Factors Affecting Stall Speed

SentryIP

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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? If you respond, can you please give the reference you used? Thanks in advance.

:cool:
 

Timebuilder

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As far as I know, the only thing that will be affected is your ability to recover after the stall occurs. Remember Jessica DuBrow, the nine year old whose parents had her trying to set a record for the youngest pilot to cross the US? Their tail-heavy Cardinal stalled after takeoff and all aboard perished. If you have an aft CG, it may be impossible to restore airflow because you are unable to get the nose down. Of course, if your nose is high, you can be closer to the critical angle of attack, and therefore closer to the condition for the stall.

Appologies for any inaccuracies, since that story must be a decade old by now...

Now, an aft CG will let you reach a higher cruising speed, but that's off topic.
 
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dmspilot00

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Yes, an airplane with an aft CG will stall at a slow indicated airspeed, and an airplane with a forward CG will stall at a higher speed. I am surprised you and timebuilder don't know this considering how much time your profiles say you have. Pick up any Cessna POH and it gives stall speeds for different CGs (no, thats not my only reference, but the only one I can think of at the moment.)
 

Timebuilder

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You're right, dms. Although there is a small difference in the POH for the 172S, the difference is more pronounced in the 182RG.
I imagine this oversight happened because my flap settings for stalls are either 0 or 30 degrees, and the difference in the 172S occurs at flaps 10.

Nice tidbit. Thanks.
 

banned username 2

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It is true, an airplane with a forward CG will stall at a higher airspeed than an aircraft with an AFT CG...

Now remember an airfoil will stall at its design Angle of Attack regardless of CG, etc.... It is Stall SPEED that can be affected by CG location, not the stall Angle of Attack...

Think of it to extremes..... With a very nose heavy aircraft, the tail needs to create more down moment to hold the nose up for level flight... this down moment is added to the aircraft weight in as the amount of lift the wing needs to create to hold the plane in level flight.... The wing now needs to create more lift for a given airspeed... the only way to do that is to increase the angle of attack.

Take a look at the VERY simplified drawing I made:



As you can see, the forward CG airplane needs more total lift... well if the airspeed is 60 kts on both planes, the only way to get more lift is to increase the angle of attack.... now with the increased angle of attack, you can see that you are now closer to the critical angle of attack than you would be with the aft CG...

I hope this helps... If you have any more questions feel free to send me a Private Message.....

Good Luck!

Falcon Capt.
 
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Timebuilder

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Thanks, Falcon, and a nifty drawing, too!

I knew why this affects the speed of the aircraft, but I had neglected the speed versus AOA aspect. It certainly makes sense that the critical AOA could occur at different speeds as total lift varies. Do you know of any knowlege test questions in the commercial or ATP database having to do with this issue? I don't recall seeing any
 

SentryIP

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Yep, easy to see when I look at the picture. Thanks Capt Falcon. I checked several of my aero texts and none of them mentioned it. Thanks for the replies.
 
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Cornelius

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What happens to stall speed when you have a full load of passengers that had chilie for lunch and are fealing the onset of gastrointestinal gaseous release? In a pressurized aircraft at half your max service ceiling, do you stall at a slower or higher airspeed?

Hmmmm.
 

jaybird

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"What happens to stall speed when you have a full load of passengers that had chilie for lunch and are fealing the onset of gastrointestinal gaseous release? In a pressurized aircraft at half your max service ceiling, do you stall at a slower or higher airspeed?"

Cornelius,

I think we covered that one in the chicken thread.

http://forums.flightinfo.com/showthread.php?s=&threadid=1033&highlight=chicken
 

ksu_aviator

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QUOTE]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. [/QUOTE]


I'm suprised noone addressed this, but I'm sure that the stall speed doesn't change with altitude. I have all my books packed away, but if you look in the Trevor Thom Private and Commercial book it will say the same thing. It will also be addressed in most multi-engine books.

The TAS of the stall will change with altitude but not the IAS or CAS. Is that what you ment? Or did you mean out of ground effect?
 

SentryIP

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Correct

ksu_aviator,

You're correct. For practical purposes, the stall IAS remains the same at all altitudes. The IAS varies slightly due to airflow changes in airplane attitude, configuration and proximity to the ground (ground effect). This variation is called position error and each airplane flight manual has correction charts to obtain CAS. Thanks for keeping everyone straight on this. Fly safe.

:cool:
 
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