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B737 moveable horizontal stabilizer aerodynamics

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B737 moveable horizontal stabilizer aerodynamics

Old 21st Mar 2020, 23:41
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B737 moveable horizontal stabilizer aerodynamics

Hi folks, having a senior moment here about the moveable horizontal stabilizer.
as far as i recall the CofP is located about 25% from the leading edge of the MAC, and behind the CofG, its position is affected by a number of factors such as the payload moving around, fuel being transferred, airplane configuration being changed - high lift devices, landing gear, thurst settings - different speeds - transonic/mach tuck-: anyways the overall location of the CofP behind the CofG causes the airplane to be nose heavy which must be compensated for by the tail down force from the horizontal stabilizer. Now the question is how does exactly that work from an aerodynamic point of view? Moving the horizontal stabilizer to a higher angle of incidence then its leading edge up will result in a decrease in the tail down force due to the fact that the camber of the stabilizer is facing downward, ultimately inducing a pitch down moment as the airplane pivots about its CofG; while on the other hand as the angle of incidence is decreased the taildown force increases inducing a pitch up moment compensating for the natural nose heavy design of the airplane, giving it stability - that would explain the larger excursion or travel of the horizontal stabilizer downward as opposed to up, or maybe i have it the other way around! I am a bit confused... also when on the trim scale the index shows 0, is that actually 4 degrees up due to the rigging of the stabilizer?
many thanks for your willingness and replies.
barrichello72 is offline  
Old 22nd Mar 2020, 00:02
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Look here
I'm just a bit lazy...
gearlever is online now  
Old 22nd Mar 2020, 07:47
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Originally Posted by barrichello72 View Post
also when on the trim scale the index shows 0, is that actually 4 degrees up due to the rigging of the stabilizer?
That's nothing to do with the rigging, it's just that the datum for the scale shown on the trim wheel is shifted so that all values are shown as a +ve number.
DaveReidUK is offline  
Old 22nd Mar 2020, 08:44
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Do you really understand how your trim works

https://www.skybrary.aero/bookshelf/books/2627.pdf
alf5071h is offline  
Old 23rd Mar 2020, 08:59
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Stab always has downward lift!

Nose heavy aircraft in trim , stab leading edge down, with trim ND ( !!) , gives lots of down force on tail!
Rear center of gravity, less angel angel of attack on stab, but always positive.and less down force on tail.
At a extreme out of trim rear CG the 737 becomes unstable, like most similar aircraft ( Cessna 172 to 777)

On another note!
I was just in the sim and did a nose down out of trim exercise: I was PM we were at ca 285 kts and we trimmed ca 2 unit ND and PF could hold it with elevator.
Then we hit the Cutoff Switch and I tried to trim nose up!!
Could only do half a turn at a time, until we reduced speed, passing ca 250kts it got better and at 230 we both could get a turn a second and the Control Loading promptly dropped!
This was a Level D , B737-800 and I was in shock!
Bad news, if this is as per the aircraft!
I now have severe doubt the Max will be certified with ANY MCAS!
I think the NGs track record will save it, just!

Regards
Cpt B
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