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High Wing and T Tail

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Rick McWilliams

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The high wing T tail configuration is fine. The low wing T tail has some problems near stall as the wing wake includes the horizontal stabilizer. These airplanes need to have extra static margin to assure positive recovery. The wake also makes the tail shake, disturbing the pilot. These airplanes were popular in the 70's.

The high trust line of your configuration can have large trim changes with power. It is best for the horizontal stabilizer to stay in the propwash or remain entirely out of the propwash. The engine thrust line should be down, so that the prop wash results in downward tail lift, which is nose up trim. This can balance the direct thrust nose down trim with power.
 

Aircar

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All good advice -- I thought of the SZD 45 Ogar just after my last post as an example of high wing pusher DIRECT DRIVE -- and T tail --there are some amphibians of the same general type and closely related such as the Westwind and Spectra (the word description can be misleading and the actual geometry makes as much difference as that between quite unsimilar layouts -the Poschel Equator started out life as a high wing T tail rear prop for example. )

There is not much extra complication for a T Tail control as Jarno notes --keeping the tail out of the propwash for the normal operating case is likely to reduce the need for beefup and save weight as against a tail exposed to the buffetting from the propwash (to get an idea of the significance try holding on to av tractor aircraft tail while it is run up --chocked of course and well away from the prop --the amount of shaking will likely apall you . The GAF Nomad developed major cracking and several in flight tail separations due to the a symmetric prop wash hitting the tail (one which failed in flight had been used for extensive ground running with one or both engines at high power during certification testing when restrained -- I attended the accident investigation hearing with the brother of the dead pilot who wanted his name cleared and succeeded --the mid fin tail in two pieces only connected by a small torsion cell (the spar ) was the weak link - a T Tail can have an uninteruppted bending and torsion structure . sailplanes nearly all use T Tails on very spindly structures and don't suffer unduly .
 

Aircar

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special note -- I note that "Autoreply" (Jarno) is -or may have just passed - on his 6000th post --quite a contribution to the life of HBA and deserving of some sort of recognition and maybe some sort of title is in order "Stalwart" comes to mind (as do 'longsuffering' and 'veteran' amongst others - anyway , my congratulations on reaching this milestone and the contribution behind it. Well done.
 
M

Manticore

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The wake also makes the tail shake, disturbing the pilot. These airplanes were popular in the 70's.
I remember a flight test on the Piper Tomahawk in which the pilot remarked that, should you enter a spin, it would be wise not to look back at the tail - unless you already intended to change your underwear immediately after landing.
 
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wsimpso1

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OH, I agree that a T-tail won't add a bunch of weight, but it unless the upper end of the vertical fin was overbuilt to begin with, you have to add some stiffness (adds weight) to the fin, and then there is another bellcrank, its mounts, a pushrod, and in most powered airplanes, a little more area to maintain takeoff and landing pitch capability. And I agree that by clever design, the pitch tube's weight could stand in for some of the elevator balance weight, and this be negated.

Still, the best reasons I can come up with for adding this particular weight and complexity (at all) to a powered airplane are looks and removing the tail for storage or trailering. T-tail makes removal a tad easier, as is done on sailplanes.

Billski
 

Aviator168

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I saw this video long time ago and it took me a while to dig it up. Anyone care to speculate what cause the accident? It is a bit hard to see. The break up seemed at the point very closed to the prop, not by flutter from the T-tail.
 
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autoreply

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Looking frame-by-frame, it clearly breaks at the beginning of the boom and just before failure occurs it slaps left/right violently. Looks a lot like flutter.
 

Aviator168

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Looking frame-by-frame, it clearly breaks at the beginning of the boom and just before failure occurs it slaps left/right violently. Looks a lot like flutter.
Amazing how quickly it broke; it took less than a second from the start of the flutter to the point of boom broken off. Question. Shouldn't it break more toward the fin instead of the attachment point between the boom and main body?
 

bmcj

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Is boom reinforced with a one piece keel (longeron/spar from front of the body to fin) in these planes?
I couldn't tell you. All I know is that the bending loads are larger at the fuselage, so the booms will usually have more reinforcement or a larger diameter near the fuselage. Of course, if this accident was caused by flutter, even if the boom did not fail, the plane hay have likedly crashed anyway. Flutter is a very serious issue.
 

autoreply

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Is boom reinforced with a one piece keel (longeron/spar from front of the body to fin) in these planes?
That varies from plane to plane. In composites usually some strips of UD are added. By tapering the boom (thicker close to the fuselage), you can have a continuous skin thickness which is strong enough everywhere (I increases with the square of the tube's radius so you can match I and the local bending moment)
 

gerome

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Hi. Can you please list down the advantages and disadvantages between a conventional tail and a T-tail. I am currently designing an aircraft and I want it to have a great maneuverabilty and i read that T-tails are more maneuverable than conventional. BTW its a single prop engine mounted at the front.
 

Aerowerx

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Looking frame-by-frame, it clearly breaks at the beginning of the boom and just before failure occurs it slaps left/right violently. Looks a lot like flutter.
I can't see it in the video. Is this flutter of the horizontal stabilizer?

How would you prevent this? Would balancing the stabilator ahead of the hinge line prevent it?

Is there a way to calculate if this would be a problem on a particular design?
 

Aviator168

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I can't see it in the video. Is this flutter of the horizontal stabilizer?
The HS and Fin combination.

How would you prevent this? Would balancing the stabilator ahead of the hinge line prevent it?
This one is bit difficult. The flutter is cost by the side wind pulse generated by the propeller. Tractor planes have this problem too. Except that the force is not as strong. Autoreply can chip in on this.

Is there a way to calculate if this would be a problem on a particular design?
Autoreply?
 

Birdman100

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Is there a way to calculate if this would be a problem on a particular design?
This is very complicated phenomenon, with a lot of variables. Only careful simulation would give valid result. If this is beyond ones capability, precautions would be stiffening the tail (Fin+HS) and balancing rudder and elevator (you can not balance the stab). It is not enough to balance surfaces statically (CG in hinge) but dynamically as well.
 

Aviator168

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This is very complicated phenomenon, with a lot of variables. Only careful simulation would give valid result. If this is beyond ones capability, precautions would be stiffening the tail (Fin+HS) and balancing rudder and elevator (you can not balance the stab). It is not enough to balance surfaces statically (CG in hinge) but dynamically as well.
Finding out the natural torsional frequency of T-tail & boom will help. With that information on hand, you make sure the prop is not going to generate wind pulses in that frequency across all rpm settings. Frequency of wind pulse is 2*rpm/60 on a 2 blade prop.
 

autoreply

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I can't see it in the video. Is this flutter of the horizontal stabilizer?
Don't know. Given the possible flutter modes (at least 9), I couldn't say.
How would you prevent this? Would balancing the stabilator ahead of the hinge line prevent it?
No. Flutter will eventually occur. When you have stiff tails, a low tail and balanced controls (all 3), you're unlikely to ever see flutter below 250-ish kts, so most people there just ignore the issue and balance their surfaces.
Long, slender tails can be worse, T-tails are even worse.

Flutter isn't just the elevator going up/down and bending the stabilizer. It also bends the tail and can couple with fin torsional movement or wing movement. Theoretically, the number of flutter modes you have to analyze is N!, so imagine what happens when you have 9 modes...
Is there a way to calculate if this would be a problem on a particular design?
NPTEL :: Aerospace Engineering - Aero elasticity

Nope, it's far from simple.
Just check whether you can ignore it (see first part of the reply) and if not, think whether you can fix it (some pultruded HM strips of UD work great). Realize that flutter is about stiffness, arms and mass, not about strength. That's why so many fast aircraft have fabric-covered controls ;)
 
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