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Old Sep 17, 2014, 11:15 AM
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elevator design

hi everyone

i was searching aerofoil design on net i found zenith microlight airplane
701 model inverted curved wing surface on the down side

i just want to know whay it is done so....
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Old Sep 17, 2014, 11:29 AM
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Because when this plane is landing, at a very slow speed and high AOA, the downwash from the wing is so high that the stab is flying at a large negative AOA, and it has to generate a lot of negative lift to counteract the wing pitching moment, so it is designed to delay any possible tail surface stall. This however is at the expense of degraded cruise performance.
It's a common feature on STOL designs

[edit] however, how beneficial that upside-down elevator is, is a bit disputed, as I found after a little search: http://www.stolspeed.com/tail-feathers
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Old Sep 17, 2014, 11:32 AM
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Quote:
Originally Posted by wingmanbunty View Post
hi everyone

i was searching aerofoil design on net i found zenith microlight airplane
701 model inverted curved wing surface on the down side

i just want to know whay it is done so....
Frankly, looks like guesswork to me - Hopefully the tailplane would be efficient at high AOA.
In actual practice the loading is 1-2-3 in order of importance.
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Old Sep 17, 2014, 12:30 PM
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Here's another thread on the subject from a while ago --

inverted Stab airfoil
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Old Sep 17, 2014, 01:36 PM
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Also, something to keep in mind: the design may or may not have been a good idea.
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Old Sep 17, 2014, 06:53 PM
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The C-130 and L-1011 have the inverted airfoil on the horizontal.
Possibly because the enhanced down-load permits a shorter tail moment arm.
The L-1011 with its full-span boost tab (which looks an elevator but isn't) has very good control qualities.
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Old Sep 17, 2014, 10:06 PM
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thanks all

what i think is increasing the size of elevator can be a solution on landing at very slow speed
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Old Sep 17, 2014, 10:34 PM
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As always, it is more to one design than what you see at first glance. Below two links to understand Chris Heintz design philosophy for his line of light STOL aircrafts:

http://www.zenithair.com/stolch801/design/design.html
http://www.zenithair.com/stolch701/7-design-tail.html

As a side note I have flown the original CH 701 and the angle of attack, climb rate and decent rate this aircraft is capable of are mind boggling: very similar to a well designed over powered model airplane. Just have a look at the photos and videos.
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Last edited by fnev; Sep 18, 2014 at 12:06 PM. Reason: spelling
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Old Sep 17, 2014, 10:42 PM
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Here is one of the video from Zenair:

http://www.zenithair.com/stolch701/videos.html
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Old Sep 18, 2014, 10:24 AM
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@fnev

thanks i gone through the link , very useful information
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Old Sep 19, 2014, 02:20 PM
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One has to keep in mind that full size aircraft are typically set up with a radically forward CG location compared to model setups. Other than aerobatic planes full size designs typically balance at or forward of roughly 20% of the MAC. This makes for a high negative lift load on the stabilizer. This makes the use of an inverted lifting airfoil more appropriate. It also makes the generation of lift during the slow speed takeoff and landing phases more reliable.

The tail would need to generate this lift anyway. The use of the inverted camber airfoil simply makes the job of the tail easier. It also delays any stall and possible loss of lift at the tail during the take off and landing. And as a side benefit it would lower the drag at the tail during this same mode.

During cruise the tail would likely generate slightly more drag than you'd get with a symmetrical airfoil. But on a plane which is this draggy that's a fair trade to achieve the exceptional short field landing and takeoff abilities.

Looking at model examples we can see the use of lifting airfoils in free flight models which typically trim out with radically REARward CG locations. The high positive lift values needed to trim these models work out better with a lifting section instead of a flat section.
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Old Sep 19, 2014, 06:40 PM
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Airfoils ---the shapes which will work are infinite.
The shapes which work well only a few million. The best shape at a finite speed and load is---wait for it
- one.
better yet if we change he size -the shape which is best changes.
or
change the loading
the best one is again, different.
What is the best compromise ?
Is a puzzlement
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Old Sep 20, 2014, 08:18 AM
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@richard hanson

puzzlement

i think that the right kind of word .

i was one how start this topic and still i come to no conclusion

airfoil s

if the design concept of inverted airfoil and full size rudder was good ,
who can answer batter then the designer himself i mean look at the zenith
latest design it is all change in the elevator and rudder concept
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Old Sep 20, 2014, 02:31 PM
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I remember seeing an experimental sailing yacht with a wing sail many years ago on TV.

The airfoil camber could be reversed, (see the picture), not sure you would want to try it on a plane though.
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Old Sep 21, 2014, 08:59 AM
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BMAtthews,

"...full size aircraft are typically set up with a radically forward CG location..."

Why? I can picture such an aircraft stalling at low altitude, dropping the nose dramatically, then crashing before the pilot can recover.

Jim R.
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