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flap question

Old 20th Dec 2011, 05:38
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flap question

when flaps are extended to say first level above the speed it is allowed, what risks are we taking? poh always or may be the most limits the speed regardless of the flap levels, (ie white arc at 70, flaps extended say one notch at somewhere between 80-90 knots for aerodynamic braking)

1. higher torsion on wing structure
2. flap hinges may be damaged,
3. the main spar(s) may bend,
4. skin may buckle or excessively bend btw ribs due to pressure distribution change,

any other consequences that i cant see? some instructors does it and defend themselves like "if the wings will be damaged let it happen now, the aircraft is stronger than you think", as an engineer and amateur i dont agree with this statement,

thank you

Last edited by rapidshot; 21st Dec 2011 at 05:48.
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Old 20th Dec 2011, 07:37
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You are right and they are wrong.

The wing is stronger than the limits suggest, because of design safety margins. Those margins however are not there to be used, they are there because of inadvertent overstress, or in case of unknown pre-existing damage or flaws within the structure.

Your list is fairly complete, although particularly if it's a pushrod system I'd also add in potential damage to control linkages,

G
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Old 20th Dec 2011, 08:11
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I once went out of the white arc in my early stages of flying. I had the first stage of flaps out on the Ikarus and was turning final and just started to run the speed up a bit too much. I didn't notice until the wind noise increased quite a bit, the aircraft started to float a lot and it became quite bumpy and more violent to control. No structural damage was done, far from it but it wasn't a nice experience.
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Old 20th Dec 2011, 08:13
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I'd say damage to flap mechanism, especially (on electric flaps) the actuator, is the most likely. But the answer must be hugely type dependent.

However if actual damage occured at 90kt when the flap limiting speed is 70, I would question the other design limits of that aircraft
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Old 20th Dec 2011, 08:42
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What you also need to factor in, is that the pressure distribution along the wings length becomes different. After all, the area where the flaps are (typically inboard) now delivers significantly more lift than the outboard areas. This leads to different bending moments/locations within the spar, compared to the normal configuration, and might lead to localized increased stresses. (Although in general, if you bring the lift in, the bending moment at the wing root will decrease.)

This effect is very significant in sailplanes where you use airbrakes, typically mounted fairly inboard, to destroy the lift at that location and provide a huge amount of drag. Suddenly all the lift needs to be generated by the outboard portions of the wing, increasing the wing root bending moment etc.

So that's another reason there are flap limiting speeds. But it's also the reason there are flap limiting G factors. In the R2160 I fly, the limits are normally -3 to +6G, but with flaps extended they're only 0 to +2G. (Flying inverted with flaps deployed would move the lift to the outboard portions of the wing, increasing the wing root bending effect.)

Oh, and to add to your list: The flaps themselves may be damaged due to torsion effects. On most aircraft I've seen, the flaps are actuated with some sort of actuating mechanism that's attached to the inboard edge of the flaps, not to the center. This will generate a torsion force throughout the flaps themselves. And at some airspeed, the flaps will not be able to withstand that torsion.
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Old 20th Dec 2011, 08:57
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Try lowering the flaps on a C152/C172 on the ground. Firmly push up on the flaps.

One side is rigid the other can be pushed up, presumably bending the mechanism.

That is only one reason. As has been said it is type specific.

Not mentioned yet is trim change which can be dramatic on some types.

D.O.
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Old 20th Dec 2011, 09:11
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On the Chipmunk they are pulled down by cables. The cable is about as thick as a bicycle brake cable, and there's one per flap (in the final part of the flap drive mechanism).

If a cable snaps, you'll get assymetric flap deployment.
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Old 20th Dec 2011, 10:44
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Generally, what everyone else said. However...

Be aware that it's not all about the white arc. For example the C172 (N onwards?) has a couple of 'Vfe' speeds - 1 for flaps 0-10, (115kts) and a lower one for 20-30 (85kts). I'm pretty sure white arc starts at 85kts (for example only, this is from memory, the numbers may be wrong, don't go use it!)

Check the POH, know the aeroplane. This might be the root of some of what you post.
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Old 20th Dec 2011, 10:56
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Mark1234 has it right. On the a/c I normally fly, a C172RG, the first stage of flaps - 10deg - can be set at 110kts. However, the white arc starts at 100kts and denotes the range of any flap setting, in this case up to 30degs.

Always check the POH for the a/c you fly, NOT for some generic 172 or whatever! Also be wary of third-party checklists. Always cross-check them with your POH and amend as required.
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Old 20th Dec 2011, 19:34
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I had the first stage of flaps out on the Ikarus and was turning final and just started to run the speed up a bit too much. I didn't notice until the wind noise increased quite a bit, the aircraft started to float a lot and it became quite bumpy and more violent to control. No structural damage was done
Perhaps not from that one occurance, but over time the knotch on the flap handle which locks it in place wears away. Then, the flaps may still engage but every now and then will slip, jolting back to a clean wing. I had this happen to a student on their first solo, luckily they had the prescence of mind to go around and then fly a flapless approach, but many would get very distracted and start faffing with the flap handle.

Any limitation, Vfe, Vne, Vno, will have some saftey factor applied, but that does not mean it is OK to regulalry execeed this limitation, even if it's just by a few knots. Bear in mind most SEP aircraft will make it well over 10,000 hours in their life, and you don't want to have to replace the flap mechanism for no good reason.
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Old 20th Dec 2011, 19:48
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over time the knotch on the flap handle which locks it in place wears away. Then, the flaps may still engage but every now and then will slip, jolting back to a clean wing.
I've had this happen a few times in the Chipmunk over the decades. Usually, on that aeroplane, it's the detent that drops into the slot that shears (and no, I wasn't exceeding VFE - I've seen those teeny actuating cables!).
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Old 20th Dec 2011, 20:02
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Another way to think of this is like this: Imagine if you hear via the media that the plane you were flying the previous week fell out of the sky, for unknown reasons and the two where were in the aircraft are now dead etc. Limitations are there for very good reasons. Only a real knob would tell you it's OK to bust them. And if you want to take this further, how much can you bust them by? Is it 10% or is 10 kts? How about 20 kts? Rapidshot, I'm with you. It's not acceptable and speed excursions should be written up in the Tech. Log.

PM

Last edited by Piltdown Man; 20th Dec 2011 at 23:29.
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Old 20th Dec 2011, 22:11
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Another possible effect: a long while back I went a few knots over Vfe in a Mooney Bravo (and I really mean a few - maybe 5, and for a few seconds). Presumably the flaps are held down by the motor, because the breaker popped but (oddly) the flaps stayed put. We could not reset the breaker. We landed OK and by then I guess the breaker had cooled down because it reset OK too. We inspected and there was no damage.
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Old 21st Dec 2011, 04:35
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some instructors does it and defends themselves like "if the wings will be damaged let it happen now, the aircraft is stronger than you think",
And this coming from a "Professional Pilot"?!

Limitations are not targets, generally speaking I always try and take a few extra knots "for mum", both in my professional job and when flying the clubs PA28.

It sounds like you have a much more professional attitude than perhaps your instructor(s) do. I would suggest a discussion on the ground next time around and if they continue to fly outside the aircraft limitations a discussion with the CFI needs to take place.
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Old 21st Dec 2011, 05:59
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i think they do it just to land 5 to 10 seconds earlier, may be less, they dont try to stabilize the approach by any other means but use flaps to slow down and increase rate of descent due to tight traffic patterns,
i believe such professionals were never taught how things change rapidly with the square of speed
anyway, i wont be flying with such pilots again, thanks to all for valuable opinions
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Old 21st Dec 2011, 06:40
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Originally Posted by peterh337 View Post
I'd say damage to flap mechanism, especially (on electric flaps) the actuator, is the most likely. But the answer must be hugely type dependent.

However if actual damage occured at 90kt when the flap limiting speed is 70, I would question the other design limits of that aircraft
At first approximation, aerodynamic loads go with the square of airspeed, and on an all-metal aeroplane the safety factor is 1.5.

So, if the flap limiting speed is 70, I'd expect most likely to start seeing some permanent damage at about 70 x (1.5^0.5) = 86 knots.

Sorry Peter, but actual damage at 90 from a limiting speed of 70 - is quite likely in any aeroplane designed to current regulations (by current, I mean anything since the late 1940s).

On the other hand, the flap limiting speed is usually around double the stall speed with full flaps: so a flap limiting speed of 70 implies a stall speed about 35, which probably makes it a microlight. Anybody capable of getting a microlight past 85knots, with the flaps down, should probably not be flying that or anything else unsupervised.

G
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Old 21st Dec 2011, 13:37
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As noted, the white arc refers to FULL FLAPS. So you can put in full flaps when in the white arc. We can go to 20 degrees upto 150 kts IAS in our aeroplane, then 25 below 130 Kts and below 120 can go full flap, which is the white arc.
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