Vertical stabliser
Join Date: Mar 2000
Location: Arizona USA
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Lets look in the distant past to see some of the other considerations that govern vertical stabilizer size on a swept wing jet transport airplane.
Type, B707 Intercontinental (707-320/436, straight-pipe powered).
There was a definite problem with this airplane regarding the vertical fin size, not driven by the engine out case, but yaw stability.
These aircraft had a parallel type of yaw damper (which needed to be switched OFF for takeoff/landing).
When this airplane was submitted to the British ARB for certification, DP Davies demanded that the rudder power and yaw damper be modified.
The rudder power Q-feel system was re-designed, and the normal 13 inch ventral fin was increased to 39 inches in width, to provide increased yaw stability.
When this series of airplanes were fitted with fan engines later on, Boeing once again redesigned the vertical fin...this time by increasing the height and, a series type full time yaw damper was fitted.
However, in extending the height of the vertical fin, Boeing just barely avoided another problem...IE: too large a vertical fin, and the spiral stability certification standards could not be met.
Aircraft design...a very large compromise, in many areas.
Type, B707 Intercontinental (707-320/436, straight-pipe powered).
There was a definite problem with this airplane regarding the vertical fin size, not driven by the engine out case, but yaw stability.
These aircraft had a parallel type of yaw damper (which needed to be switched OFF for takeoff/landing).
When this airplane was submitted to the British ARB for certification, DP Davies demanded that the rudder power and yaw damper be modified.
The rudder power Q-feel system was re-designed, and the normal 13 inch ventral fin was increased to 39 inches in width, to provide increased yaw stability.
When this series of airplanes were fitted with fan engines later on, Boeing once again redesigned the vertical fin...this time by increasing the height and, a series type full time yaw damper was fitted.
However, in extending the height of the vertical fin, Boeing just barely avoided another problem...IE: too large a vertical fin, and the spiral stability certification standards could not be met.
Aircraft design...a very large compromise, in many areas.
Join Date: Jun 2009
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Sounds like those Boeing guys had their hands full. Increasing the height increases the tail aspect ratio giving a steeper lift curve slope and making it more effective. Think Grumman Bearcat. Even with that big prop and short fuselage, it required minimal rudder on takeoff. Tails on the WWI fighters have tiny aspect ratios. They can be deflected to extreme angles without stalling and when they did, it is a gradual break in the lift curve - good things to have in a tail.
Thread Starter
thanks to everyone
thanks for the replies (bar one): I am better informed now. Was half expecting someone to suggest it is vital to have a clear surface to paint logo on. Will we never see a 'vee' ?
Join Date: Feb 2008
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However many engines there are in the tail, will we ever see todays huge diameter turbofans mounted that way in a future airliner? Mounted close to the fuselage and back far enough so that their thrust lines could be canted towards the CG, the tailpalne issues would be much reduced; except maybe getting a rough time from reverse thrust and then there's the worries about rubbish thrown up from the main gear and water ingestion. I suppose mid-mounted engines something like a scaled up A10 Warthog, so that the wing (swept though) would provide a shield.
If we are stuck with under wing pylon-mounted engines as in all current designs, I wonder why twin Vertical Stabs fitted to the end of the HS are not more popular? They seem to work very well for the Antonov 225 and I think they look very good. I realise that in this case, the reason behind the design, was to put the VS in clean airflow when carrying external loads on top of the fuselage, but this configuration would seem to have a lot to offer in engine out/exterme conditions that might come close to overloading a single cantilevered fin.
Mounted at about their midpoint to the HS, the rudder forces would be a straight push/pull with little bending moment, and I would have thought that being downstream of the engines (but not too close), they would benefit from the increased local flow velocity to enhance rudder authority in low speed/high thrust situations and they wouldn't be masked by the fuselage at high AOA. I suppose that they may work out heavier, which wouldn't please the "beancounters", but you can tell them that their expensive new hangers wouldn't need to be so high and there's plenty of vertcal billboard area to paint their Logo on!
If we are stuck with under wing pylon-mounted engines as in all current designs, I wonder why twin Vertical Stabs fitted to the end of the HS are not more popular? They seem to work very well for the Antonov 225 and I think they look very good. I realise that in this case, the reason behind the design, was to put the VS in clean airflow when carrying external loads on top of the fuselage, but this configuration would seem to have a lot to offer in engine out/exterme conditions that might come close to overloading a single cantilevered fin.
Mounted at about their midpoint to the HS, the rudder forces would be a straight push/pull with little bending moment, and I would have thought that being downstream of the engines (but not too close), they would benefit from the increased local flow velocity to enhance rudder authority in low speed/high thrust situations and they wouldn't be masked by the fuselage at high AOA. I suppose that they may work out heavier, which wouldn't please the "beancounters", but you can tell them that their expensive new hangers wouldn't need to be so high and there's plenty of vertcal billboard area to paint their Logo on!
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Why not a triple tail, like the Lockheed Constellation, and the "Cardboard Connie", the Bellanca Cruisair/Cruisemaster?
Auxiliary fins were added to the 1937 Bellanca Cruisair because it would tend to go flat in a spin, and not recover by itself within the required six(?) turns. Some 40 years later, they found it could be predicted which planes would spin more readily by the intersection of the fuselage and aft area of the wing root. The more continuous the surface between wing roots, the more spinnable. It was evident on the Grumman Yankee/Traveler/Tiger of the 1970s.
GB
Auxiliary fins were added to the 1937 Bellanca Cruisair because it would tend to go flat in a spin, and not recover by itself within the required six(?) turns. Some 40 years later, they found it could be predicted which planes would spin more readily by the intersection of the fuselage and aft area of the wing root. The more continuous the surface between wing roots, the more spinnable. It was evident on the Grumman Yankee/Traveler/Tiger of the 1970s.
GB
