Relationship between Flaps / Lift Coefficient

[DTTogaLI]

When I put more flaps

- Vortices
-L/D Ratio

+CLmax
+Effective AoA
+Minimum Glide Range

And Lift Coefficient?

Maintaining the same AoA, the Lift Coeficient will increase...

And not maintaining the same AoA...the Lift Coefficient remains the same?
- - -

Super Stall is related to T-Tail or Swept Wings?
- - -

Witch is a stall warning devices

AoA indicator or Stall Strip?


tnx

[Charlie Zulu]

Hi DT',

A superstall is related to swept wing aircraft. As the tips stall first the Centre of Pressure moves inboard and forward. This produces a nose up tendancy which further decreases speed and increases AoA. This is known as a deep stall (superstall) and is of course not very desirable feature of a swept wing aircraft.

The T-Tail element is there to confuse people... as the main wings are stalled the tailplane can effectively be blanked out of the free stream airflow making the tailplane ineffective or total loss of control in regards to pitch. This, together with swept wings, is not good!

However my groundschool instructors have said swept wings are the main contributor to the deep stall.

As a wing of an aircraft stalls at a certain AoA no matter the speed then the AoA indicator can be used to detect the upcoming stall...

As for the stall strips. I believe they are referring to the strips that are located near the root of a wing to "induce" a root stall first (desirable). Have a look at a Piper Tomahawk... they have stall strips near the root on the leading edge.

Increasing the flap selection will decrease the L/D ratio thus reducing the glide range.

I'll come back to you with the CL max and lift coefficients, although I believe if you increase the flap setting but you keep the AoA the same, the lift coefficient will increase.

However if you are staying in straight and level fligth at a constant airspeed then increasing flaps will momentarily increase the lift coefficient but you will need to lower the pitch attitude and thus the AoA to ensure the lift co-efficient remains the same, otherwise a climb would ensure.

Hope this helps for now and I'm hoping this is correct as I have PoF and Perf exams in two weeks time!

Best wishes,

Charlie Zulu.

[Oktas8]

Regarding stall warning devices DT-


The AoA indicator is an indicator, not a warning device. It gives an output (a number) regardless of how close to the stall you get.
The stall strip serves to generate buffet, giving tactile warning to the pilot of the stall. It generates an output (buffet) only when you get close to the stall. It is a stall warning.

One more point - stall strips, also called flow strips, are not only found close to the wing root. PA28-161 aircraft (for example) have them outboard of the flaps.

I'm not sure what you mean by your comments about maintaining AoA or not when extending flaps. Is it helpful to draw a diagram? If you extend flaps, the CL curve moves up and to the left - thus for any given AoA, you get more CL, and CLmax occurs at a lower AoA. What CZ said is true anyway.

Cheers,
O8
(Good luck!)

[Charlie Zulu]

Oh that was my fault for replying having just woken up and not reading the question correctly.

Can't believe I forgot about the natural buffet that stall strips create. But isn't that also true of wings without stall (flow) strips? Don't they produce a buffet (stall warning) as the laminer flow seperates into turbulent flow further forward as the AoA increases?

I was thinking that the AoA value can be used with an air data computer to send a warning to the flight deck (it has been a while since I did Inst')... ? An indicator though is just that, an indicator. Oops, silly me.

What Oktas said above is correct.

[Super Stall]

I'm related to my parents, the rest is nothing but rumour !!:p

Sorry couldn't resist, back to the topic......

[Piltdown Man]

The important thing is to get the exam bit right! The F70/100 has literally, matchstick size stall strips close to the root of the wing. Their job:- To trip the air into buffet and thus help stall the root before tip. Stall warning comes from the stall vanes (AoA jobbies), the output of which is fed into a little black box. The buffet is also a bit of a give away, but it's not easy getting to the buffet without turning a few things off. However, if exam feedback says that this doesn't happen, then it doesn't. Since when have exams reflected real life?

[Send Clowns]

Superstall requires both swept wing and T-tail. The T-tail is not their just to confuse, but neither is a Dash 8, for example, susceptible as it has no sweep. There must be pitch up at the stall (from a swept wing) but also the elevators must be in the disturbed air, so a T-tail is required to cause a superstall.

With trailing-edge flaps:

Vortices generally weaker I seem to remember (due to shedding at the flap not jsut the wing tip)
L:D ratio reduces

CLmax] increased
Effective AoA increases at the moment of deployment unless co-ordinated with nose-down pitch; subsequently as required by speed
Minimum glide range (? not max?) reduced
CL depends on the AoA as well, but yes for trailing-edge high-lift devices CL increases for a given AoA

[Charlie Zulu]

Hi Send Clowns,

I understand that a T-Tail would be required as it is more prone to be within the disturbed air from the stalled wing.

When I see these type of questions in various sources of feedback, they all tend to contradict each other.

Would you know what the JAA like as the answer to "which aircraft would be more prone to a super stall"? To which there are "Swept wings" as one answer and "T-Tail" as another.

I have seen feedback questions with both Swept Wings and T-Tail in the same answer so that would be the logical one, but if they are listed individually?

Are they asking in terms of the pitch up moment needs to be present (swept wings) or plainly that a T-Tail aircraft is more prone to have lack of pitch moment when its swept wings stall with their nose up tendancy (BAC1-11 for instance)? Ie, as you said a Dash 8 with straight wings would pitch down and thus one would hopefully not get to the "super stall" stage.

Best wishes,

Charlie Zulu.

[redsnail]

The JAA exam answer for the super stall question is "swept wings".

Handling the Big Jets say both swept and T tails are involved. Unfortunately, DP Davies didn't write the exam paper. (A bit difficult now)

Having flown both the T tails (Dash 8) and a swept wing (HS125) I know they are both susceptible to the deep stall phenonema. Hence both fitted with stick shakers and stick pushers.

A straight winged aircraft isn't susceptible to a deep stall unless it has a T tail. That is, it does behave in a normal way with the stall untill the disturbed air blankets the tail and thus renders it useless in recovery. The pusher prevents you from going that far.

A swept wing aircraft is susceptible to a deep stall. Tail position isn't as important.

There was a brilliant discussion about this a couple of years ago on this site (possibly this forum).

I'll let the more learned folks explain the guts of it.

[High Wing Drifter]

I recall the "ramshorn" effect. Because the roots of a swept wing stall last and because the centre of lift moves forward as it travels towards the root then the nose pitches up (as mentioned before in this thread). In addition (this is the deep stall bit), the unstalled air flow is effectively channelled straight down (remember that downwash is an attribute of lift) onto the top of the tailplane, pushing it down, causing the nose to pitch up further. The problem gets worse the deeper into the stall the a/c goes, hence the deep/super stall with swept wing and no T-tail.

[Send Clowns]

I would definitely say for the exam swept wing if the choice is between that and teh T-tail, as I have flown a couple of T-tails that were not susceptible (PA-44 and Be-76); however it might be a classic case of querying the question.

[machonepointone]

I can see how a swept wing can cause this phenomenon, but I don´t see how the T tail can cause it. I appreciate that the T tail can remain in the disturbed air from the wings and prevent recovery, but not necessarily be the cause. There is a well known film clip of an F100 Super Sabre flying along a runway in a super stalled condition before finally departing and crashing. This aircraft had swept wings but it also had a low fuselage mounted tailplane.

[A320rider]

so whats the dif between deep and super stall?

[High Wing Drifter]

I appreciate that the T tail can remain in the disturbed air from the wings and prevent recovery, but not necessarily be the cause
Thought I already explained how.
so whats the dif between deep and super stall?
Same thing as far as I can determine.

[machonepointone]

High Wing Drifter

From what I read, your explanation means that the initial cause of the pitch up is due to the forward movement of the C of P found in a swept wing. Only after that does the T tail have an effect. I therefore stand by my original statement, the T tail does not necessarily cause the pitch up but may either aggravate it or prevent recovery.

[High Wing Drifter]

Yes, that's what I said, the T-tail does not cause the pitch-up. As you suspect, the T-tail exacerbates the problem of getting the nose back down again to reduce alpha, to unstall the wings. However, the ramshorn effect, affects a/c with convetional empenage configuration (no T-tail) and also exacerbates the problem by creating a strong downward force on the tail plane of a non T-tail configuration, also making it difficult to reduce alpha. Hence my final line of
hence the deep/super stall with swept wing and no T-tail.