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Author Topic: Real Airfoil Thickness  (Read 30449 times)

Offline phil c

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Real Airfoil Thickness
« on: December 20, 2012, 06:11:42 PM »
The interesting discussion led me to wonder why everyone leaves off the flaps when talking 'bout stunt airfoils.

An 18% base airfoil turns into 13.9-15% depending on the flaps(30-20% of base chord).
A 20%  base foil turns into 15.4-16.7 %.

And everyone seems to have forgotten that the flaps move, meaning that they need to be streamlined into the airfoil at around 22deg. of movement so they don't stall(from Al Rabe and Igor).  Serge's modified 20%'er seems to have the best balance.  The mods thin the leading edge a bit and move the rear curvature back a bit.

When I get Profili unlocked I'll try to learn how to compare a moving flap airfoil with a fixed flap one.
phil Cartier

Offline ash

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Re: Real Airfoil Thickness
« Reply #1 on: December 20, 2012, 08:17:56 PM »
I hear that it's standard practice in full size aviation to ignore the flaps in aerofoil proportion figures, so people with full-size aviation background carried the convention over to aeromodelling.

Unlike full size, however, our flaps go both ways and are always part of the lifting surface, so in my unqualified opinion the convention is irrelevant and unhelpful to us.

Someone with a qualified opinion will be along any moment now to reveal the truth.
Adrian Hamilton - Auckland, NZ.

Offline Howard Rush

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Re: Real Airfoil Thickness
« Reply #2 on: December 20, 2012, 08:30:33 PM »
When dealing with each other, we should specify which definition we're using.  Include the flaps for Profili.
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Offline Tim Wescott

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Re: Real Airfoil Thickness
« Reply #3 on: December 20, 2012, 09:23:47 PM »
I hear that it's standard practice in full size aviation to ignore the flaps in aerofoil proportion figures, so people with full-size aviation background carried the convention over to aeromodelling.

That can't be right -- that only makes sense to me for planes with Fowler flaps or other such mechanisms that increase the wing chord when they're deployed.
AMA 64232

The problem with electric is that once you get the smoke generator and sound system installed, the plane is too heavy.

Offline Howard Rush

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Re: Real Airfoil Thickness
« Reply #4 on: December 20, 2012, 10:13:10 PM »
That's the sort of flaps one sees in full size aviation.
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Online Brett Buck

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Re: Real Airfoil Thickness
« Reply #5 on: December 20, 2012, 10:23:57 PM »
I hear that it's standard practice in full size aviation to ignore the flaps in aerofoil proportion figures, so people with full-size aviation background carried the convention over to aeromodelling.

   Hmm, I don't know, the convention I have seen for 40+ years has always been to include the flaps, with some exceptions here and there.

   

Offline RandySmith

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Re: Real Airfoil Thickness
« Reply #6 on: December 20, 2012, 11:26:57 PM »
The interesting discussion led me to wonder why everyone leaves off the flaps when talking 'bout stunt airfoils.

An 18% base airfoil turns into 13.9-15% depending on the flaps(30-20% of base chord).
A 20%  base foil turns into 15.4-16.7 %.

And everyone seems to have forgotten that the flaps move, meaning that they need to be streamlined into the airfoil at around 22deg. of movement so they don't stall(from Al Rabe and Igor).  Serge's modified 20%'er seems to have the best balance.  The mods thin the leading edge a bit and move the rear curvature back a bit.

When I get Profili unlocked I'll try to learn how to compare a moving flap airfoil with a fixed flap one.

Well most give moment numbers by measuring from a leading edge of a lifting surface, and a hingeline.... so leaving out the flaps  when figuring wing percentage kinds fits right in!! ;D ;D ;D ;D ;D

Randy

Offline Tim Wescott

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Re: Real Airfoil Thickness
« Reply #7 on: December 20, 2012, 11:34:34 PM »
That's the sort of flaps one sees in full size aviation.

Snob.  All the light planes that I've stood next to, or flown in, have had plain flaps (well, or no flaps at all).  And they are too full size!!
AMA 64232

The problem with electric is that once you get the smoke generator and sound system installed, the plane is too heavy.

Offline Geoff Goodworth

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Re: Real Airfoil Thickness
« Reply #8 on: December 22, 2012, 05:30:20 PM »
I think that for our little aeroplanes, there are two distinct issues.

First, when assessing wing performance, area, aspect ratio, etc, I always include the flaps.

Second, when establishing an airfoil to use—which I then truncate and attach the flap—you cannot include the flap in the considerations. That is a given, it doesn't work the other way.

In my head, they are quite separate issues.

Offline phil c

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Re: Real Airfoil Thickness
« Reply #9 on: January 28, 2013, 09:11:53 PM »
Geoff,  you can't ignore the fact that a CL stunt plane doesn't use one airfoil  The airfoil changes with the amount of control.  So the airfoil in level flight is almost perfectly symmetrical.  In a typcial corner of a tight radius, the flap is angled about 20+ degrees, giving the airfoil an extra 7 or so degrees of angle of attack.

And in maneuvers, especially planes with relatively wide flaps, it's entirely possible to stall the flap(get separated flow and lots of drag).  We can move the controls much faster than the air can respond.  I've seen many flyers with a slightly slow, slightly overweight plane make is literally drop out of the sky by wildly pulling full control in a bad situation. Coming around into the top loop of the vertical eight the wind slows the plane down and it starts to get soft on the controls and fall in a bit.  Instead of bailing out by letting the plane go upwind and down until they get some control they yank on the up line and the plane almost stops and falls to the ground.
phil Cartier

Offline Tim Wescott

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Re: Real Airfoil Thickness
« Reply #10 on: January 28, 2013, 09:46:38 PM »
From the standpoint of telling someone how to construct the plane the method of saying "so-and-so 18% airfoil, plus a so-and-so long flap" makes sense.  It doesn't do a bit of good for analysis, of course.

So -- the whole flap stall thing makes sense.  Can you point to any posts that show how to make the flap?  Just make it thicker than the fixed portion TE, or what?
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The problem with electric is that once you get the smoke generator and sound system installed, the plane is too heavy.

Offline Howard Rush

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Re: Real Airfoil Thickness
« Reply #11 on: January 28, 2013, 10:25:14 PM »
Here's how I make flaps: http://stunthanger.com/smf/index.php?topic=23058.0 .  TE of the fixed part of the wing is 1/4" radius; flap LE is 3/16" radius.  That's for an Impact.  Other airplanes may be different. 
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Offline phil c

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Re: Real Airfoil Thickness
« Reply #12 on: January 30, 2013, 06:06:22 PM »
From the standpoint of telling someone how to construct the plane the method of saying "so-and-so 18% airfoil, plus a so-and-so long flap" makes sense.  It doesn't do a bit of good for analysis, of course.

So -- the whole flap stall thing makes sense.  Can you point to any posts that show how to make the flap?  Just make it thicker than the fixed portion TE, or what?

Al Rabe came up with the idea for scale stunters.  He likes large flaps and basically drew up the airfoil with the flap at about 30 deg. deflection and faired a curve from the high point(~30% of the fixed portion) and through the top suface of the flap.  Igor Burger came up with the same idea.  He posted his airfoil selection here, which I found on the hard disk.
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Offline phil c

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Re: Real Airfoil Thickness
« Reply #13 on: February 25, 2015, 10:03:57 PM »
I hear that it's standard practice in full size aviation to ignore the flaps in aerofoil proportion figures, so people with full-size aviation background carried the convention over to aeromodelling.

Unlike full size, however, our flaps go both ways and are always part of the lifting surface, so in my unqualified opinion the convention is irrelevant and unhelpful to us.

Someone with a qualified opinion will be along any moment now to reveal the truth.

Designers of full size planes ignore the flaps when the plane is flying flaps up.  But you can bet a dollar they take a great interest in the flaps when they are extended.  I was always amazed flying in the early 727's.  I'd book a seat just behind the wing to I could watch in turn into four interconnected wings angled up to 45deg from the fuselage.  It sort of disassembled for landing, less so for takeoff.  But I'm sure the designers looked long and hard at how the various flaps worked together.  Then later they had to restrict the use of full flaps.  With full flaps it was too easy for the pilots to get behind the flight curve when landing.  There were several situations where the flaps caused more drag than the engines could handle at critical times.
phil Cartier

Offline Chuck_Smith

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Re: Real Airfoil Thickness
« Reply #14 on: May 16, 2015, 06:53:01 AM »
Flaps are pretty misunderstood. Generally on full scale they're used to increase the sink rate and lower the landing speed. Current practice on most full size is that they also significantly increase the wing area when deployed.

On our controline models I pondered for many years why on earth I wanted a movable surface on the wing that increases the moment opposite of the horizontal tail, which would reduce the total pitching moment. I also knew that a Flite Streak, Shoestring or Sneeker would turn pretty danged tight without a flap.

Years of study and application later I kindof sortof think I understand it now.

Like all things aeronautical, you have to remember that what you do in the front of the airplane affects the back of it, i.e., you alter the flow field around the aircraft, not just the wing. That's the key thing to remember.

Besides shifting the lift vs AoA curve to the left flaps do two other major things at the airplane level:

i) they change the wing's angle of attack since the airfoil's centerline has been shifted.

ii) and this may be the most important on a CL ship...they alter the flow field around the horizontal tail. They change the trim angle of the horizontal stab.

Number ii above explains why, many many years ago while flying a Top Flight P63 with an ENYA .35 ( with the square black plastic venturi insert! )  I had the elevator horn break and was still able to maintain some control with the flaps only. In airfoil only theory, the controls should have reversed. In practice, they did not.

Later in life, on my full-size sailplane I did some flight testing, since they flaps went +90/-10 degrees. I "locked" the stick with my hand and used the flaps to see if I could maintain pitch control with no elevator input and lo and behold... flaps go up, nose goes down. Flaps go down, nose goes up.


Also, a comment was made that flaps can only move so much before a stall. Gross over-simplification. It's dependent upon the flap chord. Case in point my sailplane again: with 90 degrees of flap the wing was not anywhere near stalled.

I will admit however, that in a aircraft with no power and 90 degrees of flap deployed, the view out of the bubble canopy is...impressive! You get a very good view of your intended landing area.

But back to flaps on a CL plane - and I have no numbers to back this up - here's what I think happens:

Flaps go down, nose down pitching moment of the wing is INCREASED but the change in AoA plus the increase in downwash on the tail causes a net overall increase in the nose up pitching moment. In other words, the total nose-up pitching moment of the aircraft is increased even though the wing's contribution is against it.

YMMV.

And you know, now that I think about it, increasing the wing's AoA will INCREASE the nose-down pitching moment, since the CG is ahead of the center of pressure, so it must have more to do with the downwash.

Crap, we already know that. When an airplane stalls the nose drops. Your flight instructor told you that's because the wing isn't making lift anymore. But that can't be, because the lift is behind the CG, so the nose would go up if the lift vector went to zero. We also know from wind tunnel data that a stalled wing is in fact, creating tremendous lift.  The reason the nose drops abruptly is due to the change in moment and flow field! Whoohoo!




« Last Edit: May 16, 2015, 07:10:42 AM by Chuck_Smith »
AMA 76478

Offline Howard Rush

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Re: Real Airfoil Thickness
« Reply #15 on: May 16, 2015, 11:56:29 AM »
This, I presume, is a joke.  
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Offline Chuck_Smith

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Re: Real Airfoil Thickness
« Reply #16 on: May 16, 2015, 02:28:10 PM »
This, I presume, is a joke.  

Howard, to which part are you referring?
AMA 76478

Offline Howard Rush

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Re: Real Airfoil Thickness
« Reply #17 on: May 16, 2015, 03:57:10 PM »
Well, you are correct that flaps are pretty misunderstood. 

You give flaps credit for changing angle of attack because you define angle of attack funny.  Then you go on to say that the aerodynamic effect of flaps is to make downwash to help the tail pitch the airplane.  On the list of what flaps do for a stunt plane, that's in sixth place or so.  If you are actually being serious, then explain what Igor's nonlinear flap mechanism does. 
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Online Brett Buck

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Re: Real Airfoil Thickness
« Reply #18 on: May 16, 2015, 11:30:15 PM »
It's funny when it's not happening to me.

    Brett

Offline Howard Rush

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Re: Real Airfoil Thickness
« Reply #19 on: May 21, 2015, 03:47:07 PM »
Now that I think of it, Chuck has a point.  Downwash may do more than I thought.  I disconnected the balance tabs on the flaps on my white dog a couple of years ago.  Yesterday, after getting the dog trimmed pretty well, I hooked them back up.  The result was a softer turn.  The tabs are inboard on both wings, in front of the stab.  My guess is that the change in downwash from the tabs had more effect on pitching moment per unit handle deflection than the reduction in hinge moment from the tabs.

Downwash would also put the tail at a little more favorable angle of attack to rotate the airplane.  If one were to have a really long tail moment arm, the reduced downwash (it peters out the farther back from the wing you go) and the effect of flying in curved air (Cmq) could cause the tail to operate at a really high angle of attack.

Chuck refers to downwash with respect to flap, rather than downwash with respect to angle of attack.  The latter is destabilizing and evil and tainted my opinion of the former.
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Offline Ted Fancher

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Re: Real Airfoil Thickness
« Reply #20 on: May 27, 2015, 03:58:52 PM »
Alas,  I still believe flaps are part of the wing.  I fully understand, however, the consternation over how to deal with complex types of flaps (fowler, etc.) that not only deflect but also extend thus increasing wing area, aspect ratio and--I still believe for lay purposes-alter the basic, "flaps up" airfoil and its characteristics to include pitching moment and lift/drag coefficients.

I'm out of my element here but would be willing to bet that each flap setting from zero to max deflection (as many as six or maybe more in the various complex aircraft I flew) was designed to produce lift/drag/pitching moment etc. characteristics of the total wing appropriate to the designed speed range of the aircraft for each setting...i.e. high speed cruise, take-off and terminal area maneuvering, efficient rate of climb or descent (at various speeds as required by ATC), approach and landing.  Each flap setting configuration is thus"designed" to produce a complete (but different) airfoil consistent with the needs of the aircraft weight/thrust available equation for the particular stage of flight.  Once the various airfoils are determined and plotted it would seem the balance of the engineering effort would be mechanical...i.e. how to move the movable hunks from one position to the next.

I've no reason-nor the expertise-to debate the manner in which aero engineers go about configuring their equations for designing those multiple airfoils produced by complex flaps; I only attempt to make the case that the airfoils that results from those equations and the resulting cross section of the wing make up "the" airfoil by which the aircraft is supported during those multiple and independent lift/drag demands of all the stages of flight between takeoff and landing.

I'd pretty much bet the mortgage that a 747 couldn't take off and land on today's runways if you simply discarded the flaps and leading edge devices to save weight.  And I'd double down on that if they somehow figured out how to take away the wing and just employ the flaps and leading edge devices.  It's sort of like making babies.  You've pretty much got to have both the XX's and XY's to make it fly.

Ted

Ooops.  I should have specified that "extending" the flaps "reduces" the aspect ratio of the wing (by increasing the average chord) in that first paragraph.  I failed to note that the verb "increasing" was directed at all that followed.  sorry about that.  Amazed Howard didn't roast me on that one! 
« Last Edit: May 28, 2015, 11:57:38 PM by Ted Fancher »

Offline Air Ministry .

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Re: Real Airfoil Thickness
« Reply #21 on: June 10, 2015, 06:07:06 AM »
the REAL airfoil Thickness of THIS is 27% with a Real A/R of 8.5:1 .



a bit like flying in glue , but seeing it weighed twice what I hoped ( about 2.2 kilo )
the olde E = Mc2 bit was about average . Had it working satisfactorilly ( maybe ) up Qld
on the 10 x 4 three blades , but the 10 x 6 were a dead loss , Despite succeding in a wingover
in comp. on its second voyage into the lufte .

Also a ' sub compact ' is a trifle little for transport , though the new Ute'll manadge it .



Flaps are intregal to airfoil , thus about a inch thick inboard , 3/4 in outerish a way .
As theyre maybe 2/3 of the span span .

Offline Howard Rush

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Re: Real Airfoil Thickness
« Reply #22 on: June 10, 2015, 08:02:52 AM »
That is very cool. 
The Jive Combat Team
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