Friday, July 13, 2018

Test flight by James Shumaker (KSNS)

My friend Tom Jones owns and flies a Rans S-6S out of KSNS -- more on Tom and his activities in a future post. He invited me to come to the KSNS Airport Tenant Appreciation Day recently. I dropped off an Airball system with him and flew back to my home airport of KRHV.

He introduced me to Jim Shumaker, who flies a Citabria out of the same airport, and does aerobatics! Jim was the first of these two folks to take Airball up for a spin!

I just waved my arms and mumbled a few words about how Airball works, and showed them how to physically mount the stuff. Apart from that, Jim was able to get the whole thing working independently, days after I'd left. Here is Jim's airplane:

This is his verbatim feedback, which he has kindly given me permission to post here:
I'm a bit late getting back to you about my flight with the airball but I did some Aerobatics with it and found it was consistent in showing angle of attack so the strap on installation worked very well. It indicated Critical angle consistently whether I was flying at 50 mph power off or 80 when in a tight turn.

I was unable to get the slip skid to center at different airspeeds. It would move to the left when it was calibrated for at low speed and then a higher speed was flown. It would move to the right if it was calibrated at cruise and then the plane slowed down.
However, The ball getting small when getting closer to the critical angle of attack did not work for me. Generally things are emphasized by getting larger, not smaller. When the ball was small and the slip skid was near the outer limit the ball disappeared. It Probably would not happen when the ball was at its largest size. I tried narrowing the limits from 15 degrees slip/skid to 10 degrees, but that was not enough. When rolling out of an Immelmann turn the ball had disappeared off to the side so I could not tell my angle of attack at all and an Immelmann is one maneuver where the angle of attack would be very useful. 
Also, when the model airplane demonstration where the string with the blue cone was used for an illustration of the wind vector, the circle it represented moved left and right as indicated but it got LARGER as the vector got shorter and looked SMALLER as it got further away. Just another reason that larger when slower would make intuitive sense. If you are getting closer to stall it should look closer. 
In Acro the angle of attack would be useful when at very low speeds and unusual attitudes where it is hard to tell where the wind is coming from. It could even be negative and that is where it might be most helpful. I've been in spin entries that developed into spiral dives because the critical angle of attack was not maintained. It would have been useful to know the angle of attack then. but with the ball out of sight to the side it would be useless. The ball should always be visible no matter the angle of slip/skid or angle of attack. 
The size of the ball does not need to change as much as it does to be a useful indicator of the speed vector.
I am grateful to Jim for having done this flight and his feedback is invaluable.

Critical angle consistency: I'm glad to hear that. This is indeed the idea, and it is good that someone who's willing to do an acro tight turn can verify that!

Slip skid centering: Jim has noted the flow asymmetry problem we've been dealing with, and indeed noted that it changes with different flight conditions. This sort of seals the deal for us. We need to at least have the option of using an accelerometer for our yaw sensing. I would like to at least run some test flights with an accelerometer and aerodynamic yaw sensing and compare the data just so we can see it, but I think the laws of nature have put us pretty firmly along that decision path.

Smaller versus larger: Noted. :) Jim is an expert pilot and, I suspect, sees the Airball display as a source of hard data, since he already has a firm intuition of flight. We have had feedback from CFIs and less experienced pilots that the "larger ball = stronger wind" visualization really feeds their intuition. It may be that one Airball may not fit all. Perhaps the display can be customized for different levels of expertise, from the most "intuitive" and "cartoon" version, to the most "information dense" version. We will keep listening.

Ball out of sight: We have a nomenclature bug. :) We talk about yaw angle "limits" when we really mean the range of the screen. So Jim reasonably took the "limits" to be a limit on the excursions of the ball, and in so doing actually ended up increasing their excursions. We need to do a better job of nomenclature and provide a "quick start guide" to users.

AoA useful in acro: We are glad to hear that. One feature of our probe -- and our system in general -- is that it is not opinionated as to how you fly. Our probe is just a 5-hole alpha-beta probe. If the airflow is coming from the bottom, it reports a "positive" AoA. If the airflow is coming from above, it reports a "negative" AoA. This is in contrast to some other AoA systems that display "zero" in cruise and never go "negative". And the display Just Works [tm]. So supporting acro (including -- say -- inverted flight) should be a simple matter.

I would like once again to thank Jim for his feedback and we look forward to having him try out improved models, which are in the works!

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