Factors Affecting Stall Speed
The indicated stalling speed varies with the square root of the load factor. The stalling speed in either a climb or a descent is lower than in straight and level flight because less lift is required, i.e. a lower load factor.
Hate to quibble checkboard, but no.. .. .In most aeroplanes, the ACTUAL stalling speed varies with the square root of the load factor. The IAS:CAS curve is often non-linear at low speed, so the relationship between INDICATED stalling speed and load factor will be different.. .. .Also some aircraft, contrary to what you read in most textbooks, for reasons of aeroelasticity or flight protection system intervention, will not display a square-law loading to stall speed relationship.. .. .Having said that, your statement would hold true for, say, a PA-28. But it might not for an A320 or a microlight.. .. .Aint life complicated.. .. .G
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Checkerboard:. .. .In a descending flight, where you have a constant descent rate, the lift equals the weight. Only when initiating the descent you have a slightly lower g-load. This applies as well when increasing your descent rate.. .. .The amount of lift required to keep you flying in a descent is equal to the amount in level flight. If you have less lift, you will accelerate towards the earth.. .. .To stop the descent you will increase the g-load momentarily, and there produce some more lift, to acclerate in the opposite direction; after that the equilibrium is restored again.. .. .P77
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vmommo,. .. .Thanks. I got my answer at the beginning of this thread, but I'm enjoying reading all the posts. Thanks everyone.. .. . <img border="0" title="" alt="[Cool]" src="cool.gif" />
Touché Genghis, I was using the KISS principle, given the nature of the original question.. .. .Yes, at large angles of attack there may be a significant pressure error, giving rise to a difference between CAS and IAS, perhaps I should have used CAS (or RAS)! In large modern aircraft, the Air Data System will display CAS in any case. <img border="0" title="" alt="[Wink]" src="wink.gif" /> . .. .Yes, some aircraft have high angle of attack protection systems that change the aircraft configuration by extending slats etc - I thought that was outside the discussion.. .. .Pegasus77 I'm afraid that just isn't so. The lift does not equal the weight in either a steady climb or descent. In a steady descent the lift = weight . CosØ, Where Ø is the angle of descent. Note that in straight and level flight, the angle of descent is zero, and CosØ = 1 (i.e. Lift = Weight in S&L) at any angle of desent, the Lift < Weight.. . . . <small>[ 09 March 2002, 03:57: Message edited by: Checkboard ]</small>
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Ft. .OK, I am being somewhat obtuse here for which I apologise. Assuming one is considering indicated airspeed only and that this is only a measure of forward airspeed, then any condition where the angle of attack approximates to 90 degrees could be said to be a stalled condition at zero IAS. Vertical take-off or landing in an aircraft such as the Harrier, or the fall-through from a tailslide might be two examples. Well , someone did ask the question .... <img border="0" title="" alt="[Smile]" src="smile.gif" /> . .Rgds. .TheAerosCo
</font><blockquote><font size="1" face="Verdana, Arial, Helvetica">quote:</font><hr /><font size="2" face="Verdana, Arial, Helvetica"> The lift does not equal the weight in either a steady climb or descent. In a steady descent the lift = weight . CosŲ, Where Ų is the angle of descent. Note that in straight and level flight, the angle of descent is zero, and CosŲ = 1 (i.e. Lift = Weight in S&L) at any angle of desent, the Lift < Weight. </font><hr /></blockquote><font size="2" face="Verdana, Arial, Helvetica">This is true in theory, Checkboard, but abiding by your previously quoted KISS principle, it's overkill.. .. .A typical descent might be perhaps 5 degrees (500 ft per mile). That makes the load factor 0.996, and thus makes a 0.2% difference to the stalling speed. To all intents and purposes, lift equals weight.. .. .By contrast, the IAS vs CAS issue can account for several knots of difference in airspeed.
The approval codes for light aircraft requires that the ASI is within 5 knots full range, but in my experience many struggle to meet that - especially near the stall.. .. .Many STOL aeroplanes will stall at 45°+ nose-up, and most microlights or smaller light aircraft around 30° nose-up (attitude, not AoA), even without power - so altered AoA are big players in what you actually see on the gauge. With such a high AoA, whatever's happening to the wing, usually the pitot is so far from pointing directly into the airflow, that it drops to something silly (I've seen as low as 8kn) IAS at the stall, when the aircraft is probably doing around 30-50.. .. .As for the KISS principle, I think that probably died somwhere early on page 2. Commendable approach though.. .. .G. . . . <small>[ 09 March 2002, 20:44: Message edited by: Genghis the Engineer ]</small>
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Bookworm.. .. .I don't think that is overkill. Consider that whilst most flight is at relatively shallow gradients, when stalling is intended it is either for primary training or more exotic manoeuvres (aeros or ACM). In the latter cases the flight path is quite possibly more vertical, so any model ought to reflect that.. .. .As an aside, the purist could argue that since straight and level flight is not actually a state of uniform motion in Newtons terms, that L <> W even in level flight.... .. .CPB
Fair enough CPB. .. .I just hear a lot of 'theory' of effects quoted where no one has run the numbers to work out the magnitude of the effect. On many occasions, some effects outweigh others by an order of magnitude or two.. .. .I wouldn't put it past some wannabee to think "OK, I'm in a 45 degree bank turn in a 5 degree descent. Turn raises load factor, descent lowers load factor. So the stall speed will stay about the same." In fact, the stall speed increases by about 40%...
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Something I don't think anyone has mentioned yet is stall speed is also affected by the rate of deceleration. The higher the rate of decleration the lower the stall speed......or is it the other way round.
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While we all are in daydreaming mode ..... .. .I don't think that anyone has mentioned the stall scenario associated with a very high pitch rate ( 60-70 deg/sec minimum rings a bell in the fog at the back of the mind). The initial stall leads to a vortex above the wing which causes reattachment and permits further alpha increase.. .. .I recall reading an interesting article on this subject in the RAeSJ several years ago. Apparently a problem with helicopters more than mere aeroplanes which have some difficult generating the sorts of pitch rates required.
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NoseWheelFirst,. .. .Higher deceleration rate cause the airplane to stall earlier (higher stall speed) due to the pitch dynamics that disturb the flow over the wing <img border="0" title="" alt="[Smile]" src="smile.gif" /> . .. .---------------------. .An Enginer who knows how planes fly but don't know how to fly a plane <img border="0" title="" alt="[Razz]" src="tongue.gif" />
