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Bearhawk 5 control balancing/flutter

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  • Bearhawk 5 control balancing/flutter

    Background: Knowing that the structure of the Bearhawk 5 was not the same as the other models and that Bob had increased VNE 5 MPH versus the 4 place, I called Bob to see if the Engineering guidance for the other models to reduce indicated VNE by 4MPH for every 1000 ft above 8000 was the same for the model 5 (the document predated the 5). Knowing that this sort of guidance is due to a flutter concern, I wanted to confirm what he intended for control surface balancing, and what was the surface that determined the flutter limit.

    Summary: Bob confirmed that the same guidance should be used on the model 5: at any altitude above 8000 ft the indicated VNE should be reduced by 4 MPH per 1000 ft. He intends for control surfaces to be balanced 100% +/- 10%. And if the control surfaces are balanced as instructed, the surface that will flutter first is the wing due to its torsional flexibility.

    Discussion: Flutter is not something you ever want to test in the air unless you have a parachute and big brass *****. So its important to learn from those who did the calculations or the vibratory testing of the airframe. The oscillations initiate and sometimes propogate so fast that the pilot has no time to respond. Bob shared that got into flutter once and instinctively and correctly pulled the nose up to slow the plane and stop it. He said he lived(obviously) but the airplane was substantially damaged. In faster airplanes flutter can be a concern. Slow airplane designers often dont require balanced controls. For instance on my Just Highlander there were no balanced control surfaces. My Lancair IV flutter analysis was deemed so critical that we were required to change out nutplates on an aft inspection panel from #6 to #8. The tail would otherwise fall off prematurely, granted we are talking at Mach 0.58.

    I did inquire what balanced controls meant to Bob and he confirmed it means the control balance so that it maintains a level attitude (neither the nose drops nor does the tail but honestly with his +/-10 spec, that may happen). He did say its not a big deal if the nose drops. I will share a link that shows a simple flutter demonstration and why erring weight on the nose is better than erring weight aft. I did want to confirm he was balancing for flutter and not for stick forces. He confirmed it had nothing to do with stick forces and everything to do with flutter. I asked him the method he used to determine the flutter analysis. He said he used the CAA formulas from the 40’s. I asked him what safety factor is in the calculations. He said he knew there was a safety factor built in but did not know what it was.

    Thanks again to all of you who share so freely. I am spending all my kit delivery waiting time reading the great stuff you have published which causes me to think and ask questions.

    Here is the simple video discussing balancing and how it impacts flutter.

    Here is a link of someone with big brass *****, someone who intentionally flew to the point of flutter. Not sure of the outcome.


    Dennis R

  • #2
    Very useful post, thank you!


    • dramsey
      dramsey commented
      Editing a comment
      You are welcome! Thanks for your work on this blog. I am thoroughly impressed with the knowledge and the helpfulness of the bearhawk builders group. The search function is very helpful as is bearhawk tips.

  • #3
    Great Info Dennis. I have always found Bob a great wealth of knowledge and if you ask the right questions, you will be amazed at the things you learn.


    • #4
      Can anyone speak about these "CAA formulas from the 40s"? Any good online sources? I'm curious what happens if you add more balance weight than you need.

      Edit: Thanks for the link!
      Last edited by Have-Purple; 11-27-2023, 09:45 PM.