Flaps and tip stall
 

If flaps provide more lift at lower angle of attack and they are usually inboard of ailerons, do they negate the effect of washout by causing the tips to stall first resulting in loss of aileron control at lower landing speeds?
ie for the lower landing speed they provide, do you also get less roll control than landing without flaps at higher speed? Therefore more chance of problems in a crosswind?
If so then you should probably only deploy flaps on final and use rudder only for corrections. True?
Is this the reasoning behind flaperons?

Just curious!:eek:

RE: Flaps and tip stall
 

Oops! Just realised, flaps give you more lift at lower angle of attack meaning the tips are less likely to stall. Now I have confused myself!![&:] Can anyone clarify?

RE: Flaps and tip stall
 

I had for many years an 8 Ft span Sr Telemaster I flew off of both wheels and floats. This design, like many trainers, has full span ailerons. I tried flaperons as I've had a computer radio for some time and like to experiment. I found it did indeed land slower, but it was also less controllable (roll) and tended to have a much more abrupt stall, especially with full flap selected. This is why most full size designs have the flaps inboard, or if they mix flaps in with aileron, the ailerons only come partially down (as compared to the flap position). I was uncomfortable enough with it that that I stopped using it. On the other hand, I'm all for flaps if they're inboard, and the plane still has normal ailerons.

RE: Flaps and tip stall
 

Many control line stunt airplanes have flaps rigidly coupled to elevator. Usually these flap's chord tapers toward the tips. The tips are thus at a lower angle of attack with the flaps deflected, and less likely to tip stall.

RE: Flaps and tip stall
 

Both replies make a lot of sense. Thanks. I read the washout thread which also helped. I really shouldn't read aerodynamics threads at 1.30am!! (I'm in India at the moment!)[:-]

RE: Flaps and tip stall
 

So what if you have outboard (ie not strip) ailerons with a Flaperon mix ie. you can extend the flap whilst still having aileron functions? Is this a nasty config with unpleasant stall characteristics?

RE: Flaps and tip stall
 

Many STOL type aircraft have the ailerons function as flaperons. I've noticed that most of them mix in much less travel than the inboard flaps, i.e. when the flaps are fully extended to approx 40 degrees, the ailerons might be half that or less. Several fighter types use flaperon too (eg. F-18), but when the flaps are fully down, the horozontal stabilators (pivoting one piece control surface) provide roll control. If you have seperate servos for the ailerons, and a computer radio capable of mixing the ailerons to flaps, go for it - it's fun to experiment.

RE: Flaps and tip stall
 

Actually, you are all correct - partially. When flaps are lowered, the wing angle of attack is increased along the span of the flaps. The wingtip angle of attack becomes less than the root, the root stalls first. Increased lift by flaps only goes so far, then lift no longer increases and induced drag increases. On a Cessna 172 for example, you get more lift than drag up to 20 degrees of flaps. 30-40 degrees gets you loads of drag and you can drop into a short field easily. If you have to cob on the power to go around, you have to dump the flaps only to 20 degrees, then milk them up the rest of the way slowly.

Long story short - flaps increase the AOA (and the lift), causing the root of the wing to stall before the tips.

RE: Flaps and tip stall
 

Therefore, by definition, outboard aileron/flap mix is a potentially bad configuration as the tip will stall first if a stall is going to occur at all!

Thanks for the analysis.

RE: Flaps and tip stall
 

How did you draw that conclusion? From zero to approximately 20 degrees flaps add more lift. As they are deployed and the angle increases, gradually they create more drag than lift. As they do so, the air over the flap can become turbulent (stall). Doesn't mean that the whole wing is stalling. There are whole families of aircraft that have sucessfully employed flaperon (DeHavilland STOL designs for example). The trick is to have a wing that has a gradual stall, not one that has rapid onset. A thicker wing with a heavily rounded leading edge can produce that characteristic. Flaperons are not bad - they just have to be designed and used correctly. Attached is a pix of (what I think is) a Helio Courier STOL design...

RE: Flaps and tip stall
 

flaps don't add lift
they just reconfigure the airfoil
Try this: trim for a set cruising speed level flight (for the pilot-in-the plane- guys)
Now add flap - a little teesy bit- what happens?
the plane balloons?
Does it continue to climb?
Or does it gradually slow down.
Ad a littl more (still under 20 degrees)
what happens?
More of the same?
If the flaps added lift - the plane would continue to climb with NO power increase needed.
They do keep the fuselage at a low AOA-and the big wing and the little wing (I love that one) at low AOA
but the drag is always higher-always-

RE: Flaps and tip stall
 

Happy with your rebutal. However, my point was that if all you have are wing tip flaperons (not strip flaperons) then if a stall occurs, and it will occur at the wing tips because of the flap induced higher angle of attack, it will be tip stall rather than a full wing stall. It could also make control more difficult (Yaw) if your aileron movement (with flaps extended) moves in and out of the drag zone (>20 degs in this example). As the wing stalls you will aggrivate the condition by applying more opposite aileron to lift the stalled wing, producing increased drag and yaw which then develops into a spin (inner wing moving slower through the air than the outer wing). In this case you will need to catch the stall as best you can with opositr rudder to yaw the stalling wing and increase airspeed over that wing (not a natural response I suggest).

Come back to me if you wish.

RE: Flaps and tip stall
 

Nope - Flapperons are a really mixed bag of results .
On control line Stunt - they are fine (how would you snap roll those?) -
I still have a UReely made of Bakelite - with 70 ft wires --oldie.
On super light park flyers -again - great -but- on a model with high wingloading -- a trap -waiting to be sprung.

RE: Flaps and tip stall
 

We agree then!

RE: Flaps and tip stall
 

Ok then, some of the above is confusing but here's what I make of it:
Inboard flaps when deployed increase lift (at a given airspeed, Dick), the aircraft balloons until the increased drag reduces the airspeed and hence the lift. Flaps lower the AOA (even during the balloon, I know this first hand from the bit of flight training I've done) To maintain the same lift you need to apply more power but achieve lower airspeed. Thus you can get the same rate of decent at a lower airspeed (but not less power) or a steeper glide angle with no power.
A lower AOA makes it less likely the tips will stall.
My question now is, at the point of stall, does the inner part of the wing with flaps deployed stall first because the higher cambered airfoil has a lower stall angle? (assuming an eliptical wing with uniform section and no washout which in theory would stall at the same time along its length without flaps)

RE: Flaps and tip stall
 

theory and fact sometimes agree -and -sometimes not . a really short span wing may simply --settle evenly - as spanwise flow increases
a longer wing may start " dancing" -as each panel stalls-call it tip stall -whatever .

As long as we understand that flaps simply reconfigure the airfoil- and angle of attack of the wing is NOT the same problem on all planforms , then the stall and it's effect can be better understood.
as an example : add inboard flaps to the demispan of a Cassut box wing aircraft ( aspect ratio of 3.3 to 1)
do the same thing to a -(you pick it) a 7-1 aspect ratio lightcraft - both with same wing loading and area .
The extremely low aspect ratio setup will act differently-and remain more stable
N/Y?

RE: Flaps and tip stall
 

Are you saying that no type of flap adds lift? My other curiosity is, are you an Aviation Engineer?
Reason I ask is, any flap that increases wing surface area as it is extended ---should--- increase lift.
Your thoughts.[8D]

RE: Flaps and tip stall
 

If the airspeed stays the same before and after the lowering of flaps the flaps can cause an increase in lift.

I will agree with you that they add more drag than lift. That is why on many full scale aircraft a power out procedure does not begin with lowering the flaps.

RE: Flaps and tip stall
 

And an alarming change in the deck angle!

RE: Flaps and tip stall
 

Aviaton Engineer ?

nope - This kind of question tho is simply a logic question - No math required.
As for other answers about speed etc.. sure if you maintain speed then lift goes up- but that requires more power doesn't it.
So the flap does not increase lift but-it increases drag as it reconfigures the lifting plate (wing).
It is of course useful -applied correctly.
Flaps only exist because most full scale aircraft are simply a collection of compromises
Not enough power and too much weight
If weight could arbitrarily be changed and power were infinite -during flight - there would never be a need for flaps .

RE: Flaps and tip stall
 

Dick, someone is going to have to advise all those Boeing and Airbus engineers that they've got it all wrong! All along they've been thinking flaps add lift. They've been adding all that extra weight, mechanical complexity and cost to airframes all these years building in flaps when you say it really doesn't create lift. What were they thinking! :^) Maybe we need to use a different angle here - if you can land slower with flaps, they must be adding lift right. After all, you're supporting the same weight at a lower air speed. Do they add drag? Of course flap deployment does - you're sticking more surface out into the airstream. You compensate by adding power. Simple physics says that you don't get something for nothing right. Dick, I'm no engineer, but I've been formally trained as an aircraft mechanic (for whatever that's worth), and we were taught flaps add both lift and drag. Makes sense I think. I think any credible book on aerodynamics will back this up. I'm betting that there's a few real aircraft engineers sitting in the wings (Tall Paul) smiling right now...

Dick - I'm watching you... ha, ha.

RE: Flaps and tip stall
 

Yeah, I gotta join the chorus that says flaps increase lift. At least all the airfoil graphs I've ever seen that showed flaps listed the amount the Coefficient of Lift was increased by the flap's deflection.

It really does appear that classical aerodynamics teach that the maximum lift that can be created by an airfoil will be increased with flaps.

RE: Flaps and tip stall
 

trick questions and trick answers !
IF you increase area ( roll out the flaps ) - you can add lift and NOT appreciably add drag .
But simple flaps simply change WORKING angle of attack of the wing.
The efficient speed for this angle is lower - make the flap large enough and lower it enough and you will eventually have a "wing" that is pointing 90 degrees to direction of flight.


How can maximum lift be increased ???
If it can be increased - it was never at max.
Hows a bout this------
"an optimised airfoil is one where the shape changes to optimize lift at a given speed.
The textbook statement " flaps add lift" is simply a partial truth--
It should be " flaps add lift but at the expense of adding drag "
OR
"flaps permit a re optimized shape for lift for slower speeds, at the expense of increased drag."
It is sorta like the instruction " to go up -add up elevator "
Ir should also say " to go down, add too much up elevator".
Semantics--------

RE: Flaps and tip stall
 

Trick argument back at you............

Ask NACA..... chuckle......

Actually, the plots show the max CL from the airfoil and the max CL obtained when plain flaps are deployed. Most that I've seen were just that, plain flaps. simple flaps. whatever the name...... But the application is very much like we'd see with a lot of models whether or not the flap is a plate that is flat against the bottom of the airfoil or is the rear of the airfoil. But that said, I've seen a couple where the CL was reported for the "rear of the airfoil" deflected and not deflected. And the plots weren't only the old NACA ones.

If you'd like to twist the definition of flaps, kewl. I'll go along with that. But really, most everyone is discussing having them things that stick down versus not having them things that stick down sticking down.

And a couple of the plots I've seen show something like a ClarkY giving around 1.? CL for max CL of the airfoil, but having a dotted line showing a simple flap hinged somewhere around .8C. And the CL for that airfoil with that flap deflected had it's own traces on the plot and the traces went WAY WAY up the chart above the "non flapped" line.

I'm on your side all the way about slats and flaps that extend, but gotta argue (with tricks or not) that almost every book I've read states that flaps increase the CL you'd expect from a specific airfoil by a very significant amount over what you'd get with that airfoil without flaps.

RE: Flaps and tip stall
 

OK, you made me go dig in my book pile. The first one I came to was Andy Lennon's R/C MODEL AIRCRAFT DESIGN, PRACTICAL TECHNIQUES FOR BUILDING BETTER MODELS

He mentions the increase of max CL in almost every section but most sections deal with the special purpose models he was having fun designing. The general section on wing design has a piece on slotted flaps. I got no problem ignoring that. The next section is on plain flaps. Plain flaps are still flaps. And the ones he shows in a diagram are basically 25%C "rear of the wing" movable surfaces that are obviously hinged at .75C. He talks about their effect on symmetrical airfoils, particularly an E168. He says they add an additional CL of 0.62 at 60degree deflection.

I'd interpret that as saying, if you built a wing using an E168, if you made the rear of it into plain flaps, the airfoil that once gave you X amount of lift would now give you more when you used the flap part as a flap. And you can compute lift of the wing unflapped with the original CL and the flapped amount with the original CL plus 0.62.