PPRuNe Forums

Go Back   PPRuNe Forums > Flight Deck Forums > Tech Log
Forgotten your Username/Password?

Tech Log The very best in practical technical discussion on the web


Reply
 
Thread Tools
Old 2nd Feb 2007, 18:41   #1 (permalink)
 
Join Date: Nov 2005
Location: Dublin
Age: 52
Posts: 57
Flaps and slats on aoa

Whilst maintaining a contant attitude,what is the effect of flaps and slats on the angle of attack??????????Dont think the answere is as obvious as it may seem but would appreciate a hand out with this one!
Captain Greaser is offline   Reply
Old 2nd Feb 2007, 20:27   #2 (permalink)
 
Join Date: Sep 2002
Location: La Belle Province
Posts: 1,893
Constant attitude is a strange constraint to apply; do you really mean that?
Mad (Flt) Scientist is offline   Reply
Old 2nd Feb 2007, 20:35   #3 (permalink)
 
Join Date: Nov 2005
Location: Dublin
Age: 52
Posts: 57
Thats what the question says!!!!!!!!!
Captain Greaser is offline   Reply
Old 2nd Feb 2007, 23:15   #4 (permalink)
 
Join Date: Jun 2004
Location: Australia
Posts: 1,853
If maintaining a constant ATTITUDE, Slat extension will cause a DECREASE in Angle of Attack.

If maintaining a constant ATTITUDE, Flap extension will cause an INCREASE in Angle of Attack.

Regards,

Old Smokey
Old Smokey is offline   Reply
Old 3rd Feb 2007, 10:02   #5 (permalink)
 
Join Date: May 2000
Location: USA
Posts: 2,486
Well...

In the original question, altitude is not a constraint, only attitude; neither is airspeed or power. While Old Smokey is correct for the immediate transient effect, I'm not sure what the final, stable result will be.

Seems to me that at constant attitude, AOA will depend on whether the airplane is climbing or descending in its final configuration... Constrain power as well, and added drag will determine airspeed and descent rate. At constant attitude, AOA will increase in a descent and decrease in a climb.

Add power and constrain airspeed, and the lift provided by flaps will cause the airplane to climb, decreasing AOA. With slats, lift available depends on whether the increase in camber offsets the decrease in AOA...
Intruder is online now   Reply
Old 4th Feb 2007, 13:05   #6 (permalink)
 
Join Date: Nov 2004
Location: uk
Posts: 1,188
From my vague memory of my ATPL exams the slats have no effect on the chord length or camber of the surface, they re-energise the boundry layer and delay the formation of turbulent airflow over the wing. Not a direct answer to your question but one of thos little facts I remember from the ground school.
smith is offline   Reply
Old 4th Feb 2007, 14:35   #7 (permalink)
 
Join Date: May 2003
Location: france
Posts: 192
attitude & angle of attack vs flaps/slats

hello everyone,
a few simple reflections on the matter. attitude = angle of attack + slope.(always true even in level flight were slope = 0)
in our example attitude = constant, so only angle of attack & slope(altitude) can vary.
nobody talks about the power or thrustlevers, so, initially, assume they remain constant as well.
flaps & slats are by definition high lift devices, increasing lift & drag, enabling us to take off & land at reasonable speeds & runway lengths, basically we bolt another wing to the aircraft when extending these high lift devices.
for take off we use only partial flaps in order to take advantage of the increase in lift & minimizing the drag penalty. for landing we use more or full flaps, to use both, lift + drag. by definition AoAttack is the angle between the wingchord & the relative wind, so by setting flaps & slats you modify the wing cross section by increasing camber & surface & so i agree with old smokey(who i consider a wise man), by constant attitude, slats decrease AoA, whilst flaps increase AoA.
kind regards,
bm,

Last edited by blackmail; 4th Feb 2007 at 18:17.
blackmail is offline   Reply
Old 4th Feb 2007, 19:06   #8 (permalink)
 
Join Date: Sep 2002
Location: La Belle Province
Posts: 1,893
OK,for the question as posed there is no single answer - the result is aircraft dependent, and there are insufficient constraints specified to permit a single solution to be determined.

Consider only the flaps case. The initial stick-fixed transient may be to pitch up OR to pitch down - it depends on the raw pitchingt from the flaps and the effect of the change in downwash on the tail lift. Left's assume the more normal case, where the aircraft pitches down in response to flap deployment.

Next we have to wonder if the increased lift due to flap deployment is offset by the decreased AoA. Again, aircraft dependent. Let's assume the flap effect is greater. So we have a net increase in lift, and of course an increase in drag too. If we do nothing, the aircraft will start to loop upwards, due to being above 1.0'g', and will slow down, both due to the drag and due to climbing. There will be some speed where the increased lift s offset by the reduced speed. There will also be some (other) speed where the drag has reduced enough to be back in trim in the speed axis. The only variable that we have (if we dont change the aircraft configuration) is flightpath angle, which we can adjust to balance the two different speeds (it will affect the drag-trim speed, but not the lift-trim speed).

Whatever that flightpath angle ends up being, its almost certain that it will not be such as to give the original pitch attitude - nothing we`ve done has been towards that end. Therefore in order to maintain the same pitch itude we have to change some other paramet9s) - elevator, or thrust, or speed, or whatever. There will be many many combinations which achieve the desired pitch attitude - some may involve higher speeds and lower AOAs but with a climb - others a low speed and high AOA but a descent. Wheteher the AoA increaes or decreases is a choice, since the question doesn`t constrain what we do to maintain pitch attitude.
Mad (Flt) Scientist is offline   Reply
Old 4th Feb 2007, 19:26   #9 (permalink)
 
Join Date: Nov 2004
Location: Dubai
Age: 38
Posts: 35
I take it you have an intervew with the Emirates...

Last edited by pltskiracer; 19th Feb 2007 at 15:43.
pltskiracer is offline   Reply
Old 5th Feb 2007, 17:26   #10 (permalink)
 
Join Date: Sep 2005
Location: Ontario, Canada
Age: 46
Posts: 65
keep it simple

Like a few people noticed, not in so many words, this is a nice academic question. Let's treat it academically and hence:
1. everything takes place in a wind tunnel
2. the model is bolted at fixed attitude
Now, we can all answer the question, and (me thinks) agree with Old Smokey.
Cheers,
(Just Balsa Models).
PS: Do all slats extend & droop the leading edge?
balsa model is offline   Reply
Old 5th Feb 2007, 18:23   #11 (permalink)
 
Join Date: May 2000
Location: USA
Posts: 2,486
I suppose we have to define AOA, then...

Strictly speaking, AOA is the angle between the mean chord line and the local airflow. However, when speaking of indicated AOA in an airplane, the AOA gauge is fixed to the fuselage, so indicated AOA is relative to the mean chord line of the clean wing only. So in that case, the AOA does not change.

If we go with the strict definition, Old Smokey is correct for MOST airplanes. Slats USUALLY extend forward and downward. Flaps USUALLY extend aft and downward. However, Fowler flaps MAY initially extend aft only, so the AOA may not change at all. Also, if slats and flaps extend downward the same amount, the chord line will not change, so neither will the AOA -- only the camber changes (plus thickness and all those other details...).
Intruder is online now   Reply
Old 5th Feb 2007, 18:26   #12 (permalink)
 
Join Date: Aug 2000
Location: UK
Posts: 3,556
If you define AoA with reference to the actual chord line and consider the change in chord line when flap or slat is extended, the Old Smokey's answer is clearly correct for the "bolted at fixed attitude" interpretation of the question.

However, in every case I've seen where aerodynamic coefficients are considered against flap or slat extension as a parameter, the chord line used as a reference against which to measure AoA is the original chord line of the wing without the high lift devices. Thus the answer is, trivially, that the AoA does not change.

M(F)S's interpretation is, of course, more interesting.
bookworm is offline   Reply
Old 5th Feb 2007, 18:49   #13 (permalink)
Anotherflapoperator
Guest
 
Posts: n/a
Not a simple one really, eh?

In simple terms perhaps, the Trailing edge flaps effect is to increase camber and therefore alter lift AND centre of Pressure. In a fixed other variable case, lift increases and CP moves aft. The downwash effect change is seperate and airframe dependent.

Leading edge devices are required because modern Supercritical wings do not respond well to high AoA. Slats act in a similar way to slotted flaps in energising the boundary layer to keep airflow attached at higher AoA, whereas L.E. flaps alter wing shape to alter camber and chord, re-shaping the wing for low speed flight entirely. Some devices function in duel roles, allowing airflow to remain attached at smaller angles of deployment and when fully deployed, reveal a low speed leading edge beneath and function purely as a slat.

To answer the original question, the effect of flap on aircraft attitude is to reduce it at constant airspeed, and if angle of attack is measured from the airframe, then it will reduce. The effective AoA of the wing increases, but providing you qualify your answer, it doesn't matter.

Secondly, slats are there to give the wing a higher AoA ability and do not alter lift significantly in themselves. Different designs will, and some wil lactually create more drag and no lift increase when deployed at the smaller AoA expected at initial flap lever operation.

To answer a tech interview question, as long as you qualify your answers succinctly and without rambling, you can answer several ways. They are looking for a good fundemental knowledge not blind faith in a narrow argument.

This is where multiple choice really falls down, IMHO!
  Reply
Reply
 
 
 


Thread Tools


Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off



All times are GMT. The time now is 01:37.


vBulletin® v3.8.7, Copyright ©2000-2014, vBulletin Solutions, Inc.
SEO by vBSEO 3.6.1
1996-2012 The Professional Pilots Rumour Network