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Factors Affecting Stall Speed

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Factors Affecting Stall Speed

Old 3rd Mar 2002, 06:30
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Question 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" />
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Old 3rd Mar 2002, 07:00
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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
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Old 3rd Mar 2002, 07:18
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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>
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Old 3rd Mar 2002, 07:32
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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.
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Old 3rd Mar 2002, 08:09
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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)
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Old 3rd Mar 2002, 08:32
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Thanks for the replies and references. Fly safe.
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Old 3rd Mar 2002, 10:48
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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" />
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Old 3rd Mar 2002, 13:02
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Flight at either zero G or in the vertical - but I ain't paying your price!
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Old 3rd Mar 2002, 20:05
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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.
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Old 3rd Mar 2002, 20:22
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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.
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Old 3rd Mar 2002, 20:36
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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...
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Old 3rd Mar 2002, 20:58
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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). .. .Iím 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.
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Old 3rd Mar 2002, 23:57
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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.
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Old 4th Mar 2002, 01:43
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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" />
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Old 4th Mar 2002, 01:53
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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" />
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Old 4th Mar 2002, 02:07
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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.
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Old 4th Mar 2002, 02:25
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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
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Old 4th Mar 2002, 03:04
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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'.
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Old 4th Mar 2002, 03:55
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I agree with John. There is only such a thing as 'stall speed' in unaccelerated flight. we should be talking about 'stalling alpha' here.
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Old 4th Mar 2002, 04:24
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Angle of Bank <img border="0" title="" alt="[Wink]" src="wink.gif" />
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