Friday, September 27, 2013

Chicken Whisperers Chief Designer on Flutag Record

Gary has been actively researching the record to get better data and an email was forwarded to the Free Flight Mailing from a chief designer on the project. From the email which I will post below it will be apparent that the record was even more impressive in that the launch height was only 22 feet not 30 feet. It is also interesting to see that model airplane prototypes were used.

Comments from Bob Parks:

"Actually, the platform was not 30 ft.  In Long Beach it was about 20 ft above the water (from photo interpretation, and when we flew.  Tide went down by a foot during the event).  The Long Beach rules this year gave a 10 ft height over the platform limit, so we had a 2 ft high cart, so launched at about 22 ft.  The record from Germany last year appears to have had a 27 ft high platform, and they had an 18 ft high cart, so launched from 45 ft, about double what we had.  I have not tried to estimate the overall height for the B-25 from Minneapolis in 2010, but they had a 15 ft high cart, and the platform seems to be higher than 20 ft.  Anyway, we more than doubled the L/D of the previous record holder.  I dont know the wind from the prior 2 records but we had some headwind.. great for launching but not for glide range.

Note, a big limit on the performance of these "craft" (to use the Red Bull term.. I think its more as in "arts and crafts"  rather than "aircraft" ;-)  ) is that we had a 28 ft wingspan limit.  Combine that with the speed you can get by pushing and running on the platform (15 mph is a good push speed, we added about 5 mph to that with headwind and pilot running, so about 20 mph airspeed) and if you dont want to dive to gain more speed, you are stuck in a situation with high span loading (or low aspect ratio if you prefer), and lots of induced drag.

If we really had the 4 mph headwind the whole time and flew at 20 mph airspeed, the air distance is about 322 ft, for a net L/D of 14 or so... not bad for all the pilot drag and an aspect ratio of 4!  Note, the duration that someone mentioned in this thread confirms those numbers.. about 22 fps ground speed, and we should have been flying around a trimmed CL of 1.0 to 1.1, at 29 fps (~20 mph), so 4 to 5 mph headwind.

Our glider weight was 85 lbs (heavy, but quick and cheap and LOTS of structure margin), with a 130 lb pilot, so 215 total flying weight.  Projected wing area was 196 sq ft.

Anyway, with that flight condition, without the winglets, the lift to INDUCED drag ratio is only 12:1 or so.  Winglets and taking advantage of ground effect helped a lot.

We did look at diving to get to a higher airspeed, which would have given better L/D  (my estimate for our glider was best L/D speed was about 26 mph), but when you look at the altitude loss to get the speed, its not a net advantage, particularly with the low platform.  And then the flight dynamics ( we did some 3 degree of freedom simulations) mean that the bottom of the pullout is double the altitude loss compared to what you need to get the acceleration, AND its some very critical piloting for something you dont get to practice.  So, we decided to keep it simple and get enough wing area to just fly level off the platform..  a very fortunate decision since the platform was way lower than they said it would be!

Hey.. we won the dance part of it also..  ;-)    Given all the constraints, I would say we had a "real" glider!  Yes, given more time (2 months from when the rules came out until the flight) and money, we could have done somewhat better, but the low launch height, low speed launch and span limit really constrain what you can do for performance.  And you have to question how much you WANT to put into it when you know its going into a dumpster right after the flight.  We know we could do a lot better if we did it again... but you really have to ask if that should be the purpose of the event..

We did a small RC model (with weight shift control for pitch) to test overall handling.  Certainly good for morale!

And, of course, we also tested the actual plane the week before the event.  Just very short hops on a hang glider training hill to check trim and get used to it, since we did not want to risk damaging it too much... but we did learn a LOT, made some improvements to the plane and cart etc.  The main pilot (Laura), backup pilot (Marshall) and our test pilot (and my co-designer) Zach all flew it.

Bob Parks
Chicken Whisperers Chief Designer  (and very long time FF and RC modeler)" - My Initial Post

Bill Kuhl

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