Trimming Park Hawk

[anotherjustin]

HI.

What experience do people have trimming Park Hawks?

Mine requires about 50% right servo travel (40 degree rudder rotation) for straight powered flight, but it strangely then goes the opposite way during the glide.

I find that the recommended method of adjusting the leading edge tension by tightening the tab at the wingtip on the slow wing actually appears to have the opposite effect and makes the turn worse. Conversely, tightening or loosening the opposite (fast) wing has little effect at all.

If I balance the bird between the leading edge and the middle TE carbon "finger" (thus taking up slack in the TE), the TE on the slow wing displays about 1cm more billow than the TE on the fast wing. Thinking this TE tension may be to blame, I tightened the TE by peeling back the tip LE tape, and taking a 2mm tuck forwards in the tip fabric. This again seemed to make the turn worse, despite making the TE appear more symmetrical.

The main body slack appears very similar, and by inspection the whole wing looks remarkably symmetrical, except for the observation about the TE.

What other experience do people have trimming the wing on these machines?

Ta,

Justin, UK

[Ed Couch]

Justin, I had trouble at first trimming out the Park Hawk, always wanting to turn right or left depending on which side of the sail I had up. Tried the method you described with no luck. After a little reading and mind bending I figured what the hell and cut a slice about a quarter of an inch long just ahead of each batten in the wing opposite the turn to kill the lift on that wing. Did it two battens at a time flying and trimming as I cut. Finally got the trims in the middle and all things flying straight. This may sound extreme but it works and my trims are centered. One of the problems I'm having is an aft center-of-gravity, when the wings lock in place the bird pitches up immediately and to me is just not controllable. Would rather pigeon the wings than have them lock. At least the bird goes where I tell it to. Its a hoot any way you look at it but still takes a little fiddling with. ed

[anotherjustin]

Hi Ed.

OK, that's interesting. I also had another few goes last night armed with my roll of duct tape.

I found that if I applied duct tape to the top surface of the wingtip of the FAST wing, taking a ~2mm tuck in the thin ripstop portion of the top surface just behind the leading edge tape. This completely cured/reversed the problem. The effect of the tape is to cause the wing shape to distort slightly, sort of setting the leading edge up at a few degrees with respect to the rest of the membrane.

Trimming here with tape seems to be quite sensitive, and an almost impossible turn one way can be corrected or turned to a turn the opposite way.

What I am expecting is that the trim might alter as the sail stretches, since it became worse over the first few flights since the bird was new.

Now I need some way of making that mod more permanent. - Or I might experiment with your more radical method!

Unfortunately the last dozen or so flights have had a bad turn, and during trimming I have done many three second spiral flights and heavy landings, and head-over -heeled the bird a few times. When this happens, the tailplane flattens against the ground on the inverted bird, which has now stripped my elevator servo gear (HS55). Any recommendations for a tougher (light) servo wiich can withstand the odd turn-over on the ground?

Rgds,

Justin

[Ed Couch]

Justin, I used the Hitec 81 servos which are quite a bit larger than the 55's and I to have blown a set of gears when she sit down on her tail while landing. If you follow some of the older threads here there is a discussion of the various servos used. Again I am carrying quite a large difference of weight in the tail between the two servo types and I can feel it. The heavier duty servos do help with the gear stripping but you pay the pipper when you add the weight, whats better is up to the individual, I followed the recommendation of the discussion group and all things aside I would probably put a 55 on the pitch control and a 81 on roll control. I had four of the 81s laying around not being used so I went that way. As for the cutting of the wing, the slices are not noticable in the air and the material is Ripstock nylon cloth it doesn't tear at the cut with use. Solved the problem and made flying the bird a lot easier.

[uglyplane]

Ed,
can you post pix of the cuts you made? Also, Should I try my first flight without any mods?
Thanks,
Bill

[Ed Couch]

Bill, don't do anything until you've attempted several test flights, most of the birds fly great out of the hole. Only as a last resort would you cut the wing. Flip the sail over if your trims can't handle the turn. Try again, then and only then try slitting the wing at two of the battens as called for on the high wing. Only a couple of quarter inch tics and then try a couple more flights. If this doesn't correct the problem, do it again at two more of the battens. Take your time and test fly each time you cut anything. All my trims are centered and the bird flies fine now after minor surgery. A sweet little machine that will always draw a crowd whenever its flown. eec