Aerodynamicists Question

[VH-XXX]

Here is a question that I hope someone can answer.

Let's say you had a current MTOW of 1,000 kg's with a stall speed of 50 knots. Easy round figures to start with.

You somehow lower the stall speed of the aircraft, eg. wing extension, wing-tips, vortex generators, flap modification etc. As a result you have lowered the stall speed by 5 knots, to 45 knots.

I then need to work out what the weight needs to be to stall the aircraft back at 50 knots.

Eg., 1,200 kg's.

This is probably not possible and I realise flight testing would be a good start but it would be good to start with some numbers to work back from.

[ForkTailedDrKiller]

Ohhhhh! My head hurts!

Dr :8

[matt_mccarty]

L=1/2. ρ.V^2 .S.Cl
In this instance, ρ and S (density and reference area) remain constant, by making the aerodynamic addition you are increasing the Cl at which stall occurs.
If the new stall speed is 45kts (23.2 m/s) at 1000kg mass (weight of 9800 N), then the new Cl (the Cl at which stall occurs) can be found (along with the constants ρ and S)
1/2.ρ.S.Cl=L/V^2
= 9800/(23.2^2)
= 18.3

Then the lift at a speed of 50kts (25.7m/s) can be found as well.
L=(1/2. ρ .S.Cl).V^2
= (18.3) . 25.7^2
= 12087 N
This is equal to a mass of 1233kg.

[john_tullamarine]

.. or, if you want to do it without any brain strain -

Recalling that

(a) at the stall, CL = CLmax and is presumed to be constant

(b) L = W, if we make the dubious presumption of level flight

Then, where "1" indicates before, and "2" after ..

CLmax = L1/(1/2.rho.V1^2.S) = L2/(1/2.rho.V2^2.S)

so, cancelling out the common factors

W1/V1^2 = W2/V2^2

from which you get either

V2 = V1.sqrt(W2/W1), or

W2 = W1.(V2/V1)^2

So, with the example figures given, and presuming that the speeds are CAS, not IAS

W2 = 1000.(50/45)^2 = 1234.6 (call it 1235)


This only has some validity for a constant configuration comparison.

Also, and this is very important, PEC gets interesting around the stall so IAS won't hack it (we've all seen a Cessna with naught on the clock at the stall - means nothing due to PEC) so you have to do the sums in CAS to get anything in the way of sense out of the sums.

I think that this was where Matt was coming from ?

[The Green Goblin]

Simple,

You will stall when the aerofoil exceeds the critical angle :ok:

Weight and Airspeed have zilch to do with it!

[john_tullamarine]

It would appear that The Green Goblin attended a different uni and/or PoF lectures at the flying school to most of us ?

[FGD135]

Well done matt and John!

I got the same answer - actually 1234.567 kg - a coincidentally interesting number!

[The Green Goblin]

It would appear that The Green Goblin attended a different uni and/or PoF lectures at the flying school to most of us ?

A consequence of going to slow is exceeding the critical angle which results in a stall.

A consequence of being to 'heavy' is exceeding the critical abgle by attempting to generate additional lift which results in a stall.

So therein lies your answer!

As a result you have lowered the stall speed by 5 knots, to 45 knots.

No, you have just reduced the minimum steady in flight speed to 45 knots from 50, Vs being the minimum steady in flight speed :E

[ReverseFlight]

Quick calculation in the field:
50/45 = 1.111 recurring, square it to get 1.2345678
Then multiply by 1000 kgs = 1234.5678 kg :ok:

[rmcdonal]

It would appear that The Green Goblin attended a different uni and/or PoF lectures at the flying school to most of us ?

No The Green Goblin is correct. Vs is just the minimum speed that an aircraft flying at the critical angle can maintain S+L flight.
A stall occurs not when you go below the stall speed, but when you exceed the critical angle.

[VH-XXX]

Thanks guys, there's a couple of good ideas there. I'll get cracking on flight testing to try and back up the findings before writing my "amendment" :ok:

[Jabawocky]

I know what you are doing! :}

[VH-XXX]

I know nuffink !!!

[john_tullamarine]

A stall occurs not when you go below the stall speed, but when you exceed the critical angle.

There is some validity in that but the posters miss the thrust of the thread. A bit like saying "the sky is blue" which doesn't give any information as to the what, when, how or why.

Either way, and all jokes aside, I really do suggest that you both revisit the books and then go ask for your money back from whomever did your initial principles of flight instruction - coz either you wuz ripped orf or weren't paying all that much attention

... we are talking flow angles in terms of the parameters which affect the indirect measurement of those angles.


Mr McDonal doesn't indicate which airline it is for which he flies. You have the option of asking your ops engineering section to run over the engineering again for your interest. I am quite sure that they will opt for a variation on your story.

[training wheels]

Simple,

You will stall when the aerofoil exceeds the critical angle :ok:

Weight and Airspeed have zilch to do with it!

Try explaining that to the guys who wrote the CASA Flight Instructor's Manual. (http://www.casa.gov.au/wcmswr/_assets/main/aoc/training/guides/fim.pdf)

From the said manual, page 33;

RELATIONSHIP BETWEEN CRITICAL ANGLE AND STALLING SPEED
Explain that for a given weight at ‘1’ g every angle of attack including the critical angle, has its associated indicated airspeed. As the angle of attack of the wings invariably cannot be observed, reference is therefore made to an aeroplane’s stalling speed.

FACTORS AFFECTING THE STALLING SPEED
The basic stalling speed of an aeroplane, such as referred
to in an Operations Manual or Owner’s Handbook means
the indicated airspeed at which the aeroplane will stall
from straight and level flight, with power off.
Explain that the stalling speed will vary, depending on:
(a) Weight
(b) Power
(c) Flap and/or Slat position
(d) Manoeuvre
(e) Ice on or damage to wings

[Oktas8]

I love it. Green Goblin & rmcdonal (who?) correcting Mr Tullamarine on a point of P of F. Can't work out whether it's funny or tragic. But it's certainly priceless, either way. :ok:

[The Green Goblin]

As the angle of attack of the wings invariably cannot be observed, reference is therefore made to an aeroplane’s stalling speed."

Answered your own argument there mate!

Come on guy's! I can't believe something so trivial as this is causing such confusion!

This is why 'stalling speed' is so en grained in peoples collective thoughts. As an instructors we teach the lesson on stalling by demonstrating it with a decreasing airspeed. Don't forget when we demonstrate stalling we get you to look out the window at the increasing angle of attack relative to the horizon.

You can stall at high speed, you can stall at low speed, you stall anytime you exceed the critical angle of attack.

Vs is simply the minimum steady flight speed. Reduce the airspeed anymore and you will need to increase the angle of attack to generate more lift. Increase the angle of attack anymore and you will stall.

Either way, and all jokes aside, I really do suggest that you both revisit the books and then go ask for your money back from whomever did your initial principles of flight instruction - coz either you wuz ripped orf or weren't paying all that much attention

... we are talking flow angles in terms of the parameters which affect the indirect measurement of those angles.

You're clutching at straws mate :D

FACTORS AFFECTING THE STALLING SPEED
The basic stalling speed of an aeroplane, such as referred
to in an Operations Manual or Owner’s Handbook means
the indicated airspeed at which the aeroplane will stall
from straight and level flight, with power off.
Explain that the stalling speed will vary, depending on:
(a) Weight
(b) Power
(c) Flap and/or Slat position
(d) Manoeuvre
(e) Ice on or damage to wings

Obviously we generate lift three ways. Through angle of attack, airspeed or camber. So to support that increased weight we need to adjust one of these three things to support it. An increased weight will result in an increased Vs simply because we require a larger angle of attack to generate the additional lift for a given airspeed. Notice when you're aircraft is heavy it will not get the same TAS? Nor climb as fast? It would't be because you're taking a larger bite of the air would it?

I'm still struggling to comprehend how smart people get something so simple, so wrong!

[GADRIVR]

Goblin.....you are a **** stirrer...though correct....love it.
A stall can happen at any old time. Just pull the stick, exceed critical angle, hey presto....stalled aerofoil.

[Deaf]

Of course another consideration is that

L=1/2. ρ.V^2 .S.Cl

is an oversimplification/half truth/something/certainly not worth too many significant figures.

Cl varies with Reynolds number ie with rho, V and chord (a big compnent of S) and that variation is different for different airfoils.

Now if the reduction in stall speed is achieved with wing extension then we have a change in aspect ratio; whole new sets of things to play with.

[RHLMcG]

As an instructors

You're an INSTRUCTOR ?? ... Christ Almighty !! The standards have plummeted to abysmal depths !!

[training wheels]

An increased weight will result in an increased Vs simply because we require a larger angle of attack to generate the additional lift for a given airspeed.

So initially, you said weight had zilch to do with it, but now you say it does.

I'm still struggling to comprehend how smart people get something so simple, so wrong!

And I guess you can include yourself in that as well. :p

[VH-XXX]

Now now, this was a nice thread and I got the answers I needed so play happy :ok:

[FGD135]

I'm still struggling to comprehend how smart people get something so simple, so wrong!
You need to realise, The Green Goblin, that you are talking about different things.

You are talking about when the wing stalls. The others are talking about the relationship between the stall and the aircraft's weight/airspeed - and the importance of understanding those relationships.

Another relationship regarding the stall is that associated with angle of bank/load factor.

Every textbook on aerodynamics emphasises those relationships and I think you will find that every syllabus of instruction related to flight principles also insists on some coverage of those relationships.

Important for every pilot to know these relationships? What do you think?

[john_tullamarine]

I normally don't come back into a thread which goes wildly off the rails but this one is a tad frightening (and I mean that quite sincerely) in respect of standards and (a dreadful level of) general knowledge ..

Some observations -

(a) You can stall at high speed, you can stall at low speed, you stall anytime you exceed the critical angle of attack.

Certification (ie Flight Manual) stall data is for a slow speed (max 1 kt/sec) speed reduction to stall with specific trim conditions, CG, etc., and, as such, is a not all that relevant to real world operational risks. However, to a reasonably practical (and conservative) accuracy, a "normal" low speed stall will/should approximate the certification figures.

High(er) speed stalls fall into at least two categories -

(i) an accelerated stall where the load factor provides a higher speed observed at departure - higher load factor works much the same as a higher weight

(ii) for very high pitch rates (recollection is 70-80 deg/sec or more - there was an interesting RAeS paper on the topic several years ago), quite a different situation where a leading edge vortex forms and significantly higher angles than "normal" are achievable - generally not a problem on fixed wing but may be a concern with some of those devices which thrash the air into submission (humble apologies to JE, NL, et al)

(b) An increased weight will result in an increased Vs simply because we require a larger angle of attack to generate the additional lift for a given airspeed

I am a tad confused by this .. if we are at stall angle (and, as you have observed, stall angle is a pretty important consideration in stalling) how do we then fly at a higher angle just because we have a higher weight (other than by generating a very high pitch rate as described in (a) (ii), above .. and that is probably quite irrelevant) ? .. or have I missed some underlying and important physical principle somewheres along the way ... ?

(c) Cl varies with Reynolds number ie with rho, V and chord (a big compnent of S) and that variation is different for different airfoils.
Now if the reduction in stall speed is achieved with wing extension then we have a change in aspect ratio; whole new sets of things to play with

.. all of which is interesting and factual ... but quite irrelevant to the present discussion on weight and speed variations at constant altitude with a constant configuration

(d) You're an INSTRUCTOR ? .. and following impertinent comments ..

I might have been a tad more polite than our contributor ... but the underlying sentiment so expressed was not too far short of the mark, I suggest

(e) FAR 25.333 (http://ecfr.gpoaccess.gov/cgi/t/text/text-idx?c=ecfr;sid=8987618240385c25f225d40e30ef0744;rgn=div8;vie w=text;node=14%3A1.0.1.3.11.3.164.7;idno=14;cc=ecfr) gives a nice picture of the low speed stall lines. For a certification style of stall you just can't operate outside the envelope in the curvy sections as they represent the stall condition. For interest only, I guess ?

[RHLMcG]

JT - you normally don't get catty.

[john_tullamarine]

JT - you normally don't get catty.

On occasion, technical incompetence of such exquisite proportion requires respectful acknowledgement.

[18-Wheeler]

I love it. Green Goblin & rmcdonal (who?) correcting Mr Tullamarine on a point of P of F. Can't work out whether it's funny or tragic. But it's certainly priceless, either way.

Yes but he/she is correct*, although for the intent of the question it's not the desired answer as such.


*Okay, going to throw yet another spanner in the works, even the stalling angle will vary with air density & speed so even the statement of an aerofoil stalls when its angle of attack exceeds a fixed angle isn't quite correct. For practical purposes it is though.

[freshy1234]

The goblin is being qualitative
JT is being quantitative.
The underlying principal of both arguments is the same.

[Meikleour]

GG

By failing to adequately read the posters actual query you have started a completely pointless technical spat!!

The poster referred to a situation where the aerodynamic qualities of an airfoil had been altered to change the critical AoA. He simply wanted to know how to derive the new weight which would result in the same stalling parameters as the unmodified aerofoil!!

[VH-XXX]

Ohhhhh! My head hurts!

Dr http://images.ibsrv.net/ibsrv/res/src:www.pprune.org/get/images/smilies/nerd.gif


Dr. FYI there's another thread currently running with information where you can get some medical relief from this condition, albiet not the legal kind from a chemist.

[The Green Goblin]

Quote:
Originally Posted by The Green Goblin
An increased weight will result in an increased Vs simply because we require a larger angle of attack to generate the additional lift for a given airspeed.
So initially, you said weight had zilch to do with it, but now you say it does.

No, the aerofoil will always stall at the roughly the same angle of attack but will achieve that angle at a variety of different weights/speeds. The underlying message I am telling you is that an aerofoil will stall at a given angle of attack not a given speed or weight.

JT - you normally don't get catty.

On occasion, technical incompetence of such exquisite proportion requires respectful acknowledgement.

Then answer me this Mr Moderator. No technical talk just A/B/C.

An Aerofoil stalls at a particular:

A) Airspeed
B) Angle of Attack
C) Combination of airspeed and weight depending on the conditions.

GG

By failing to adequately read the posters actual query you have started a completely pointless technical spat!!

The poster referred to a situation where the aerodynamic qualities of an airfoil had been altered to change the critical AoA. He simply wanted to know how to derive the new weight which would result in the same stalling parameters as the unmodified aerofoil!!

No he wanted to know what speed he would stall at for a given weight. I was merely pointing out that airspeed did not matter, he would stall at a particular angle of attack regardless of airspeed or weight.

From the PMs I'm getting many people agree :E

[glekichi]

No he wanted to know what speed he would stall at for a given weight. I was merely pointing out that airspeed did not matter, he would stall at a particular angle of attack regardless of airspeed or weight.

Please GG, stop stirring!

Airspeed does matter in this case, because, despite the fact we are all aware that the aerofoil will stall at a particular angle regardless of weight, the original poster wants to know what speed this will occur at in straight and level flight.

There is no need to expressly state that we are talking about straight and level flight because that is implied whenever the term 'stalling speed' is used.

[FGD135]

No he wanted to know what speed he would stall at for a given weight.
So what did you do? You posted an "answer" that had nothing to do with the question.

His question was perfectly valid and was well answered by other posters.

You're a clown GG.

[bluesky300]

I might spend too much time upside down, but straight and level flight is certainly NOT implied whenever I talk about stalling speed!

[Brian Abraham]

glekichi, thanks for your injection of reality and sanity.

Have to agree with the others who imply that GG is about as helpful as a hip pocket in a singlet. Have look at how the people who certified most of the aircraft which we fly have to say on the subject.

V S means the stalling speed or the minimum steady flight speed at which the airplane is controllable.

V S0 means the stalling speed or the minimum steady flight speed in the landing configuration.

V S1 means the stalling speed or the minimum steady flight speed obtained in a specific configuration.

V SR means reference stall speed.

V SRO means reference stall speed in the landing configuration.

V SR1 means reference stall speed in a specific configuration.

V SW means speed at which onset of natural or artificial stall warning occurs.

Of course, the FAA never had GG as an instructor, so they may well be remiss in their definitions. Don't see any mention of critical angle, and how do you measure that in your aircraft? Only ones I have experience on that had AoA all flew off carriers, as in navy floating type.
From the PMs I'm getting many people agree How many? We need to know how many prats can dance on the point of a pin. :p

bluesky, yours maybe V SR1. :p Defer to JT.

[rmcdonal]

You have lost me here John. I’m not sure what I have said that is incorrect. Aircraft stall at the critical angle, Vs is just a measurement of that angle based on a set of parameters (S+L, WT, ISA etc).
Hence the reason some aircraft have angle of attack gauges. However as most GA aircraft are not fitted with this kit they use Vs as a guide as to when the aircraft is about to stall.

Yes I know that is not what the topic question was about, however I don’t enjoy being told I’m incorrect when every manual on the topic I hold shows otherwise.

It is possible to stall at any speed, it is the Angle of Attack that determines when it happens.

[GADRIVR]

This is even more fun than sitting at Schoies on a Friday evening and listening to the usual suspects go at it!!!!!:E

[The Green Goblin]

Let's say you had a current MTOW of 1,000 kg's with a stall speed of 50 knots. Easy round figures to start with.

You somehow lower the stall speed of the aircraft, eg. wing extension, wing-tips, vortex generators, flap modification etc. As a result you have lowered the stall speed by 5 knots, to 45 knots.

I then need to work out what the weight needs to be to stall the aircraft back at 50 knots.

Quote:
No he wanted to know what speed he would stall at for a given weight.
So what did you do? You posted an "answer" that had nothing to do with the question.

His question was perfectly valid and was well answered by other posters.

My answer was a factual correction. I was pointing out that regardless it will stall at a particular angle, and I will point out I am correct.

You're a clown GG.

Now now, don't back yourself into a corner and result to personal insults. A clown I may be, but at least I don't take life too seriously. Perhaps you need to go to clown school too :p

Have to agree with the others who imply that GG is about as helpful as a hip pocket in a singlet. Have look at how the people who certified most of the aircraft which we fly have to say on the subject.
Of course, the FAA never had GG as an instructor, so they may well be remiss in their definitions. Don't see any mention of critical angle, and how do you measure that in your aircraft? Only ones I have experience on that had AoA all flew off carriers, as in navy floating type.

The aircraft I fly has an instrument to measure the AoA and it is definitely not very suitable for carrier operations. It would need the whole pacific to get airborne. :ok:

So am I wrong Mr Abraham?

Regardless of if an aircraft has or has not got an instrument to measure the AoA, it is still the determining factor in a stalled aerofoil. If you understand the underlying principles then when you approach your minimum steady in flight speed for your particular configuration you are armed with the knowledge of what you can and cannot do in the event of a mishap.

Anyway this topic is starting to become laborious.

[john_tullamarine]

You have lost me here John

I've reread the earlier posts and acknowledge that I have lumped you (somewhat unfairly) in with The Green Goblin's unhelpful chamaeleon posting pattern.

My humble apologies for being a little heavy handed in my commentary. Your observations are, indeed, typical of pilot level training in the topic. I was a little concerned with, and probably misinterpreted, your apparent emphasis on a simplistic explanation of the thread subject and overreacted by including you in my earlier post.

Every now and again even the most relaxed of us can get just a tad irritated and frustrated with difficult and gratuitously argumentative folk such as The Green Goblin. This was particularly so as I am aware of the original poster's circumstances from a side email discussion with him and the stalling angle distraction was not overly pertinent or helpful to what he was after ...

As a side comment, the usual emphasis on stalling angle is a bit simplistic as the inferred value is associated with a particular set of constraints which may not be applicable in a set of real world circumstances. Likewise, Vs involves a few more considerations than simply being an indication of a specific stalling angle.

Pass, friend.

[FGD135]

The Green Goblin, about your initial post to this thread you said:


My answer was a factual correction.


What exactly were you correcting? Do you believe posters were somehow making suggestions contrary to what you initially posted?

[M14_P]

bluesky300, you took the words right out of my mouth and GG, beers, my shout.

[Zoomy]

This is awesome,

To throw yet another spanner in the works, why does the centre of pressure move aft after the critical angle is reached?

I agree with all the posts that regard stalling nothing else but exceeding the critical angle by the way. Speed is simply a tool by which we can recognise the onset of the critical angle and it will vary with varying other mentioned factors ie weight/load factor.

Bloody awesome thread mate.:ok:

[djpil]

This report illustrates three type of stall with chordwise pressure distributions provided.
- trailing edge stall
- leading edge stall
- thin aerofoil stall
NACA Tech Note 2502 (http://naca.central.cranfield.ac.uk/report.php?NID=4466).
".. it is believed that they illustrate the stalling characteristics of most practical airfoil section."

[hueyshuffle]

Only 'somewhat' related. But if those wing mods mentioned in the original question change the aspect ratio of the wing then the downwash will also be modified. Thus changing the geometric angle of attack for a given effective angle of attack. The Effective Angle of Attack, however, is what determines when the wing stalls (stall occurs at AoA > critical angle (effective)).

The thread is off topic already so i figured i'd help with the diversification.

And a question.....i've recently done my instructors rating and we got taught Bernoulli's Theorem etc..yet the examiner had a chat with me about coander and downwash and i've also heard of circulation...Can anyone with some experience please inform me of what the dominant theory of lift production is? If someone asked you what provided the upward force, what would you tell them?

[john_tullamarine]

If someone asked you what provided the upward force, what would you tell them?

The wing cleverly throws a lot of air down and, as a result, wants to go up .. something Newton said a long time ago, as I recall ?

The effectiveness of this process (ie lift is desirable but drag is a bore) is a measure of the cleverness of the designer and the then present literature state of the art.

When you come to describe what's going on it's no different to any typical technical thing .. you can have the simpler or the more complicated explanation according to your needs and interests.

For the typical pilot a combination of the reaction and Bernoulli seems to be reasonably adequate ? One needs to have a passing bit of an idea about pressure distributions because pressure is the way a fluid transfers a force to a solid.

[triton140]

The wing cleverly throws a lot of air down and, as a result, wants to go up ...

Such a simple explanation!

I'ts always puzzled me why we always talk about the prop producing thrust by pushing the air back (Bernoulli never gets a mention) but when we get to the wing we tie ourselves up in knots with Bernoulli, circulation, coanda and other exotica.

Are they not both aerofoils? And does not the wing, like it's smaller rotating cousin, push lots of air down?

[john_tullamarine]

I'ts always puzzled me why we always talk about the prop producing thrust by pushing the air back

The considerations and explanations relate to one's vantage point.

If that be

(a) a substantial distance, then the "gross" effects are obvious - air gets thrown/pushed/shoved/whatever this way or that and there is a reaction (which is the thing we are after). This viewpoint is simple and appeals to me as a pilot as it gives me most of what I need to know to point an aeroplane in a desired direction etc ...

.. ranging through to ... being

(b) right up, close and personal, where we find ourselves looking at what is happening at/near the lifting surfaces which are actually performing the magic things. Then the mathematicians come into their own ... This viewpoint appeals to me as an engineer as I can use the mathematics to predict loads, etc., which provide useful information for doing engineering stuff.

but when we get to the wing we tie ourselves up in knots with Bernoulli, circulation, coanda and other exotica

knots - that was a pun, right ?

Bernoulli - useful for getting a feel about fluid flow and pressures - good pilot stuff

circulation - engineer stuff and not of much interest to the pilot except for handwaving whilst talking about trailing vortices at the bar on Saturday night in the presence of adoring neophytes

(Henri) Coanda useful for handwaving explanations of a number of attractive party tricks at the bar on Saturday night in the presence of adoring advanced neophytes (but not of much interest to the pilot).

[glekichi]

I have only ever heard props described in the correct manner; as an aerofoil with lift and torque vectors , etc.
(Unless we're talking about a TIF, in which case the lift of the main aerofoil tends to get explained in rather simple terms too.)

[gutso-blundo]

Hmm... Seems like a role-play is on order:


XXX - At what speed can I legally travel down the freeway?

Others - 110km/h

GG - No no no, that's just what the car speedo is saying. What you need to do is take into consideration the rotational velocity of the engine's crank shaft, then multiply that by the respective gear ratio (which is dependant on many things, such as which gear is selected, and if it's an auto box a factor for slippage must also be applied), then multiplied by the diff ratio. Finally you must calculate the car's tyre radius (which of course will vary depending on tyre pressure, load on the tyre, tyre temperature and tread level) and multiply that by the rotational speed of the axle, thus giving you a speed roughly equivalent to the calibrated speed shown on the speedo. The two are unrelated.

Others - :{:ugh:

Both technically correct, only one answers the question that was asked...

[toolowtoofast]

What if the auto had a lock-up converter? Wouldn't it be just like a manual gearbox then?

[gutso-blundo]

Ahh, touché :ok:

[Atlas Shrugged]

Where's Planky when you need him?? :E