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 Post subject: A little knowledge is...
PostPosted: Thu May 06, 2010 2:21 pm 
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Here's a question from the design-challenged (that'd be me) to those who know more (that'd be pretty much everybody else):

Would it be possible to smooth out the under-camber of the wings by a small amount in order to acheive an increase in cruise speed at the cost of some low-speed capability? And has anybody tried a few degrees of reflex on the flaps?


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PostPosted: Thu May 06, 2010 9:07 pm 
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I'm certainly no expert on airfoils, but trying to modify the existing ribs would probably not be a good idea. There are a lot of flying characteristics you would be changing besides speed. You could be making big negative changes in the basic stability of the wing and stall characteristics. That kind of thing is best be left to the experts.

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PostPosted: Fri May 07, 2010 10:40 am 
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I'm no expert on the Highlander but there are some basics of aeronautical engineering that you have to remember. Aircraft are just a series of compromises flying in close formation. Changing the airfoil will put you in the test pilot category. It's possible to approximate what effect changing the airfoil will have, but even the best sometimes get it wrong. Just look at the tail of a new Cessna 162 LSA and the wreckage of the two they spun in. Even minor changes really need to be properly considered before proceeding.
All of that said, you change the camber every time you use flaps. Kitfox went to a more standard type airfoil and gained some cruse speed, increased the stall speed, and upped the wing loading somewhat in the process. With the more standard control surfaces on the Highlander, flutter needs to be looked at as speed increases as does stress and airflow separation.
Changing the airfoil is something that has been done successfully by many builders of all types of experimental aircraft. It has also caused accidents when not properly engineered. You really need to consider your threshold for risk vs. reward and be sure that doing some aeronautical engineering is something you want to get in to.

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PostPosted: Fri May 07, 2010 1:22 pm 
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One could get a small increase in speed that way with the under camber reduction but they'd spend far more time with the alteration's installaton than the time they'd make up flying a few knots faster.

Even the reflex flaps idea which is much easier to install, I personally abandoned as too much work for only a possible few knots gain, as evidenced by relatively similar airfoils that employ them.


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PostPosted: Thu May 20, 2010 12:12 pm 
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Reflexing the flaps (and ailerons) works for me. A small, but measurable increase in cruise speed, but more importantly, a reduction in trim change with change of speed. Only took one afternoon to accomplish.


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PostPosted: Thu May 20, 2010 12:43 pm 
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Keith,
I need more info on how you reflexed the
flaps.

I suppose the the reflex for the ailerons is
in offcenter cabling. correct?

How much offset? How much speed?

Need info.... please...

Where are you located?

John

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PostPosted: Sat May 22, 2010 12:35 pm 
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I'm at Corvallis, Oregon and fly a Sky Raider II, GEO power. Reflexing the flaps to close up the gap between the flap and the wing is done by filing clearance in the flap hinges and giving the cables a little slack. That raised the trailing edge about 5/8" on my plane. Raising the ailerons to match is simply cable adjustment.

The mod gave me 1 to 2 mph cruise speed increase, but a noticeable reduction in attention to trim requirement. More reflex on the ailerons didn't have much effect. Still have full flaps for landings, in fact I now have 3 notches of flaps for when I really want to come down steep. I filed a third notch into my flap actuator. With three notches applied, you need to add power for touchdown if you want to avoid a carrier style landing. Flap angle measures about 45 degrees.

With the flaps set up this way, the first notch is about 10 degrees, which improves glide slightly over no flaps for extending power off glide. Still get best climb performance with no flaps.


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PostPosted: Sun May 23, 2010 2:40 pm 
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Keith,
So the flap spring pulls the flaps up the 5/8"?
I suppose the low pressure would help also, but
I would be concerned about flutter.

Could the aileron reflex be readily added once
the aircraft is flying with conventional indexing?

I have been waiting for a while to get mine painted,
so I haven't been around it much. Next time I will
look into first hand about this.

I intend to do a lot of clean up to improve top end
& fuel burn, so this sounds like it may be another
simple thing to add to the list.

Thanks for the input.

John

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"Once man has tasted flight, he
will walk the earth with his eyes
turned skyward, for there he has
been & there he longs to return..."
L. DaVinci

Duane Sorenson & Rick Norton,
Gone West, 6/8/09.
Godspeed...


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PostPosted: Mon May 24, 2010 5:01 am 
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The spring has plenty of tension to pull the flap up. Because of the hinge pin location and the constant high pressure underneath the flap, flutter is extremely unlikely. 5/8 inch isn't very much... it's hard to detect visually, but that's all I could get before the gap closed. The fabric is pushed up between the flap ribs by lift pressure, even at high speed with the nose trimmed down. Curiously, except at very high angles of attack, the fabric on the ailerons bulges down slightly even before reflexing. Washout effect? Sealing the gap with tape did not effect the bulging.

The biggest improvement in drag I found was sealing the underside of the wing to the fusalage. Particularly towards the back where the air pressure under the wing is highest, including the flap. I used foam weatherstrip on the root rib to seal to the wing rib, and fabric to cover the area from the root rib to the fusalage tube over the door. My plane has a single top stringer from the back of the triangular turtle deck window to the vertical stabilizer, so there is a large gap to the flaps. Filling that gap with a fairing along with the previously described sealing had a profound effect on low speed high angle of attack drag. I believe that Sky Raider founder Kenny Schraider would b alive today if he had these improvements on his plane. He died when his Sky Raider stalled in a high bank low altitude maneuver. I experienced the same phenomena before adding the fairings, but had enough horsepower (95 HP) to save myself. I believe he had a Rotax 503. Needless to say, I sought out a fix. Another part of the fix was to correct the cg location for the undercambered airfoil. Anything forward of 35 percent causes excess drag on my plane by using elevator deflection to hold the nose up. So, lead in the tail it is. My plane flies best with 40 to 45 percent back of leading edge CG. Lots of lead!

Your mileage may vary.


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PostPosted: Mon May 24, 2010 12:52 pm 
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Keith,
Fairing the flap to turtle deck gap in
another mini project on my list, after
I get it in the air.

A root rib gap seal should be easy, as well.

I really appreciate your detailed input.

John

_________________
"Once man has tasted flight, he
will walk the earth with his eyes
turned skyward, for there he has
been & there he longs to return..."
L. DaVinci

Duane Sorenson & Rick Norton,
Gone West, 6/8/09.
Godspeed...


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PostPosted: Wed May 26, 2010 6:34 pm 
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I posted this under another topic but this might help:

Here is what I found yesterday when I was looking on Barnstormers. I found a "sold" ad with a link to the owner's webpage and its said "New wing for Highlander" under products. You can check it out too @ http://thunderridgeair.com/Our_Products.html. I called Stan and he has a new Highlander wing designed by Harry Riblitte (who designed the Kitfox wing) on his Highlander. Here are some of the characteristics of his Highalander with this new wing:
1. The wing chord is 4" longer
2. The wing folded width is 8'2" instead of 8' of stock highlander
3. There is an increase in cruise speed of 10-12mph
4. Stan get's 117mph @ 5200-5300rpm with the 100hp Rotax
5. The stall characteristics are 5mph better
6. Troy has already flown his highlander and likes it!

Stan is now modifying his Kitfox wings with this new design as this new design by Harry is not the flat bottom wing on stock Kitfox of today. Stan suggested that Troy will be able to incorporate this wing design in Highlander and I hope to talk to him at Oshkosk about this in a few months. I think it all depends on him.


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PostPosted: Thu May 27, 2010 10:08 am 
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So the new wing AND a few degrees of reflex have been tried separately, and they work separately, but no one has tried them together, which could give a 12-14 mph airspeed boost on the high end at minimal reduction on the low end.


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PostPosted: Thu May 27, 2010 6:31 pm 
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Well, my point is that this is not a design idea anymore. It has been demonstrated in an actual Highlander(s) operational today. The question is what are the downside that people like me overlook (all the time). I can see the folded width go up as one of them and added weight (maybe) but Troy gets to say the last word on this one for sure.

That 15mph gain puts Highlander right with Kitfox and with Jak's Bushking engine at 117Hp or Jan's Viking at 115HP this thing will jump up like a rocket..........Oh well!, at least I can dream


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