Ultralight Flaps

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Dana

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Why do you say it would separate? Just to clarify, the "dirty stuff" I am referring to would just be a couple of lever arms and some mounting tabs... i don't mean to imply that it would be completely devoid of decent airflow. Single-surface airfoils work well at low Reynolds numbers and at low speeds. In fact, they are quite efficient in that regime, no? Some of the low-Re eppler airfoils are actually profiled in a way that isn't too dissimilar to a single-surface airfoil. No?

Perhaps though, the LE should be closed off though to reduce drag, but the rest of the flap could still be a single sheet.
The way you originally drew it as a split flap is one thing, but then you were talking about separating it from the rest of the surface, and as I imagined what you were describing it sounded like an upside down Quicksilver single surface wing with a void behind the leading tube, which would make for horrible flow in an adverse pressure gradient.

Single surface airfoils work quite well... for a narrow AOA range... unless they have something to create a large leading edge radius, in which case they're not really single surface any more.

Now if you could somehow make a sliding upper surface that telescopes aft and down to increase wing area and camber...
 

Grimace

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The way you originally drew it as a split flap is one thing, but then you were talking about separating it from the rest of the surface, and as I imagined what you were describing it sounded like an upside down Quicksilver single surface wing with a void behind the leading tube, which would make for horrible flow in an adverse pressure gradient.

Single surface airfoils work quite well... for a narrow AOA range... unless they have something to create a large leading edge radius, in which case they're not really single surface any more.

Now if you could somehow make a sliding upper surface that telescopes aft and down to increase wing area and camber...
I haven't rigidly defined the nuts bolts and angles at this point. Before I even go that far, the first question is whether a single-surface sheet of CF would even make a reasonable flap (whether with or without a leading edge). My "idea" is that I think this would be feasible (still waiting to be told/convinced otherwise), reasonably light, and require relatively little time to make, and wouldn't be very expensive (maybe 7 - 14 yards... so less than $400 in materials). And it would take up minimal space, making multiple surfaces like you mentioned a possibility.

Sliding back from underneath the top surface and extending backwards? Yeah. I think someone could do flaps like that, out of carbon, two elements, and still keep the weight very manageable... probably not tracks though, too heavy. Something like a pushrod and guide rods would be easier to manufacture.
 
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Grimace

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So, let's say I could create a lightweight and compact set of dual slotted fowler flaps for an ultralight glider. Would that even be desireable? 1) I would think you would be in jeopardy of severly nose-down attitudes. 2) it would create a lot of drag. Would this even possibly be justifiable? (I know it depends on max Cl/Cd but since the shapes would be a bit unconventional, I'm asking if anybody thinks the answer is at least "maybe" before I spend a lot of time on the idea. I don't think I've ever seen fowlers on a glider.
 
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BBerson

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Before I even go that far, the first question is whether a single-surface sheet of CF would even make a reasonable
All split flaps are single surface that I've seen. At least the Grumman Goose is single surface and I think C-310.
But not any use on an ultralight glider, except for a dive brake.
 

Grimace

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... few airfoils can really accommodate the entire speed range and stay in the low-drag operating range of the airfoil. But these types of gliders have hard-surface wings, or wings with mostly hard surface and a relatively small area covered in smooth plastic film. They never use single-surface airfoils or "traditional" fabric covering like the airchairs do.
Right. But my thinking is that there is a middle ground. This wouldn't be a glider built to mission requirements, but rather an answer to the question - How much performance can I squeeze out of an aircraft that is ridiculously simple to produce? If I could make triple-slotted fowler flaps out of a flat 2D mold and duct tape, well darnit, I'll include a ridiculous flap arrangement. But if it requires specialized jigs and a million dollar facility, then that's out. Are there any complex features I can include cheaply in an innovative way? I think I can do retractable gear and keep it under 10 lbs. So that's an add. I can't do (don't think I can do) a complex wing shape without large molds, so that's an omit (though I haven't thought this part all the way through). I think I can add about 100lbs of water ballast easily, so that's a maybe (because, though legal, I wonder how the FAA would view a 500lb ultralight. Would I be inviting controversy?) So now I had the idea of a completely thin flat CF flap in lieu of the traditional built-up flap. If you and Dana and others tell me "hrmmm... I dunno... maybe..." then I will fiddle with the idea. But if you convince me I am out in left field, then I can concentrate elsewhere. I'm not asking you all to design something for me. I'm just trying to see if this idea has some merit worth exploring. My ideal would be for you to convince me that a flap needs to be fat and thick and take up the entire rear third of a wing, even in an ulralight... because then I would simply move on to the next challenge.
 
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Grimace

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All split flaps are single surface that I've seen. At least the Grumman Goose is single surface and I think C-310.
But not any use on an ultralight glider, except for a dive brake.
It doesn't have to be a split flap. I agree split flaps are by and large single surface and less than optimal. My focus right now is on whether you can just have a flat sheet of a few layers of CF as a flap. Do you really need to build it up, or not? If I can slap down a few layers of CF and easily squeeze it into the rear third of a wing, then options are opened. It could be set up easily to have a gap though. But if it is thin enough, then maybe squeezing another element in there might be doable.
 

BBerson

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A thin sheet won't do it. The Grumman split flap has a lower skin with a full network of ribs and stringers. Just no upper skin.
 

Dana

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It comes back to what are you trying to achieve? If your aerodynamic design indicates a need for flaps to achieve your mission, then by all means work out the best way to incorporate them. "Best" can mean a combination of cost, simplicity, weight, performance, reliability, etc. If, OTOH, you want to add flaps just because you have a cool way to build them, you may be better devoting your effort elsewhere. Though I'll add that in homebuilt aviation, "because it's cool" can be a valid reason, just look at some of Mark Stull's designs... but remember that "coolness" may impact the flying characteristics in other undesirable ways.
 

BJC

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So, let's say I could create a lightweight and compact set of dual slotted fowler flaps for an ultralight glider. Would that even be desireable?
If you mean a high performance ultralight glider like the Sparrowhawk, the answer is no. https://www.compositesworld.com/articles/sparrowhawk-all-carbon-composite-sailplane
1) I would think you would be in jeopardy of severly nose-down attitudes. 2) it would create a lot of drag. Would this even possibly be justifiable? (I know it depends on max Cl/Cd but since the shapes would be a bit unconventional, I'm asking if anybody thinks the answer is at least "maybe" before I spend a lot of time on the idea. I don't think I've ever seen fowlers on a glider.
For a hang glider or Goat class glider, where minimum sink is more important that penetration speed or L/D, it seems to me (no study or calculations involved) that you would be better served by adding wing area than a complicated flap structure.

Tell us more about the type of glider that you envision and exactly what you want to gain by adding flaps.


BJC
 

pictsidhe

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It doesn't have to be a split flap. I agree split flaps are by and large single surface and less than optimal. My focus right now is on whether you can just have a flat sheet of a few layers of CF as a flap. Do you really need to build it up, or not? If I can slap down a few layers of CF and easily squeeze it into the rear third of a wing, then options are opened. It could be set up easily to have a gap though. But if it is thin enough, then maybe squeezing another element in there might be doable.
Work out the loads on your flap, and do the maths. Or test and check deflection is within acceptable limits. As someone else pointed out, ribs will make it easier. I'm currently planning split flaps for my project, with ribs and no upper surface on the flaps. This was a very common way of building split flaps. I also want to continue ribs on the upper wing surface back through the flap. Though I may decide that this is a PITA and put plain flaps on instead.
 

Grimace

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A thin sheet won't do it. The Grumman split flap has a lower skin with a full network of ribs and stringers. Just no upper skin.
First, the Grumman weighs a lot more than an ultralight. Second, somewhere I have a bar made of 8 plies of carbon fiber. Of course, I'd need to do proper testing on any composite structure, but believe me, it's plenty strong for an ultralight flap.
 

Grimace

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It comes back to what are you trying to achieve? ... "Best" can mean a combination of cost, simplicity, weight, performance, reliability, etc. If, OTOH, you want to add flaps just because you have a cool way to build them, you may be better devoting your effort elsewhere.
To my mind, something that hasn't been done before and actually provides a new way of doing things is reason enough as long as it is appropriate. I'm not talking about JATO bottles on a Lancair... but maybe on a Wilga... ;) I was just looking for an easy way to do flaps, then I realized that perhaps the easiest way is also uniquely efficient in terms of space. So why wouldn't you explore that? If it improves the design over an unflapped version, then it's worth tinkering with, in my opinion.
 

Topaz

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Right. But my thinking is that there is a middle ground. This wouldn't be a glider built to mission requirements, but rather an answer to the question - How much performance can I squeeze out of an aircraft that is ridiculously simple to produce? ... If you and Dana and others tell me "hrmmm... I dunno... maybe..." then I will fiddle with the idea. But if you convince me I am out in left field, then I can concentrate elsewhere. I'm not asking you all to design something for me. I'm just trying to see if this idea has some merit worth exploring. My ideal would be for you to convince me that a flap needs to be fat and thick and take up the entire rear third of a wing, even in an ulralight... because then I would simply move on to the next challenge.
To my mind, something that hasn't been done before and actually provides a new way of doing things is reason enough as long as it is appropriate. I'm not talking about JATO bottles on a Lancair... but maybe on a Wilga... ;) I was just looking for an easy way to do flaps, then I realized that perhaps the easiest way is also uniquely efficient in terms of space. So why wouldn't you explore that? If it improves the design over an unflapped version, then it's worth tinkering with, in my opinion.
What I'm hoping to understand is, fundamentally, how you feel split flaps improve the performance of a ultralight-style fabric-covered glider wing? It's irrelevant to me whether or not it can be done (I think, if I understand your drawings, it certainly can be). What's relevant to me is what it brings to the design that gives that design higher performance, against some metric that is, as yet, unspecified.

I understand that you're averse to designing to a specific design mission and, for a homebuilt, that's perfectly okay. But when you say,

... How much performance can I squeeze out of an aircraft that is ridiculously simple to produce? ...
... what do you mean, exactly?

Are you trying to get better L/D? A split flap won't do that. It might allow a higher wing loading while maintaining a desired stall speed but, as noted before, Part 103 gliders have no regulatory stall speed or maximum speed limitations in the first place. As such, it might be better to accept a lighter, simpler, easier-to-build wing structure with no flaps, increase the wing loading by making a smaller wing, and take the "penalty" of a higher stall speed. A key thing to remember is that raising the wing loading will generally increase the speed at which L/Dmax occurs, which is one reason sailplanes use water ballast, but it won't really improve L/D itself.

Are you trying to get a lower minimum-sink rate? A split flap won't do that, either. What you need for that is span and low wing loading. Split flaps might allow you to maintain some arbitrary stall speed with an increased wing loading, but that's the opposite of what you want to reduce the minimum sink rate.

As I noted above, sailplanes have flaps generally to change the camber of their wing airfoil, so as to keep the "drag bucket" in the CL versus alpha curve of the airfoil centered around whatever CL the wing happens to be operating at a given portion of the flight. But camber-changing flaps are essentially plain flaps, and split flaps won't do that job without also adding a ton of parasite drag. That latter bit is exactly what you don't want. And a fabric-covered airfoil on a wing of typical ultralight construction won't support laminar flow in the first place, so there won't be any drag bucket to move up and down the CL versus alpha curve anyway. So camber-changing flaps have no purpose on a fabric-winged, ultralight-style glider.

My question is, fundamentally, not about whether your flap mechanism will work. I suspect it will. Rather, my question is, "Why do you want flaps on your fabric-winged glider?" It's another way of saying, "What are you hoping to achieve here?"

If you want to increase gliding performance as you say, span, aspect ratio, and wing loading are the parameters to play with, along with general drag reduction. Flaps (of any kind) aren't going to help you improve gliding performance on a traditional ultralight-style wing.[/QUOTE]
 

Bille Floyd

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It doesn't have to be a split flap.
...
My focus right now is on whether you can just have a flat sheet of a few layers of CF as a flap. Do you really need to build it up, or not?

If I can slap down a few layers of CF and easily squeeze it into the rear third of a wing, then options are opened. It could be set up easily to have a gap though. But if it is thin enough, then maybe squeezing another element in there might be doable.
The L-13 glider, had a retractable fowler flap ; i started a Thread
on it , to find out how it works :
https://www.homebuiltairplanes.com/forums/threads/blanik-l-13-glider-fowler-flap.32546/#post-500658

The Flap on my Exxtacy rigid, flying-wing , has a carbon Nomex honeycomb
sandwich flap , (non retractable) ; when i add flap , i also get twist, and
the flap gives me a WAY wider speed-range.

The Gostbuster glider , looks a Lot like my Exxtacy ; but it's
flap was fully retractable. Side by side , (Exxtacy vs Gostbuster)
with equal wing loading ; when my buddy retracted His flap
he would pull away from me, (with Ease) , and when deployed
we had near the same sink-rate because both our gliders, had the
same aria, and deg-down settings for flap.

I think flaps work good !

Bille
 
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lr27

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If you want it simple and easy, I suggest either plain or Junkers flaps. Unless you want spoilers too, deploying flaps adds drag, which may be useful for landings. However, if you droop the ailerons too, they'll be less effective just when you need them the most. So it may be handy to have a way to droop the flaps much more than ailerons for landings. If you use flaps for glide path control, keep the speed above the "clean" stall speed. Otherwise, if you retract the flaps to stretch the glide, you'll stall. I guess once you're sure you'll make the field, you can fly slower.

On some airfoils, plain flaps could be simply triangular in cross section without losing much. Junkers flaps would have to be shaped like little wings.

If you go to NASA's NTRS site, or to the Magic NACA Archive, you can find a number of older papers with useful, digestible information about flaps.

I think if I was doing an ultralight glider, and I wanted to minimize the effort for the performance, I'd use the effort the flaps and their controls would take to build a better wing instead.

A coreless, flat carbon fiber lamination used as a flap would be either too floppy, too heavy, or both.

The idea that the Reynolds numbers on ultralight are just too low for fancy aerodynamics is just wrong. Even on RC gliders, a properly chosen and precisely made airfoil can make a BIG difference. That's why serious competitors, at least in certain events, she'll out the big bucks for "moldies", which are made with methods similar to those used in full scale, composite racing sailplanes.
 

lr27

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If you want it simple and easy, I suggest either plain or Junkers flaps. Unless you want spoilers too, deploying flaps adds drag, which may be useful for landings. However, if you droop the ailerons too, they'll be less effective just when you need them the most. So it may be handy to have a way to droop the flaps much more than ailerons for landings. If you use flaps for glide path control, keep the speed above the "clean" stall speed. Otherwise, if you retract the flaps to stretch the glide, you'll stall. I guess once you're sure you'll make the field, you can fly slower.

On some airfoils, plain flaps could be simply triangular in cross section without losing much. Junkers flaps would have to be shaped like little wings.

If you go to NASA's NTRS site, or to the Magic NACA Archive, you can find a number of older papers with useful, digestible information about flaps.

I think if I was doing an ultralight glider, and I wanted to minimize the effort for the performance, I'd use the effort the flaps and their controls would take to build a better wing instead. Maybe one with sheeting back to 40 percent or so. Eppler's 749 airfoil was designed for that much sheeting and to be used in ultralight sailplanes. It's supposed to have relatively high lift, and reasonably low drag over a wide speed range. It's also quite thick. Unfortunately, it has quite a bit of pitching moment. It's predecessor, the Eppler 748, is similar.

Another airfoil I found interesting for this purpose is the 20 percent Ara-d. Similar virtues but maybe a little less sensitive to roughness. Plus a much smaller pitching moment. It's designed with a blunt trailing edge, and ought to do better with a thinner one.

I'm not aware of any aircraft that have used these foils, though.

A coreless, flat carbon fiber lamination used as a flap would be too floppy, too heavy, or both.
 

Bille Floyd

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If you want it simple and easy, I suggest either plain or Junkers flaps. Unless you want spoilers too, deploying flaps adds drag, which may be useful for landings. However, if you droop the ailerons too, they'll be less effective just when you need them the most. So it may be handy to have a way to droop the flaps much more than ailerons for landings. If you use flaps for glide path control, keep the speed above the "clean" stall speed. Otherwise, if you retract the flaps to stretch the glide, you'll stall. I guess once you're sure you'll make the field, you can fly slower.

On some airfoils, plain flaps could be simply triangular in cross section without losing much. Junkers flaps would have to be shaped like little wings.
...

A coreless, flat carbon fiber lamination used as a flap would be too floppy, too heavy, or both.
The Junker , is a bit similar to the Fowler Flap , when deployed ; Fowler
gives a , smaller sq / cleaner profile when retracted. I'm going for the Fowler.

I like the flap down 20-deg , with aileron down 5-deg settings ; but i'll use that
for thermaling, because i still get 15-deg more twist in the flying wing with
everything maxed out for min sink, ( it's a Really Good idea) Thanks !!!

For landing the flap could be down 20-deg and the aileron up 3 to 5 deg ; that
doges the glider out big-time, and makes for a real stable platform for landing, with
a L/D of crap .


I agree with Not reducing flap to extend glide ; unless i have
altitude to increase speed First , ( another prudent Idea) Thanks !!!

And Yes, the airfoiled Flap, must be rather rigid ; i have 4 sheets of
1/4" Nomex, and a roll of 3k carbon twill with uni, that should help
insure it happens.

I referred to Ailerons a few times above ; there will be no ailerons, only
elevator out there , and i'll turn the glider with the Stock spoilers.

I LIKE the way you think , "lr27" ; hoping for More input, please !!

Bille
 
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