C of G loading question
Thread Starter
Joined: Feb 2010
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From: Chicago
C of G loading question
The following is driving me nuts as I have a different view to that of a piece i saw written recently.
It concerns the following question :
If C of G moves fwd, what does the elevator do?
The answer given was :
Moves DOWN in order to counter balance the nose down.
Is this correct???
I would ve thought that it should move UP instead in order to avail of the resulting downward airforce it would encounter which would "push" the tail downwards thereby pulling the nose "upwards"!!!
thanks guys n gals
D
It concerns the following question :
If C of G moves fwd, what does the elevator do?
The answer given was :
Moves DOWN in order to counter balance the nose down.
Is this correct???
I would ve thought that it should move UP instead in order to avail of the resulting downward airforce it would encounter which would "push" the tail downwards thereby pulling the nose "upwards"!!!
thanks guys n gals
D
Joined: Aug 2005
Posts: 149
Likes: 0
From: England
Could be very badly / poorly worded question.
The moveable horizontal stabilizer itself may be set leading edge down to keep nose-heavy aircraft in trim with elevator neutral, but you are correct, with a fixed horizontal stabilizer, the elevator would need to be raised to command a downward force on tail to counter increasing downward force at front.
The moveable horizontal stabilizer itself may be set leading edge down to keep nose-heavy aircraft in trim with elevator neutral, but you are correct, with a fixed horizontal stabilizer, the elevator would need to be raised to command a downward force on tail to counter increasing downward force at front.
Joined: Feb 2001
Posts: 243
Likes: 1
From: Milkway Galaxy
Sure, the elevators should move up,
but my explanation is a little bit different:
CG moves fwd, then the lift induced moment about the cg increases becuse of increased arm between Lift and CG. To counteract or balance this increased moment, tail should create a greater downward force: elevator up.
A side effect: Increased downward force is added up to Weight. So apparent weight of the plane increases. Lift, should also increase to carry this new apparent weight, this means a higher AOA for a given speed.
So a higher AOA results: More Fuel burnt, lower max alt, higher stall speed, lower max speed.
Only good thing is a more stabilized airplane.
An addition: Yes, some books confuses me too, with "elevator nose up/down"
Elevator nose up/down: Elevator movement to drive the airplane`s nose up/down=Elevator trailing edge is up/down
but my explanation is a little bit different:
CG moves fwd, then the lift induced moment about the cg increases becuse of increased arm between Lift and CG. To counteract or balance this increased moment, tail should create a greater downward force: elevator up.
A side effect: Increased downward force is added up to Weight. So apparent weight of the plane increases. Lift, should also increase to carry this new apparent weight, this means a higher AOA for a given speed.
So a higher AOA results: More Fuel burnt, lower max alt, higher stall speed, lower max speed.
Only good thing is a more stabilized airplane.
An addition: Yes, some books confuses me too, with "elevator nose up/down"
Elevator nose up/down: Elevator movement to drive the airplane`s nose up/down=Elevator trailing edge is up/down
Last edited by JABBARA; 8th October 2011 at 21:40.
Joined: Nov 2001
Posts: 126
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From: Kerikeri New Zealand
C of G foreward more down force required by tail.
if stabiliser fixed (Cessna 172) more up elevator to balance and so on
If tail has a moveable stabiliser
Pa18 etc forward c of g results in LE of stabilise moving down (aircraft nose up)
All flying tail PA28,C177 etc same rules nose heavy total elevator LE down to increase aircraft nose up force.
Confused? I am!!
Heavy metal always try and Keep the C of G as far aft as possible to decrease the fuel burn because of the previous answer.
less down force required by the "TAIL" so less load on wings, less lift reqired less induced drag, less fuel required for a given mach number. less dollars per air nautical mile flown.
Bottom line
MONEY!
if stabiliser fixed (Cessna 172) more up elevator to balance and so on
If tail has a moveable stabiliser
Pa18 etc forward c of g results in LE of stabilise moving down (aircraft nose up)
All flying tail PA28,C177 etc same rules nose heavy total elevator LE down to increase aircraft nose up force.
Confused? I am!!
Heavy metal always try and Keep the C of G as far aft as possible to decrease the fuel burn because of the previous answer.
less down force required by the "TAIL" so less load on wings, less lift reqired less induced drag, less fuel required for a given mach number. less dollars per air nautical mile flown.
Bottom line
MONEY!
