Factors Affecting Stall Speed
 

Hello All,. .. .I've been told that the center-of-gravity location of an airplane affects the stall speed. This is news to me. I'm talking about a conventional airplane with positive static stability and not a fly-by-wire airplane with relaxed static stability. I know that the stall speeds vary due to altitude, gross weight, configuration and bank angle and the only way an airplane can stall is by exceeding the CLmax angle-of-attack. This person is telling me that an airplane with a center-of-gravity near the aft limit will stall at a slower airspeed than an airplane with the center-of-gravity near the forward limit. Does this person know something I don't know? I've checked my aerodynamics texts from college and none of them have any information on this. If you respond, can you please give the reference you used? Thanks in advance.. .. . <img border="0" title="" alt="[Cool]" src="cool.gif" />

Only a humble PPLer, but it's what i've been taught,. .. .From the Ground Up, Millenium edition p.36. .. .Trevor Thom, Flying Training, p. 171. .. .Transport Canada, Filght training manual, p. 76.. .. .152

I ain't very technical and I'm sure you will get some comprehensive replys on this. But here is a VERY simplistic explanation. There is a down force on the horizontal/ elevator which holds the nose up. If an aircraft weighs 1000 lbs. and the down force is 20 lbs., the aircraft thinks it weighs 1,020 lbs.when airborne. If that same a/c weighs 1,000 lbs but has a 100 lb. downforce then the aircraft thinks it weighs 1,100 lbs. The reason there would be higher down force would be if the CG was forward and it needed more down force to hold the nose up. The heavier aircraft the higher the stall speed. Aircraft with computers actually figure their CG and will fly at a speed with this calculation, such as a L1011. The stall no matter what the weight will always happen at the same angle of attack. . .Now I sincerely hope that someone who knows what they are talking about will set me straight. . .. . <img border="0" title="" alt="[Big Grin]" src="biggrin.gif" /> . .Ps. A rear CG loaded a/c will get better fuel economy than a forward loaded CG. Why? Would a 1,020 or 1,100 get better fuel economy? Which weight would require a higher angle of attack? And the higher angle of attack will give more drag and less fuel economy. Might not should like much of a difference but an airline operating 1,000 a/c will save hundreds of thousands of $ loading their a/c with a rearward CG. . .Why do i have that feeling I am about to learn something? <img border="0" title="" alt="[Smile]" src="smile.gif" />. . . . <small>[ 03 March 2002, 03:50: Message edited by: polzin ]</small>

For what it's worth I think 'you' are correct. As far as I am aware the main factors affected by the CofG are elevator authority, stability and fuel consumption. I too would be interested on hearing more on this.

This one comes up from time to time .... Polzin has the drift here I suggest.. part of the desirable upwards lift is used to offset the balancing tail downforce ... and it is for this reason that the AFM stall speeds are normally critical (ie maximum) and specified for the forward CG limit. . .. .I wouldn't worry about referring to the pilot training books which are more geared to giving a simplistic story .. try any of the standard undergraduate aerodynamics texts to get the tale from first principles .. if you fancy a little bit of maths. One text which is reasonably readable is Aircraft Performance, Cambridge Aerospace Series, Mair and Birdsall.. .. .It is relatively simple to show, using the principles of dimensional analysis that lift and drag are functions of incidence, Reynold's Number, and Mach Number. . .. .1G stall incidence (angle of attack ...whatever term turns you on) relates to CLmax which depends on ... .. .the normal things that all pilots know about .... .. .(a) total lift = weight (although the load factor reduces a little below 1.0 during the stall process). .(b) air density (an altitude consideration). This also is relevant when looking at ASI calibrations.. .(c) airspeed. .(d) a representative area, usually the wing. .. .plus the ones which tend to get lost in normal usage ... .. .(e) Reynold's Number (a fluid viscosity consideration). .(f) Mach Number (a compressiblity and shock flow separation consideration)

Thanks for the replies and references. Fly safe.

One of the things we had to do during my flight instructor training,(23 years ago) was to write a list with 10 factors affecting the stall speed of an airplane.. .. .Don't remember half of the list now, but perhaps this is a good place to review the list.. .. .A few factors are listed above:. .. .1) Weight. .2) C.G.. .3) G loading . .. .Also. .. .4) Contamination of wing (Ice). .5) Power. (On or off). .. .Etc.. .5 more guys?. .. .Question: When is the stall speed zero?. .(In the air, not on the ground). .First correct answer wins a price. <img border="0" title="" alt="[Wink]" src="wink.gif" />

Flight at either zero G or in the vertical - but I ain't paying your price!

Correct answer Checkboard.. .Ya won a free hiking trip in the Norwegian mountains.. .Bring warm clothes. <img border="0" title="" alt="[Big Grin]" src="biggrin.gif" /> . .. .Come on dudes, 5 more factors that affects indicated stall speed.

I disagree that the wing can't be stalled in vertical flight. Sufficient pitch/'g' to exceed the critical AoA will do it, irrespective of the flight path. . .. .Think of a snap roll on a vertical line.

Well, lets see.. .Vertical flight as in the vertical part of a loop, yes, ya are loading the wing and it could be stalled.. .But stright up vertical flight, such as hanging from the prop, the load is on the engine/prop, not the wing. If ya slow to zero and just hang there, the wing could not care less and yer stall speed is not there, well....in theory anyway.. .. .Straight down as well. 0 AOA, no load on the wing, no separation resulting in stall...

Hi all. .. .My engine failed this morning after I landed on the sand at low tide. So was I unlucky that the engine failed or lucky that it happened when it did? (OK it was only a toy aeroplane before you go looking at CNN). .. .I’m having a funny day so please excuse this RANT. .. .Wings only stall because of excessive AOA which has nothing to do with speed. The factors that can affect stall speed are almost endless and all have to be in human pilots minds. I doubt 10 would scratch the surface given the humans I know.. .. .All autopilots avoid the stall by using AOA not speed . .. .Even stall warning systems do not use bl**dy speed to warn humans about the stall. . . . .And nor do stick pushers have any connection with speed.. .. .Is it not time we humans got with it?. .. .Sorry.

Easy John.. .. .We are talking about the indicated airspeed where the stall occurs.. .. .Of course the aircraft stall because the critical AOA is exceeded. Duh.. .. .Not many light aircraft come with an AOA indicator.. .Therefore we are talking about stall speeds instead.. .. .Since ya are so smart, why don't ya finish the list.. .Should be a piece of cake for ya. . .Only 5 more to go.

I think we have missed two of the main factors:. .6-configuration (flaps, slats, spoilers, landing gear). .7-density altitud (if we are talking on indicated airspeed). .Another one could be altitud: at higher altitudes Vstall increases due to compresibility factors.. .Good flight everybody. <img border="0" title="" alt="[Smile]" src="smile.gif" />

ILS:. .. .Yup, good point on configuration.. .. .But, uh density altitude does not change the stall speed indicated??. .. .True airspeed yes. . .Say yer Cessna 172 usually stalls at 65 knots indicated at max weight, straigt and level fligth flaps up. At 10,000 feet density altitude, it still stalls at 65 knots INDICATED.. .. .As for compresebiltity, well perhaps on a fast jet, but the 172 hardly runs into such.. .. .(I know, we did not specify A/C type or other specifics, the list was just a general, get yer mind thinking kind of excerize). .. .I am however not the judge in what goes on the list or not, if others agree on density altitude, then parhaps I was wrong...? <img border="0" title="" alt="[Big Grin]" src="biggrin.gif" />

Don't think they have been mentioned yet but here are a couple of obvious ones. . .. .Power On or Power Off. Especially with a prop aircraft but also true in a jet.. .. .Unbalanced Flight...... Can lead to even more interesting manoeuvres.

Towerdog. .. .I only wish I was smart then I would not get sucked in!. .. .Leading edge devices that move. .Trailing edge devices that move. .Wing plan form. .Wing aerofoil shape. .Altitude . .. .And if you want to use IAS don't forget the airspeed indicator calibration and where the static ports are and whether you have any slip and if your aircraft has a prop the swirl (not just power used) the type of prop (even Lockheed get stuffed by that) and so on and so on.. .. .I'm sorry, but my main beef is that if we are to make any sense of the expression "stalling speed" we must first write a great list of circumstances describing exactly what is going on for the speed we are quoting to apply. . .. .Used as a guide to keep inexperienced guys safe on finals (in simple aeroplanes that don't change weight much)it maybe fine For any other use it is just not a good enough concept. . .. .And we have not even mentioned all the various things/circumstances/events that different people define as "the stall" for which they then quote the "speed" . .. .When BAe and McDonnell Douglas (as were) finish up paying lawyers to decide what the stall speed is then I reckon the notion has passed its sell by date.. .. .BTW thank you for telling me to take it easy. Without out that sound advice I would probably have got wound up.. .. .You have a good day. .. .(I just have to believe that my tomorrow will be better). .. .Regards

First, and with respect to Towerdog, in neither case is the 'stall speed' zero. Why? In neither case is the wing 'flying' (other than in the legal sense), and as the wing is neither lifting nor altering the trajectory of the aircraft, it is neither stalled nor unstalled (other than inasmuch as there may or may not be some, more, or less, streamline flow - but then, what if there is streamline flow, but it's reversed, as in a tail-slide...?). So, stall speed is not zero because there can be no stall speed in the circumstances.. .. .Second (and with much respect to John Farley) the statement that 'All autopilots avoid the stall by using AOA not speed' is untrue. Autopilots do not concern themselves with flight close to the stall. They provide the human operator with a means of programming the aircraft's flight path, and rely upon the operator not programming an unsafe path which would lead to a stall.. .. .The closest we can get to an autopilot-induced stall is a V/S climb where the commanded rate exceeds the aircraft's capability, eg +9900fpm in, say, a medium jet at medium level. The autothrottles will drive to full thrust to attempt to hold the speed, the elevators will cause the nose to pitch up in an attempt to gain the RoC, but the AFCS will revert to LVL CHG and the aircraft will get no-where near the stall. If it did approach the stall, or if you did the same thing in a turboprop, for example, the autopilot would be thrown out by the stick-shaker or stick-pusher.. .. .In a modern 'bus, of course, things are a little different, as the aircraft will protect itself with reference to AoA (I'm talking of the stick-held-fully-back scenario), but here it is not the 'autopilot' which is doing this, rather there is an envelope protection system at play which happens to be part of the automatic flight controls - or, as there are no 'manual flight controls', I should say that it's simply part of the 'flight controls'.

I agree with John. There is only such a thing as 'stall speed' in unaccelerated flight. we should be talking about 'stalling alpha' here.

Angle of Bank <img border="0" title="" alt="[Wink]" src="wink.gif" />

Another point. NorthernSky, I think u misinterpreted something about autopilots here. If I'm not mistaken John wasn't talking about autopilots avoiding the stall, he was talking about the autopilots fly the aircraft. Autopilots trim for alpha, and that's a fact. Trimming for speed in a heavy airliner with a constantly changing CoG and possible Mach tuck due to center of pressure shifts above 0.615M (in a 737) would present serious problem as weight shifted etc. Trimming for alpha means that the autopilot is 'thinking ahead' of the aircraft, relying on actual performance rather than what's indicated on the speed tape which as John mentioned can have built in or pitot and static position errors leading to an incorrect reading, whereas an alpha reading would negate the need for an airspeed indicator at all! Most V speeds are alpha related... Vs0, Vs1, Vno, Vx, Vy, the list goes on. But it's safe to say that 'stalling speed' is a VERY dangerous choice of words to teach a student. Get a student to pull 3G's at 90 knots in a Cessna 150 and ask him to explain the flick roll and spin that follows if he's been taught about 'stalling speed'!

A bit pedantic, this discussion, I think. It's pretty obvious that we are talking about changes in the observed IAS during a stall, with Vs as a reference.. .. .Some factors not yet mentioned:. .. .</font><ol type="1">[*]<font size="2" face="Verdana, Arial, Helvetica">Ground effect</font></li>[*]<font size="2" face="Verdana, Arial, Helvetica">Formation flight</font></li>[*]<font size="2" face="Verdana, Arial, Helvetica">Damage to the wing</font></li>[*]<font size="2" face="Verdana, Arial, Helvetica">Compressibility</font></li>[*]<font size="2" face="Verdana, Arial, Helvetica">Climb/Descent angle</font></li>[*]<font size="2" face="Verdana, Arial, Helvetica">Boundary layer control devices ("blown" wings and vortex generators)</font></li>[*]<font size="2" face="Verdana, Arial, Helvetica">Vectored thrust</font></li>[*]<font size="2" face="Verdana, Arial, Helvetica">Gusts</font></li>[/list=a]<font size="2" face="Verdana, Arial, Helvetica">

Put simply stall speed is determined only by wing loading and the maximum lift coefficient. . .. .However, there are multiple ways to vary those two values, most of which have been discussed above.

SentryIP, back to you intial question. Published AFM stall speeds are for forward CG, which are worst case senario, in other words highest value (fastest). . .. .Also, add air density to my above post.

Stalling ALPHA?. .. .We are not talking Airbus logic here.. .. .Hey, slow down guys.. .I was asking what time it is and some of ya studs are building the clock.. .. .Checkerboard:. .. .Yer Number 4 was indeed mentioned above... .. .As for ground effect, yup, that would do it.. .. .Formation flight and damage to the wing?. .Well. Both would change the airflow and so on.... .So would a mid-air collision or a drunk pilot hanging out of the side window and on and on... .Uh slow down here guys.... .. . <img border="0" title="" alt="[Big Grin]" src="biggrin.gif" />

I don't think anybody's actually answered the question, which was is Vs affected by CG. The answer is yes.. .. .The aerodynamic stall, where flown breaks down completely over the wings is rarely reached by any aeroplane. This is because the "piloting" stall occurrs when, whilst still nibbling the aerodynamic stall, you run out of nose-up control authority. This is a function of CG.. .. .So, with further forward CG, you have less ability to pitch up, less ability to enter the aerodynamically stalled regime - and a higher perceived stalling speed. This pitch limited stall is referred to by a lot of pilots as "mush".. .. .G

ATPLwannabeboy,. .. .I fly the 737 at present, and I assure you that the autopilot does not trim for mach tuck. This is achieved by the Mach Trim system which then commands re-datuming of the Elevator Feel and Centering Unit to move the elevators. As it happens the system is armed above 0.615M, but the aircraft can be flown to (3-4-5) 0.74M (NG) 280kts/0.82M with the system inoperative, without trouble.. .. .I can assure you that the 737 autopilot system does not use AoA information.

Right, anyone else agree that the 737 autopilot DOESN'T trim for alpha? (Anyway CoP change causes an alpha reduction resulting in a nose-down moment) If it doesn't it's one strange and clunky system. However if no-one else contributes we'll just have to take Northern's word for it. Any 737 drivers care to comment? TowerDog, stalling alpha (or if u want to call it AoA) is what the 'stalling speed' is measured on, whether it's a 73clunk or an A320... we all know that <img border="0" title="" alt="[Wink]" src="wink.gif" />

ATPLwannabeboy!. .. .I´m afraid I have to agree with Northernsky. . .As he I also fly the 737 (4/5/700) and unless they (Boeing) have left it out of the books he is right!

Thanks for clearing that up, HC and grudging apologies to Northern. Any other airliners that don't trim for alpha (and I'm not talking about what it APPEARS to be trimming for)? I'm still convinced that MOST autopilots trim for alpha, and that alpha is what we should be talking about in terms of a stall.

ATPLwannabeboy,. .. .In answer to your latest question, I will go so far as to say that airliner FCSs do not reference AoA, with the exception of some oddities, and modern 'busses - A320, 330, 340 families, which use it for envelope protection. The oddities I mention would, I think, include the L1011 - though never having flown one, I'm going on what people have said about some of its modes, which I guess must have been referenced to AoA (DLC, perhaps?). Any L1011 drivers care to comment?. .. .Or to put it more simply, you may quote me on this: 'In normal flight, airliner autopilots pay no heed to AoA'.. .. .BTW, I'm not having a go at you, wannabe, or anyone else. It's important that people who fly these aircraft know what their machines are doing, and why. Too often, lack of this understanding leads to mishap, especially with regard to AFCS. I'm just trying to get the true facts across, as part of this lively and enjoyable debate.. . . . <small>[ 04 March 2002, 18:03: Message edited by: NorthernSky ]</small>

NorthernSky. .. .Thanks for putting everybody right about my duff gen. Good job.. .. .What I should have said was "All auotpilots and Flight Control Systems (that include a stall protection capability in their spec) use AoA to avoid the stall" . .. .Sometimes in an attempt at brevity it is all too easy to finish up misleading others, which is an extremely undesirable thing in aviation.. .. .Regards

An interesting thing about CG location is the difference in stall characteristic between an aft and a forward CG.. .When CG is forward the stall is very "honest" eg. plenty of pre-stall warning( Buffet). With an aft CG my expierence is that the thing barely gives you any buffet and the stall is more violent.. .. .And now to the Techies: How does this affect the way a modern "computerized" airplane presents it protections vs stall? Is the margin to stall the same in both cases, or could you expect to experience buffet at max AOA on a forward CG aircraft.( eg. Valfa max on an Airbus)?

Excuse thread creep but calling JOHN FARLEY - I cannot email you via the icon - can you email me with an email address please? Breathe is 'bouncing'.. .Mike Phillips

NorthernSky has explained why the stall speed, if we must call it that, is not zero in the vertical. However, to be pedantic I can think of a couple of different situations where it could be said that the stall (i.e exceedance of the critical angle of attack)can occur at zero indicated airspeed. Any thoughts? To give a clue, I expect John Farley will get one of them pretty quick.

8?. .. .Angle of Bank

TheAerosCo,. .I'm curious as to how you would you define angle of attack at zero airspeed?. .. .Cheers,. . /ft

NorthernSky . .. .“If it did approach the stall, or if you did the same thing in a turboprop, for example, the Autopilot would be thrown out by the stick-shaker or stick-pusher”.. .. .Not everyone fly's the fancy stuff.... .In a bog standard turboprop with standard Autopilot experiencing icing conditions. Operating in ALT mode the A/P whilst trying to keep altitude slows the A/C down. With the stall warner being ices up by that time. What will happen? It’s a killer situation, which has cost lives. No automatic devices around....other then a shaken pilot.. .. .Love me feed me

John Farley,. .. .Thanks for your kind remarks. This is a great way to debate and discuss, isn't it?!. .. .Lima, you make a valid point (and one which I understand, especially having flown the ATR - I thought it was a wonderful aircraft, btw). However, you have to realise that (a) the authorities set a standard, then, (b) a manufacturer builds an aircraft which meets the standard, then (c) people go flying in it. Multiple failures are not accounted for in these standards, and it is 'assumed' that systems will usually work as advertised.. .. .OK, bad things have happened in turboprops, but this does not mean that every turboprop which flies into icing will suffer the fate which you propose. I have some great photographs of the 'Ice detector' on the ATR, covered with sparkly white stuff (about two inches of it), to prove my mettle and my belief in the systems which were protecting me at the time. However, I was also 'ice aware', and would not have climbed the aircraft in V/S in those circumstances. Knowledge may not be power, but it certainly helps awareness.

I have seen angle of bank mentioned twice now as influencing the stall speed: this is not true. The higher g-load in a turn causes the stall speed to be higher, not the bank angle. . .. .Also mentioned was climb or descent angle. These are not influencing the stall speed. It is the AoA, where it really doesn't matter if you are climbing or descending.. .. .P77