Tailplane lift
Fleet Manager
Joined: Apr 2001
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From: various places .....
It's a design requirement that full elevator in available up till Vmo without overstressing the airframe
It's a tad late in the day for me .. you couldn't, by any chance, cite a reference for this ?
It's a tad late in the day for me .. you couldn't, by any chance, cite a reference for this ?
Joined: Mar 2005
Aviation Qualifications: Military
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From: Aus
I've found an answer that satisfies me, thanks to those who responded. 
FLIGHT, 23 February 1985, B. R. A. Burns, technical manager of BAe's Experimental Aircraft Programme
Tailplane download
In a stable tailed aeroplane the forward c.g. limit is determined by the ability of the tailplane to produce a download to balance wing lift, which acts behind the c.g. The tail must also counteract the "no lift" pitching moment generated by wing camber, which is greatest with flaps down. (A cambered wing produces lift at zero angle of attack (AoA). At zero lift, negative AoA, the wing carries a download at the front and an upload at the rear, producing a nose-down moment.)
The c.g. limit moves forward with increasing tail size at a rate proportional to tailplane maximum download capability. For this reason some aeroplanes feature devices to augment tailplane negative lift—leading-edge blowing on the Buccaneer and slats on the Phantom are examples.
The aft c.g. limit is determined by stability, and in a stable aeroplane is located a few per cent of wing chord ahead of the neutral stability point, where c.g. and a.c. coincide. The aft c.g. limit moves rearwards with increasing tailplane size, but only at a comparatively shallow rate because its effectiveness as a stabiliser is diminished by wing downwash, typically by 50 per cent.
It is a misconception that tailed aero-planes always carry tailplane downloads. They usually do, with flaps down and at forward c.g. positions, but with flaps up at the c.g. aft, tail loads at high lift are frequently positive (up), although the tail's maximum lifting capability is rarely approached.
As our tail sizing diagram shows (Fig A), there is a steep increase in c.g. range with increasing tail size as the limits move apart. In the example illustrated, a 40 per cent increase in tail size doubles the c.g. range from 10 per cent to 20 per cent of wing chord. Also the c.g. range is located well within the wing chord, so that the trimming load on the tail (and trim drag) is small.
FLIGHT, 23 February 1985, B. R. A. Burns, technical manager of BAe's Experimental Aircraft Programme
Tailplane download
In a stable tailed aeroplane the forward c.g. limit is determined by the ability of the tailplane to produce a download to balance wing lift, which acts behind the c.g. The tail must also counteract the "no lift" pitching moment generated by wing camber, which is greatest with flaps down. (A cambered wing produces lift at zero angle of attack (AoA). At zero lift, negative AoA, the wing carries a download at the front and an upload at the rear, producing a nose-down moment.)
The c.g. limit moves forward with increasing tail size at a rate proportional to tailplane maximum download capability. For this reason some aeroplanes feature devices to augment tailplane negative lift—leading-edge blowing on the Buccaneer and slats on the Phantom are examples.
The aft c.g. limit is determined by stability, and in a stable aeroplane is located a few per cent of wing chord ahead of the neutral stability point, where c.g. and a.c. coincide. The aft c.g. limit moves rearwards with increasing tailplane size, but only at a comparatively shallow rate because its effectiveness as a stabiliser is diminished by wing downwash, typically by 50 per cent.
It is a misconception that tailed aero-planes always carry tailplane downloads. They usually do, with flaps down and at forward c.g. positions, but with flaps up at the c.g. aft, tail loads at high lift are frequently positive (up), although the tail's maximum lifting capability is rarely approached.
As our tail sizing diagram shows (Fig A), there is a steep increase in c.g. range with increasing tail size as the limits move apart. In the example illustrated, a 40 per cent increase in tail size doubles the c.g. range from 10 per cent to 20 per cent of wing chord. Also the c.g. range is located well within the wing chord, so that the trimming load on the tail (and trim drag) is small.
Joined: Oct 2004
Posts: 255
Likes: 15
From: australia
Well, AC 23-19A has a different take on this.
"48. What is the design maneuvering speed VA?
a. The design maneuvering speed is a value chosen by the applicant. It may not
be less than Vs√ n and need not be greater than Vc, but it could be greater if the applicant chose the higher value. The loads resulting from full control surface deflections at VA are used to design the empennage and ailerons in part 23, §§ 23.423, 23.441, and 23.455.
b. VA should not be interpreted as a speed that would permit the pilot
unrestricted flight-control movement without exceeding airplane structural limits, nor
should it be interpreted as a gust penetration speed. Only if VA = Vs √n will the airplane
stall in a nose-up pitching maneuver at, or near, limit load factor. For airplanes where
VA>VS√n, the pilot would have to check the maneuver; otherwise the airplane would
exceed the limit load factor.
c. Amendment 23-45 added the operating maneuvering speed, VO, in § 23.1507.
VO is established not greater than VS√n, and it is a speed where the airplane will stall in a nose-up pitching maneuver before exceeding the airplane structural limits."
While the FAA did not introduce a Vo for FAR Part 25 aircraft (after American Airlines Flight 587 12Nov2001), the full control deflections are about rudder, elevator and aileron structure. Careful reading of FAR 25.331 shows that at Va, with maximum elevator deflection, loads which occur beyond the limit load need not be considered.
"48. What is the design maneuvering speed VA?
a. The design maneuvering speed is a value chosen by the applicant. It may not
be less than Vs√ n and need not be greater than Vc, but it could be greater if the applicant chose the higher value. The loads resulting from full control surface deflections at VA are used to design the empennage and ailerons in part 23, §§ 23.423, 23.441, and 23.455.
b. VA should not be interpreted as a speed that would permit the pilot
unrestricted flight-control movement without exceeding airplane structural limits, nor
should it be interpreted as a gust penetration speed. Only if VA = Vs √n will the airplane
stall in a nose-up pitching maneuver at, or near, limit load factor. For airplanes where
VA>VS√n, the pilot would have to check the maneuver; otherwise the airplane would
exceed the limit load factor.
c. Amendment 23-45 added the operating maneuvering speed, VO, in § 23.1507.
VO is established not greater than VS√n, and it is a speed where the airplane will stall in a nose-up pitching maneuver before exceeding the airplane structural limits."
While the FAA did not introduce a Vo for FAR Part 25 aircraft (after American Airlines Flight 587 12Nov2001), the full control deflections are about rudder, elevator and aileron structure. Careful reading of FAR 25.331 shows that at Va, with maximum elevator deflection, loads which occur beyond the limit load need not be considered.
Joined: Dec 2006
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From: The No Transgression Zone
Last edited by Pugilistic Animus; 20th February 2018 at 01:30.
Joined: Oct 2004
Posts: 255
Likes: 15
From: australia
Why do you need to derive VA?
The FAR's are quite clear. The applicant can choose any speed above VA min.
Where in the FAR's is there a requirement to associate ultimate load factor to VA at the stall?
The FAR's are quite clear. The applicant can choose any speed above VA min.
Where in the FAR's is there a requirement to associate ultimate load factor to VA at the stall?
Joined: Oct 2004
Posts: 255
Likes: 15
From: australia
I would be really greatfull if you can reference the section of FAR25, or FAR23 which associates maximum load factor, stall and Va.
It may be the case at Va min, but not at any other speed which the applicant can select as VA.
It may be the case at Va min, but not at any other speed which the applicant can select as VA.
Joined: Feb 2002
Aviation Qualifications: AME
Posts: 4,183
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From: UK
Not necessarily. The tailplane just needs to be producing less downforce than needed.
