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FlyingForFun
6th Apr 2002, 13:53
Hi,

Could someone please explain to me the relationship between the centre of gravity, and the stalling characteristics of an aeroplane?

The Bristol groundschool notes explaination is limited to: "If the C of G is aft of the safe range ..... there is a tendancy to stall because the tailplane moment is reduced." What does the tailplane moment have to do with stalling - I thought stalling is related to the wings, not of the tailplane? Then I thought maybe the change of moment would make the elevators more responsive, meaning it would take less back pressure to produce a stall - but surely this would require an increased moment (such as with a forward centre of gravity), and not a decreased moment?

Then, to add to the confusion, there's a question in one of the progress tests on the effects of moving the centre of gravity forward, including whether the stall speed would be increased or decreased. But the notes don't actually discuss the stall speed, only the tendency to stall. Am I right in assuming that the "tendency to stal" with a rear C of G would mean an increased stall speed? And therefore a forward C of G would result in a lower stall speed? Or is this too simple?

Any help would be appreciated,

Thanks,

FFF
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mstram
6th Apr 2002, 14:07
http://www.pprune.org/forums/showthread.php?s=&threadid=10848

Stan Evil
6th Apr 2002, 14:16
If we're just talking stalling speed then an aft cg reduces stall speed. In level flight, approaching the stall in this case, the moment produced by the wing lift about the cg must be balanced by the up or down force from the tail. If the cg is behind the wing centre of pressure then the tailplane must always be producing an up force to balance it. If the cg is ahead of the wing centre of pressure then the tailplane must produce a down force. Now, in level flight lift (from the tailplane and wing combined) equals weight. If the tailplane is producing a down force then wing lift must be greater than weight and vice versa. If the wing needs to produce more lift at a given speed then its aoa must be greater. The wing stalls at a particular aoa so, with a forward cg this occurs at a higher speed than with an aft cg.


As far as the increased tendency to stall is concerned, an aft cg tends to reduce stick force per g and the aircraft becomes less stable. So careless use of the stick may produce a g-stall. On some aircraft (like the Vulcan) fuel was moved forward to give stability and low trim drag for the cruise or back to give increased pitch control for the landing - occasionally ending up off the back end somewhere when the co-pilot forgot to stop transferring backwards. On one occasion the crew could not vacate the aircraft on the ramp until it was refuelled for fear of it tipping onto its tail!;)

BEagle
6th Apr 2002, 16:05
So after landing some Vulcan pilots would always select the 1-to-7 transfers forward to make sure that the C of G was well forward!

An aft C of G will probably make stalling characteristics more dramatic due to reduced manouevre stability from the lower tail volume. I had some fun demonstrating this effect to a group of pilots a while back in a T67A. This is only allowed to aerobat at quite low weight - and the fuel tank is in front of the cockpit. So with a chubby crew, the maximum fuel load allowed was very low - we'd weigh the crew, check the fuel level, calculate how many more litres it could take and then add fuel very carefully. This had the effect of moving the C of G aft, fairly obviously, so that stalling in the turn was noticeably more playful and it would flick very readily if provoked. But with a lean racing snake in the other seat, the fuel load could be greater, the C of G further forward and it was much more docile!

John Farley
6th Apr 2002, 16:24
FFF

Re your first query, the tailplane is responsible for producing longitudinal stability. If you give it a smaller arm by moving the CG aft it is less efficient and so a slightly smaller elevator input will be needed to reach the stall (or, if you prefer, pilots have to overcome stability when controlling an aircraft – less stability means less control needed to achieve any manoeuvre – in this case a stall)

As to your second query, moving the CG forward makes an aeroplane more stable, so more downforce is needed at the tail (to stop the nose going down) this “downwards lift” makes the aircraft appear a bit heavier to the wings and so the stalling speed goes up a tad.

Dick Whittingham
6th Apr 2002, 19:21
Our P of F notes, pp 9.10 and 12.9/10 cover this, although I don't specifically mention a possible inadvertent stall, just the twitchy handling and a possibility of overstress.

See you at Phase 2,

Regards, Dick W.

FlyingForFun
7th Apr 2002, 07:59
Thanks for the excellent replies, everyone - and apologies for the duplicate topic, I obviously didn't spend long enough searching before posting.

Some very helpful answers, particularly from Stan, and also from Polzin on the previous thread. I'll definitely be printing these out and inserting them in the relevant place in the notes!

Dick, I'm still on Module 1 (this is for Mass and Balance) so I don't have the P of F notes yet. But look forward to meeting you before too long!

FFF
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