway wrapped up in barbed wire !!

At Aberdeen I use a slightly higher threshold speed when its windy along with 40 flap so that I'm not floating down the runway, its pretty much a case of flare, kick, thump, and your down with no time to float.

Or if it all goes horribly wrong, Flare, bounce, kick, bounce, flare, kick, kick, bounce, bounce...... :)

Interesting finding out how its done in the heavy stuff though.

Regards

UA

[foghorn]

Here is an explanation I've been given, I'm not qualified to vouch for its correctness, though it makes sense to me:

Lowering flap significantly changes the spanwise pressure distribution on the wing. Since flaps tend to be on the inboard portion of aircraft wings, lowering flap causes the pressure distribution to become more centralised, which decreases stability in roll. Clearly, roll stability is more crucial in strong crosswind situations since there is more chance of gust upsetting the aircraft in the roll axis, so a lower flap setting means more stability.

The higher Vref/increased aileron effectiveness metioned before also makes sense in this respect.

[foghorn]

I was given this explanation, I'm not qualified to vouch for its correctness:

Use of flap changes the spanwise pressure distribution of the wing. Since flaps tend to be on the inboard portion of the wing, the change of the pressure distribution will decrease stability in roll. This will cause obvious problems in gusty cross-wind conditions so a lesser flap setting may improve things.