Found this paper on the topic of flaps: https://commons.erau.edu/cgi/viewcontent.cgi?article=1026&context=jaaer
I didn’t read the whole paper, just the conclusions and the recommendation, so not sure how good the paper is and how valid the conclusions are.
@UdoR how and when do you run out of rudder authority when all you are doing is straightening in the flare?
It’s just a normal landing then with in to wind aileron.
I have only ever run out of rudder authority in a taildragger (a Menéstral) and that was my own stupid fault for being too tense on the pedals and not catching the swing quick enough. I should have been more ahead of the plane.
Fortunately I ran off the tarmac runway onto the grass runway with no damage to anything or anybody.
However, if you have an accident on take off or landing your aircraft and the crosswind is shown to be above this limit, do you think the insurance company will just pay up for all the damage.
It’s not a limit. If they didn’t the court costs would kill them and they’d lose every time. I can empathize with owners who worry about it though, even if they are making up things to worry about.
My actual approach (pun intended) is not to carry in-motion hull coverage on 12 of my 13 vehicles, including my plane, so that it’s irrelevant to me. This genuinely gives me a better and more secure feeling than doing unnecessary business with insurance companies and paying them year upon year for the privilege 
gallois wrote:
how and when do you run out of rudder authority when all you are doing is straightening in the flare?
No I cross the rudders when passing the airport fence, so some seconds prior to the flare. I have much more control with that. And I did not run out of rudder authority so far. The approach until touchdown is fine. It’s after touchdown that gets increasingly interesting with increasing wind that approaches v_rotate.
I am not sure I understand cross the rudders.
And does your aircraft reach its limit of effective into wind aileron as you slow to taxing speed?
gallois wrote:
Each aircraft is different. Each aircraft ( or many)have a demonstrated crosswind landing figure. Each aircraft has a recommended speed over the threshold or on final, with or without flaps. That speed like all speeds can be calculated as a Facteur de Base for that phase of flight eg a ULM might fly final at 60kts which equals a Fb of 1. You use this 1 to calculate what the wind speed Vw and its angle to the aircrafts track is going to do to the aircraft. […]
All winds will have an effect Ve on you being able to do that but if you are landing at 120kts over the threshold and the wind is 40kts you know that the maximum effect the wind is having on the aircraft is 20kts and you can correct for this. It is very rare to have a full 90° crosswind at such a level but you can prepare for it in the planning stage and decide whether to go or not.
I’m really trying to understand how this works. The “effective crosswind” has a direct relationship to the crab ange – that is clear. E.g. an “effective crosswind” of 10 kt corresponds to a crab angle of 9.5°. 20 kt corresponds to 18.4° etc. In other words, that concept serves the same purpose as the 1 in 60 rule.
But you don’t need to know the crab angle in advance – on final you very quickly find what crab angle will keep you on the centreline. Also as you descend the wind changes so you need to change the crab angle as well. Even if it may be theoretically possible to predict these changes, it is not feasible in practice.
Now, let’s talk about the flare and touchdown. The main limiting factor for crosswind is how much rudder authority you have to point the nose in the runway direction. If the rudder authority isn’t enough for the crosswind then you can’t make a safe touchdown. (The amount of drift you can cancel out by banking the aircraft also matters, but to a lesser extent.)
This crosswind limit depends on the particular aircraft type and even on the particular configuration of the aircraft. So you can’t say that a particular effective crosswind is a general limit. You can of course calculate a “demonstrated max effective crosswind” for every aircraft type and configuration, but what’s the point? Why not simply use the unfactored demonstrated crosswind from the POH?
So my point is: I think I understand how effective crosswind works but I don’t understand how the concept helps you either in making decision about whether to attempt a landing or in actually carry one out.
Would you or anyone have some good theory sources on the topic of flaps in varying crosswind and/or gusting wind conditions?
Trying to describe this in writing is a bit tricky. I will check Kermode (Mechanics of Flight) and Anderson (Introduction to Flight) to see what comments they make, but my comments come from our old friend the coefficient of lift/alpha graph which features in all principles of flight material.
Improved roll control derives from coefficient of lift shifting up and left as you increase camber with trailing edge flaps. Your wing now achieves the same coefficient of lift with a lower angle of attack, however the outer wing with the ailerons has moved away from the critical alpha of the wing with no flaps, as the inner wing is generating more lift with less alpha. Conversely the curve as you approach critical alpha becomes sharper with increased camber. The symptoms of the stall occur closer, in a more compressed fashion, to the stall itself.
Extending trailing edge flights will shift the centre of lift further inboard towards the root of the wing, because, that is where the flaps are (more complex STOL aircraft may have flaperons and spoilers etc), and this reduces lateral stability and aerodynamic damping.
Also flaps, with increased down wash may have an effect on rudder effectiveness. The slower approach speed with flaps would also reduce available rudder.
If this is the case, using flaps in gusty crosswind conditions may reduce the demonstrated crosswind speed due less available rudder, a higher risk of wing drop due to lower lateral stability?
Big Jet Airways would regard landing without full flaps as an abnormal condition so perhaps this is a GA thing.
A_A Yes effective crosswind has a direct relationship to crab angle, time, drift and other factors.
But either we are talking on cross purposes here or possibly, you are in danger of overthinking this.
As I pointed out previously. In France most flight training was based around mental calculations, ( I stress was as tablets and glass cockpits have reduced the need for that now)
One of the first things we learnt was “facteur de base” Fb. It’s simply represents the speed of an aircraft in minutes.
So in the days of maps we would put our scale rule on the map measure the distance in Nm and multiply that by Fb to get the time without wind in minutes for the journey.
If you multiply Vw x Fb = X
X is used as the maximum effect on time it is also maximum drift.
To correct for time t =X.Cos" a"
Correction to maintain track = X.Sin “a”
So if alpha is 90°to your track or runway axe Sin alpha is 1 cos alpha is 0
When you are planning a flight and the TAF gives a wind of let’s say 60kts. Is it a threat if your POH says max demonstrated crosswind or personal limit is 25knts?
As we descend on final we know that the wind is both likely to reduce. As the wind on an airport is usually measured at a height of 10metres IIRC it is unlikely to change direction much ie back, due to ground friction. (Not accounting for buildings and trees which line the runway and can cause wind shear.)Not only that but the wind direction on the TAF will be true whilst your QFU is magnetic.
Let’s say I am flying an aircraft with a approach speed of 100kts = Fb of 0.6 I will simply multiply Vw (60kt) x 0.6 which will give me a maximum effective crosswind X (anything above or below 90° to the angle of the runway is going to have less of a crosswind effect, less X). In the above case it would be 36kts. Well above my POH demonstrated crosswind landing speed.
Is this a no go? Well not necessarily.Because this is a maximum and if the angle of the wind is less than 90° then it might be different. a no go but it deserves further calculation (remember we are on the ground here). What if the wind direction was just less than 50° magnetic from the runway axis. Well that would give us an effective crosswind of 25knts and that is within the demonstrated crosswind figure.
Of course as LeSving wrote if we did nothing we would drift downwind of the runway axis but if we took a 25° crab angle to the runway axis/QFU we should happily crab our way down the extended centre line where we decrab (for me during the flare but others have their own preferences and that’s no problem) and land.
So our rudder has to provide 25° of turn maximum. In fact it will be a lot less because by the time you flare you will already have lessened the angle of crab due to the usual progressive reduction in wind speed. However if you just de crab there is still the possibility that the wind will try and lift the wing and sort of scoot you across the runway or turn you into the wind. Both of which can be a bit of a problem especially if your aircraft is of the lighter variety or in a tail wheel aircraft. Even a heavy one like a DC3. So to avoid this you need, at the same time as you decrab to add a little into wind aileron. Not too much or you’ll start to turn into wind, but as the aircraft slows you’ll gradually have to add more. As you are within the demonstrated crosswind limit, there is no reason why you should reach the limits of either aileron or rudder deflection.
Now all of these calculations are fine in the planning but winds do tend to change direction and speed. So when you arrive at your destination and the controller gives you the wind speed using Vw which has just been given you and multiplying that by you Fb which you know will give you a maximum. If this is above your crosswind limits instead of tormenting your brain at a critical time you can have time to go away and either land elsewhere or just do some recalculations. If the maximum is below your crosswind landing limits then you know you are okay.
This is all very pedagogique and most of us, I think, have learnt to just make the calculations (other calculations are of course around and do the same thing) and assess the situation. Different pilots also have other methods of crosswind landings which they feel more comfortable with.
We are all intelligent enough to make our own decisions.
A_A you ask about an effective crosswind of 10knts. I assume you mean X and not Vw and the wind being a full crosswind being 90° to your track then you will use a crab angle (by the method I was taught) of 10°
At 20knts it would be 20°
So pretty near the 1 in 60 (as I said there are different methods of mental calculation)
If you mean Vw and not X then I can’t answer that unless you tell me the speed of your aircraft.
gallois wrote:
I am not sure I understand cross the rudders.
OK sorry in German you say “Ruder kreuzen”, because all the aerodynamical acting surfaces are “Ruder” in German language. Of course this hasn’t got the same meaning in English, I did not notice that.
What I mean is obvious and you sure know this. You cross the acting direction of the aileron with the acting direction of the rudder. E.g. when the wind comes from right side, while the rudder is engaged to the left (out of the wind) the aileron is engaged to the right (into the wind). That’s a sideslip.
gallois wrote:
And does your aircraft reach its limit of effective into wind aileron as you slow to taxing speed?
That’s a good question. I don’t know. It sits on the ground quite well, so there’s no need to give into the wind aileron (that’s a huge difference to the Cessna 172, where according to my experience you do need to give a lot aileron into the wind so that the wind-side wing doesn’t go up). It just skids sideways over the ground. In retrospect, I found the Cessna sidewind landing a lot more “vivid”, it needed prompt aileron inputs, even after landing, to prevent that it doesn’t “fall” to the side. Where the Comanche just sits down but skids. It “feels” a lot heavier and stable in this situation.
The skidding of the Comanche is nothing unusual, even on normal landings. Well you train to avoid this, but every Comanche pilot will have stories to tell about this behaviour. There is even in the original Piper checklist the item that you should retract flaps immediately after touchdown in order to minimize skidding. (That item was removed from most checklists because of the risk to activate the wrong lever and raise the gear instead of the flaps).
To give some more general input to the interesting discussion here, there is no such “general personal crosswind limit”, because this depends a lot of the aircraft. I’d not try to do a 40 knots crosswind landing in a microlight that flies at 35 knots. This just won’t work. But say in a Comanche that won’t fly before reaching about 58 to 60 knots a lot more wind is doable.
I’m a “crabber” and can only emphasize to all training this. But I don’t crab into the flare, I change from “crab style” to “slip style” when passing the airport fence. Typically the wind is a lot less already, so less force is needed for the slip. But I won’t wait until the flare because I dislike any movement that could be uncontrollable. To “throw” the plane into the right direction and time the movement about the vertical axis with the touchdown sounds like a bad idea to me. Particularly in an aircraft with very low rudder authority like a Super Dimona motorglider you won’t stop that movement. If it’s not controllable in a slip I would not proceed to land. A friend of mine tried this in our Super Dimona and catched a lamp from the runway lights. With the sideslip you have full authority through the whole landing process.
Today I landed with some 40 kts of crosswind during the approach which decreased gradually to 22 kts just before touchdown. I’m curious how this can be flown except crabbing and then just prior to touchdown wing into wind and opposite rudder.
