# Is there any way to estimate the weight difference between a trimmable horizontal stabilizer and a fixed tailplane?

I am trying to estimate the benefits of using a trimmable horizontal stabilizer (THS) instead of a fixed tailplane. In my model I use a T-tail (Sh~30 m), and I can trim the aicraft in every condition so the main reason to go for a THS would be aerodynamic efficiency.

I've estimated the drag reduction but I'm having huge problems estimating the weight increment of the system.

Please, could someone shed some light on this? I've checked a lot of references but still couldn't find anything clear.

Hard to find a general reference for this one. You're adding the weight of the hinge, the actuator, and the supporting structure now connecting only to the hinge axis.

Torenbeek section 8.4.1 gives a weight estimation for preliminary design purposes for the tail group of high subsonic aircraft: it mentions a weight increase of 10 % over the weight of a fixed horizontal tailplane.

• Thank you! I looked in the Torenbeek but missed that!
– JGG
Dec 2, 2017 at 16:16

You need a trimmable stabilizer in two cases:

1. In transsonic flight when local shocks render the elevator ineffective, or
2. In case of powerful flaps which require a wider trim range than what a regular elevator can provide.

If you can get by with a fixed tailplane, not only the weight saving but also the reduced complexity should be weighed against the increase in efficiency. If your elevator trim deflections in climb and cruise are within single digits, the drag increase is negligible. If you need more trim deflection during approach, the drag increase might even be welcome.

Make sure that with all trim deflections your elevator maintains sufficient control authority. Then a fixed tailplane is the best choice. The weight increase versus efficiency calculation should only decide wether to use a trimmable stabilizer or not if you need to achieve a specific performance figure. Normally, the extended trim range is what is pushing engineers to choose a trimmable stabilizer. If it is not needed, it is better avoided in order to ease maintenance and to avoid the new failure modes which come with more moving parts.

• Number 1 won't be a problem. However, I see the other points differently. I have control to trim the aircraft with a maximum of approximately 2/3 of the maximum deflection (~15º) in the worst-case escennarios. I think the other 1/3 (~7º) would allow me maintain sufficient control (maybe this is the first point where I am wrong). I have mainly analysed the drag in cruise and, as I see it, an elevator deflection would have a significant effect increasing the pressure drag of the HTP. Let's say it reduces by 2% the overall efficiency just in cruise.
– JGG
Dec 2, 2017 at 16:37
• Also, I strongly believe a THS is also a strategic decision: -It allows you to move more the CG. -It allows you to create derivatives of your design. Apart from that, It also reduces loads in the elevator and allows the pilot to maintain full control authority when the aicraft is trimmed (which I assume it much more safer). Of course, this is just my opinion and I may be completely wrong. Again, thank you for your time and help :)
– JGG
Dec 2, 2017 at 16:44
• @JavierGarcia: If you feel you need a wide cg range, then you do need a trimmable stabiliser, as I said. And 15° just for trim will not leave enough for control. Dec 2, 2017 at 17:04
• Thank you sir! I'll think in what you said and try to make the best decision
– JGG
Dec 2, 2017 at 17:57
• Excuse me sir, Please, could you tell me what elevator deflection is common in take off and landing manoeuvres? Do aircraft only use the THS for these so they still have the whole elevator deflection available? Thank you!
– JGG
Dec 5, 2017 at 19:10