T-Tail design advantages?

[freddriver]

Since I fly the C-5, the largest T-tail of 'em all, I can give you my .02....First, the T-tail gives the elevators clean air as opposed to the low tail configuration. Second, the moment arm of a T-tail is higher. Next, a T-tail design aids the use of aft cargo doors for roll-off and airdrop missions (I know the C-130 is an exception, but the 130 is an ancient design and built like a tank). Finally, and most importantly, it allows the Air Force airlifters to use our beloved "T-tail mafia" phrase.

 

[enigma]

I should know better than to post on a technical subject without reviewing my old texts, but here goes anyway.

The T-tail does have a couple of advantages, and also some disadvantages. The thing to remember is that 99% of the design/aero/engineering decisions made in designing an
aircraft are compromises. The T-tail is used when its advantage outweighs it's disadvantages. I have also observed that there are very few commercial aircraft that use the T-tail and the ones that do have a fairly obvious reason for doing so. Aircraft that were intended to be sold to non commercial users seem to use the T-tail more as a device to give them market differentiation.

With that out of the way, let me attempt to give some concrete answers.

Advantages: it gets the tail out of turbulent air, it allows for a longer tail arm (allowing smaller surface area, and therefore, less drag), it has some potential for improved stall recovery, and it looks cool.

Disadvantages: reduced control authority for prop planes (it's out of the propwash) , increased complexity for the control linkages, heavier vertical stab required because vertical stab must carry/transfer the pitch loads to the tailcone, heavier vertical stab required becouse the vert. stab must also transfer twisting loads to the tailcone, heavier tailcone required because the aero forces developed twist the tail (especially in the case of a high G rolling pullup)

T-tails get used most often when other design criteria force the designer to use them. For example, the Learjet was designed around a pre-existing wing and that wing had no provision for mounting engines, therefore they had to mount the engines on the tail and therefore the T-tail worked best.
The DC9 would be another example of engine placement dictating tail style. The 727 uses the T-tail for somewhat the same reason, it was easier to fit to the top of the tail and they already had to make the vertical stab extra wide in order to accomodate the third engine. The ATR turboprop aircraft use a T-tail partly because it allows the horizontal tail to be installed slightly farther away from aerodynamic center of the wing and allows for a slighty smaller tail (the tail volume issue) I'm not totally certain, but I think the B1900 uses a T-tail for the simple reason that the B200 used one and the engineers determined that they would be able to get what they wanted for the 1900 by modifying that existing tail.( if they had used the B100/B99 tail, they wouldn't have had any room on the tail to add on the stub stabs and the high aspect ratio fences that drop down from underneath the stab would have presented a tail strike problem)

It is interesting to note that the DC9 sends bleed air to heat the leading edge of the horizontal stab, yet the 737's tail is completely unprotected. I understand that the issue is not one of tail stall, it seems that the hot jetblast prevents ice from forming on the 73's horizontal stab.

regards,
8N

 

[~~~^~~~]

8N: The DC10 also lacks horizontal stab de-ice if I am not mistaken. Maybe somebody who has flown these types can clue us in on why Douglas felt de-icing the horizontal stab.

If I had to venture a guess, it would be a combination of the high sweepbank angle and operational characteristics of the airplane (speed & higher total air temp) that result in tailplane de-ice not being necessary.

As for T-Tail arrows, yes that was just a marketing gimmick to make the airplane sexier. Compare the prices of the Arrow IV and the Arrow III and one quickly realizes the III is the more desireable airframe.

 

[Draginass]

T Tails do not have good natural stall warning characteristics. With a normal Hoz Stab like a B-707, disturbed airflow over the wings give a rumble to the stab. T Tails won't necessarily. Also with T Tails, in a deep stall the Hoz Stab can become blanked out and result in a "superstall" and lack of stab authority. Hence, T Tail airplanes normally have stick shakers and stick pushers when approaching the stall AOA, to compensate for the lack of natural stall warning. As point of fact however, most modern conventional Hoz Stab airplanes have at least stick shakers for additional warning.

 

[airnik]

T-Tail arrow = a pig at the controls

 

[airnik]

This accident happened in a Trama-hawk. I think this investigation is still in the factual procedures, so the final procedures won't be out until a few months from now. Nonetheless, it's a pretty good accident scenario to learn from. The private pilot applicant of this ride was the father of a friend of mine - and the chedkride pilot was "over qualified", if that makes any sense at all. So, if at all in the least bit interested, check it out.

Here's the website:

http://www.ntsb.gov/ntsb/brief.asp?ev_id=20001212X19701&key=1

-airnik

 

[airnik]

sorry - the website is listed below.

http://www.ntsb.gov/ntsb/brief.asp?ev_id=20001212X19701&key=1

 

[Cornelius]

T-tails do rumble and shake when near critical AOA at least in the BE76 and PA44-180 providing a good stall warning. I remember way back when I was doing my MEL training, my instructor was approaching stall and told me to look back at the H-tail. It was shaking like crazy which was amazing. As far as T-tails on the heavy metal, maybe it is high enough to be outside of the cavitated air during approach to stalls. I don't remember any significant buffeting in the BE1900D (T-tail) during my training since we would recover usually at the first sign of the stall horn, mushiness, buffeting whichever occurs first.

 

[Caveman]

Airnik,

No offense to your friend, but that crash appears to be the classic stall/spin/crash/burn and had little to do with it being a T-tail airplane. No matter what you fly if you unintentionaly spin it too close to the ground you are toast.

 

[Draginass]

Cornelius - I should have clarified that my comments refer to swept wing heavys. My light airplane experience very limited and as point of fact, I have not personally flown swept wing heavies with T Tails. Only conventional swept and VG wing heavy aircraft with conventional hoz stabs.

My comments refer to approach to stall only. Fulling stalling a swept wing heavy, even for practice, is very risky.