I have a Grumman AA1. It’s been modified several times through stc’s to increase power and speed. The aircraft requires full nose down trim to fly level in cruise. Is there a way to calculate the perfect angle of incidence of a horizontal stab for a given airspeed? It’s set at -3deg now, I wonder what moving it to 0deg would do? I have all kinds of load data for the horizontal stab.

  • $\begingroup$ Sounds like a CG problem to me, and not a stab incidence issue. Was the aircraft weighed and CG calculated after all the changes? Center of gravity range for this aircraft is as follows, according to the type certificate:(+78.5) to (+81.0) at 1500 lb. (+77.5) to (+81.0) at 1430 lb. (+75.0) to (+81.0) at 1245 lb. $\endgroup$ Apr 3, 2019 at 11:04
  • $\begingroup$ Looking through an AA-1 maintenance manual PDF, I don't see anything about adjusting stab incidence - it just bolts on. Is that another STC that makes the horizontal stab adjustable, or did the engine STC (I presume you have an O-320 boat anchor on the front now) include a kit to make the stab adjustable? Are there specs? Just remember, when you change things outside of a published specification, you are now an experimental test pilot. $\endgroup$
    – John K
    Apr 3, 2019 at 14:01
  • $\begingroup$ If the plane is significantly faster with roughly the same weight/CG, the stock stab angle is going to provide more downward lift, i.e. nose up, which explains why he needs more nose-down trim to compensate. Reducing the stab angle makes sense, but I have no clue how to calculate by how much, nor what that would do to low-speed handling. $\endgroup$
    – StephenS
    Apr 3, 2019 at 16:02
  • $\begingroup$ I’m actually trying to work on a 337 with my IA and DER. There are people that have done this illegally with good results. There has to be a way to get some preliminary calculations. $\endgroup$ Apr 4, 2019 at 12:51
  • $\begingroup$ The FSDO won't consider a 337 for this without baseline weight and balance information. You haven't answered my question or even mentioned the subject in your posts. Have you in fact done reweighing? $\endgroup$ Apr 5, 2019 at 9:00

1 Answer 1


One clue may be increase in power and speed. It is not unusual to trim down as plane accelerates from climb to cruise. Since you are faster, the plane is generating more lift at a given AOA. Greater speed will require more downtrim. Your engine may be too powerful for your current airframe.

If the plane is safely flyable, you may try to isolate the problem by gliding power off to see if Hstab, elevator, and trim are working properly. CG of course must be within limits.

If everything is ok, you may need a larger trim tab rather than resetting the Hstab angle. Remember, the Hstab incidence compared to the wing AOA is critical. I would not mess with it.

If the problem isolates to the new engine, you may also check the thrust line. A few degrees can make a big difference. Your more powerful engine may need to be mounted with more downthrust. That, and a larger trim tab, may do it.

I would definitely seek out people who have done similar modifications for expert advice. Please, be safe.

  • $\begingroup$ Thank you for your input. I will not mess with the horizontal stab...you make sense. I like your idea about downthrust...just do clarify, increasing downthrust would be shimming so the motor is pointing downwards, at a negative angle in relationship to the fuselage right? $\endgroup$ Apr 4, 2019 at 12:47
  • $\begingroup$ Yes, more down thrust angle will pull nose down at higher power setting but PLEASE get expert help and do it a little at a time. I was worried that changing Hstab will give you less pitch authority, making it harder to pull out of a dive. In the immortal words of my instructor "don't wanna do THAT". Again, try the glide. Pitch down and see if nose comes up strongly. If it does, double check CG and trim tab. If it does not, you may have good shot at it with more down thrust. Good luck! $\endgroup$ Apr 4, 2019 at 16:59

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