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Sep 24, 2002, 02:51 PM
Arizona Rim Country

What the Heck it Nasa Droop

I kind of understand the concept, but putting it into practice eludes me. Ron P. did a good job of describing it but still not quite getting it.

I found this article and it helps some. Ron, you got any pictures or diagrams ?
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Sep 24, 2002, 05:06 PM
Leave me alone!
Martin Hunter's Avatar
OK, so it's not a foamie, but it's applicable to your question. My latest wing for my Cessna 180 has droop leading edges on the wings. Here's a shot

Sep 24, 2002, 08:29 PM
Registered User

NASA Droop

I too had a hard time finding info and understanding what it is it could do for me. Tom Hunt is the one who pointed me onto it.

He inital suggested as a way of adding washout, but i found it also dragged up the airframe of my flying wing without too much pitch change. (Droop does drop nose a little). it also tames the stall by alot.

Not only that, but i incorporated 5 degree negative droop, that i will fool with to see if it can negate washout drag at high speed and perhaps help with inverted flight.

the 180 droop for sure increases lift at slow speeds, but hurts on the top end unless you can retract.

See photo of what i did at:

I am using a Hitec digital version of the 225 pushing on a cable and actuating a Beckman concealed hinge.

Fool with it as i suggested, if you dont like, just glue them back on. If you have not fooled with Elmers proBond, now may be a good time.

ron parigoris
Sep 24, 2002, 08:56 PM
Registered User
LEROY's Avatar
Looks like a fixed-position version of a leading edge flap, or slat, as it's called. The traditional slat allows an increased maximum coefficient of lift as well as an increased maximum angle of attack. Kinda like flaps, only without the decrease in max AOA. Works well at slow speeds on full-size aircraft.

I can only assume the "NASA Droop" is a version of what I have described. At the very least it would increase the camber of the airfoil.
Sep 24, 2002, 09:12 PM
Oxymoron: Rap Music
GForceStress's Avatar
very similar to 'slats' on the real planes (most huge ones like 777, 747 etc)

Sep 24, 2002, 09:22 PM
Registered User

Click aircraft, then features, then wing.
Sep 24, 2002, 09:29 PM
Registered User

NASA droop concept

Hope I can shed some light on the question. Please forgive the long post.
I was the NASA team leader (now retired) on the project to develop the droop concept at NASA Langley Research Center in Hampton, VA. As part of a program to increase the inherent spin resistance of G.A. airplanes, we tried to prevent stall on the outer wing with a passive, non-moving, low-cost device that would have a minimal impact on performance. After many wind-tunnel tests, trying slots, stall strips, and full-span droop (as in Martin's model), we found that the secret was to begin the droop at the wingtip and abruptly end the droop at a distance of about 40 percent inboard toward the fuselage. The droop was not as exaggerated as some of you are using, in fact it did not cause any noticeable decrease in cruise speed of 3 full-scale airplanes (Yankee, Skyhawk, and Sundowner). In every case, the spin resistance was dramatically increased--when our pilots intentionally tried to spin the airplanes by holding in pro-spin controls for as long as 10 seconds, spin entries were reduced from 70 to 80 percent for the basic airplanes to about 5 percent!

Now, the secret to the concept is the discontinuity. It basically creates a vortex that originates from the notch near stall and sweeps over the wing making the outer wing act like a low-aspect ratio wing (with a very high stall angle of attack) while the inner wing stalls first. In fact, we found that when we faired the droop and removed the discontinuity, each of the 3 airplanes entered spins easier and spun flatter! The real surprize to the engineering community was that the rectangular wings on these airplanes had extensive spanwise flow to the tips near stall, and that the vortex acted like a fence. Today's Lancair and Cirrus airplanes use the droop concept. One word of caution: This concept was developed for wings with aspect ratios of about 7. When you go to higher aspect ratios (as on most gliders) the stall pattern is different and the droop configuration must be modified.

This was a very satisfying project, and we felt we pointed the way to significantly decrease the number of inadvertent stall/spin accidents in general aviation. I also enjoyed working with Dave Robelen, who began his now well-known status as R/C model guru with me back at Langley and flew many R/C models to validate the concept!

Within about 3 months, NASA will be publishing a book I have written --for public consumption--on NASA's contributions to current civil aircraft (airfoils, winglets, crashworthiness, lightning protection, etc.), including the complete story on this program. I'll keep everyone posted on book status. Meanwhile, an outstanding summary of applications of the concept to R/C models is given in Andy Lennon's book "R/C model Aircraft Design" from Model Airplane News--give a look at his chapter entitled "The NASA Safe Wing"!!!

BTW, thanks so much to all of you Forum dwellers for the helpful tips to get me going in this hobby!!

Martin, can you please tell me about the B-17 model hanging on the wall in the photo of your Cessna??

Sep 24, 2002, 10:57 PM
Registered User

variable drooping wings

Joe, I found your reply very interesting and informative. I have a question that i think you might be able to help me with. The concept is drooping wings that you can change the angle of droop. I was thinking in this concept that using this wing to control roll instead of alerions. In my thinking, You could control roll with less drag. My question is do you think Changeing the shape of the camber(per say)of the wing to change the high and low pressure to control roll would be as effective as using aleroins. I built a prototypye of this wing and found it to fly and roll but it was extremely sensitave. Droop deflection over 10 degrees in either way was more than enough to get roll from the wing.

And I would also like to see that B17? too Dreamer

David G
Sep 24, 2002, 11:51 PM
Registered User
Ron, can you post more photos on you NASA Droops, the more the better.
Sep 25, 2002, 10:09 AM
Registered User


Thank You, both for your original contribution to general aviation by performing your work, and for chiming in on this forum to provide a brief and clear description of the basics of the results of the work your team performed. One thing that forums like this need more of is a reduction in speculation and a corresponding increase in formal study, evaluation, and engineering.

Thank You.

Scott Winans
Sep 25, 2002, 01:02 PM
Arizona Rim Country

Ron P

On your flying wing picture it looks like you've added the droop to the out-baord leading edges. But, from what I've read the Droop should be on the in-board. I'm still just trying to get a better grip on the specifics.
Sep 25, 2002, 01:38 PM
Registered User

Hey pcaffeldt

Where did you read you want to reduce stall speed and thus put NASA LE Droop on inboard of wing?

If you want STOL then more camber and lots of the wing should have Slats or NASA Droop.

I have read about NASA Droop a bit and have not seen anywhere about putting just on the inboard section of wing.

I am using droop for a slight different reason than just keeping the tips flying.

The Tips of my wing stall before the center. Either with low airspeed or pulling Gs, the plane will enter into a spin PRONTO!

I wanted to keep the tips flying to prevent this. When you build a wing, you can prevent by putting in washout. however with too much it creates alot of drag. On an existing wing, it is kinda hard to put in twist. NASA Droop nets as more washout by lowering the LE! It also decreases stall speed a bit, and allows you to fly closer to stall because of the now more docile stall and easy recovery. It also drags up the wing to allow steepier approaches, and ability to bleed airspeed quicker, net you can land much shorter.

You see Lead Edge Cuffs, NASA Droop, Lead Edge slats and flaps and all other to net allow slower flight.

I am shoehorning the NASA Droop:

to act as a means of inducing drag without pitching my flying wing too much (if i had an elevator, adding flaps would work but flaps will pitch a flying wing nose down)

to lower stall speed a bit by effective increasing camber on the outter 12 inches

allowing me to fly closer to stall with confidence because the tips do not stall first. The center stalls first, but even when it does, a wing does not drop anywhere as violent and does not enter a spin very easily. Without droop nose slightly low and banking 45 degrees can cause a tip to stall (slight extra G force, remember holding altitude in a 60 degree bank pulls 2 gs), and a perfect 1 point landing will happen real quick. With droop if you pull too many Gs too slow, it kinda mushes and when you release back pressure it is flying again.

Leaving Droop down slows top speed. You can really feel it. If flying less than 60 MPH, I just drop 1/2 droop and it is a nice thing to fly, over 60 mph, droop up and drag goes away, too high on final with 1/2 droop, put in full droop and drop nose so as to not overshoot.

I also want to fool with the negative droop as a way of getting rid of even more drag at high speed. The wing does have 3 degree washout, and that probably creates drag at high speed, I am thinking that the negative droop will undrag a bit, also may help with inverted flight as washout when upside down is washin which is not a good thing for stalls.

If building a moderate fast wing, you could add more twist in the building stage and blunt the LE a bit and have a wing that handles OK. But at high speeds will be a bit draggy.

On a typical airplane you see flaps on the trail edge. This nets not only more drag, but a slower stall speed for that portion of the wing. It also puts that portion of the wing at a higher angle of attack compared to the tips. so when you stall, the tips will tend to keep flying and help not entering an unwanted spin.

WW2 ME109s had LE pop outs to help with slow and tip stalls. They were speed actuated. If one stuck in the reatacted position, it was far worst than not having in the first place. Many a pilot chose to wire them closed and deal with a known ill slow handeling aeroplane instead of one that may spin RIGHT NOW when going slow, or not.

Ron parigoris
Sep 25, 2002, 02:00 PM
Arizona Rim Country

Ron, Thanks for your patience

I'm building and flying Zagi type flying wings with electirc power. Tip stall is a problem. I've built some out of solid foam, some out of fan fold foam, and the next one will be out of Correx/Coroplast. I'm fiddling witht the idea of giving the NASA Droop a try as opposed to wash-out of in conjunction with wash-out.

Would I be right, then, in thinking that to add the NASA Droop to a flying wing I would make the changes to the out-board 60% of the leading edge ?
Sep 25, 2002, 02:52 PM
Registered User

Hey pcaffeldt

If you are building from scratch, i would tend towards adding a bit more twist, and or blunting the lead Edge a bit more at the tips (I have good sucess with taper blunting 1/8 inch off a THL 12 inches from the tip). That is if it is going to be stationary droop and you can twist wing.

With Coroplast, yes i think if you taper Drooped 60 or 70% to 15 or 20% chord at the tip it would give you less tip stalling.

Now articulating droop or even ground adjustable (with coroplast, just double stickey tape some thin aluminium and bend to adjust) would be a neat way to go to dial in wing. Once you see how it works, you could just make them fixed in future set at the sweet spot, but my feelings are once you fool with adjustable, you will look foward to fooling with it.

If you do a search on NASA Droop someone posted a pic of a i think coroplast jet looking wing that just bent the LE of the coroplast. Think he said it worked well.

BTW, the only difference between a tric RC 400 wing and a THL wing, is the THL has a bit of NASA LE Droop the entire span of wing. Makes the THL only a bit more draggy, but better slow, and worst inverted. I like the THL. Have sticks that range with AUW of ~18oz to 45 oz on the same wing. At 45 oz it still glides OK, and i blunted 1/8 off the entire span, and taper blunted another 1/8 off the outer 12 inches.

Oh yea, you could always add washout with twisting elevons, especial if wider at the tips.

On my fast and heavy wings, i make elevons a % of chord, and it nets wider at center and narrower at tip. To keep wing area same, i cut TE to accomodate. Now the elevons are thicker at center but the TE of elevons are the same. twisting would not be too effective like this.

I think more efficient to move large surface alot, as compared to smaller a little. Then slow control is good to move the larger surface more!

Bass works well for durable elevons on heavys that need robust elevons that do not flex.

ron Parigoris
Sep 25, 2002, 03:15 PM
Arizona Rim Country


Can you see that look of recognition starting to gleam in my eyes !

What connected most with me in your last comment was the practical application of a thin piece of aluminum sticky taped to the leading edge.

On a 24" wing, from the wing tip inward, what should the length of aluminum be ?
Sep 25, 2002, 03:16 PM
Registered User
That's what they've done since we landed on the Moon.
Sorry, couldn't resist it, but you all know it's true.
Sep 25, 2002, 03:30 PM
Registered User

Hey pcaffeldt

I was kinda sortta hinting at making a taper bend in the coroplast at ~ 60% span in from tip and ~15 to 20% chord at tip.

Now to adjust this bend, you would only need a thin piece of aluminium double stickey over the bend.

Perhaps you could even get away with some of the aluminium tape used on those Dyna whatevers, you know the machines that indent letters and numbers.

wellllll, if you are gonna use that stuff, you may as well indent NASA Lead Edge droop by pcaffeldt on the stuff!

If it is a foamie or built up LE, you could hinge on the bottom with the same stuff, and put some on the top gap to hold in place. real easy to ground adjust on the foamie, just peel off top, re position and reapply the aluminium.

You could also hinge on the bottom with 375 3M packaging tape, make different spacers out of balsa or similar, and insert spacer and tape top with 375!

Let us know how you like it.

Since you are sound like a tinkererererrer, get hold of some aluminium roof flashing, you could easily use that instead of dyna whatever tape, but you can fool with making a top central located spoiler. try about an inch and a half wide and 3 inches tall and tape to top of wing just ahead of CG. prop will sound terrable due to turbulance, but check out steep glide angle. If you dont articulate NASA Droop, perhaps you wanna fool with putting an articulatable spoiler, also mess with putting these (start smaller) on the wing tips, or extensions of tip rudders. You can use rag at tips to make turns, and if you deploy at same time, you increase glide angle.

Ron Parigoris
Sep 25, 2002, 09:22 PM
Registered User

Photos of NASA Droop

Here's a series of photos (I hope!) that give some info on the droop we developed. First, here's a modified Piper T-Tail with the droop. Note the extent of span covered, and the abrupt end of the droop. Impact on cruise speed was less than 2mph.
Sep 25, 2002, 09:29 PM
Registered User


I tried posting an 80k jpeg. Uploaded O.K., but doesn't appear. Anybody know the cure??
Sep 25, 2002, 10:26 PM
Registered User

One more time

Sep 25, 2002, 10:54 PM
Registered User

Wind tunnel pics

Here are a series of wind tunnel pics that demonstrate what the NASA droop does. They were taken using flourescent oils that give the engineer a picture of attached and separated flows. You look for dark blue areas that indicate attached flow, and the bright stuff shows stalled areas. Take a look at these pics:

First, here's the baseline model at 16deg, right on the verge of stall. Note the good stall pattern, with the wingtips still flying while the inboard wing has stalled.

The wing stalls with just a little more angle of attack (about 17 degrees). Here's a picture at 30 degrees, showing the whole wing is stalled (this was the same from 17 degrees all the way up to 30).

Now let's compare the model with the droop. First, here's the model with the droop mod at 17 degrees, showing flow similar to the baseline.

Now here's the pic that shows the big payoff of the droop . This pic is at 35 degrees, showing that the stall has been confined to the inboard wing, and the tips are still flying! Fellows, that's DOUBLING the useful range of angle of attack, and tip stall is pushed so high in angle of attack that the pilot can't trim to that angle of attack. In other words, tip stall will never occur. Hope you grab the significance of this picture in preventing inadvertent tip stall/spin-ins for light planes. To show how long it takes for R&D to be adopted, we did this work almost 20 years ago!!

In our work, we even applied the concept to Burt Rutan's Varieze, using a full-scale model in our 30-by 60-ft wind tunnel at Langley. The focus was to prevent tip stall when the airplane was loaded way beyond the aft c.g. limit. When loaded to this extreme, the design can be forced to high angles of attack, where it has tip stall and can drop a wing, or "wing rock"--the droop completely cures it.

Here's a photo of Dave Robelen (now of RC Microflight fame) and an engineer during flight tests of an RC model to evaluate the droop for a design proposed by Devore. It dramatically increased the spin resistance.

One of the most interesting things about fluid flows on wings is how you have to interact with the natural progression of stall to control it, which depends on the aspect ratio of the wing. Here's a series of pics of a model showing the different stall patterns that occur for a rectangular wing with no washout. Note the different stall cells that develop as aspect ratio is changed, and how more cells are present for the higher aspect ratios!

As you would guess, that requires a different approach to the droop concept. here's apic of a Sweitzer powered glider, showing the droop configuration that worked. Ain't nature wonderful (and complicated)?!!
Sep 25, 2002, 10:59 PM
Registered User

Forgot punchline!!

Sorry guys--I'm gettin punchy tonite. Here's the payoff picture:
Sep 26, 2002, 09:04 AM
Arizona Rim Country

Spinner & Ron P

I really appreciate your spending the time and resources to explaining NASA Droop.

And Spinner - the old saying is true - your pictures were worth thousands of words !

Thanks Again !
Sep 26, 2002, 12:45 PM
Registered User

You're Welcome!

pcaffeldt, thanks for the good words. I hope the info has helped. I think the real key is to understand that we were driven to look for low cost, low maintenance, and negligible performance degradation for general aviation pilots. Large droops are unacceptable because of performance penalties, moving slats (a la Me-109) are unacceptable because of mechanical complexity/maintenance and handling quality issues as pointed out by several forum members, and full-span droops do something different. Let me explain this.

Putting a drooped leading edge, or slat all the way across the leading edge will increase the maximum lift capability of the wing and delay the stall to a higher angle of attack. As long as the pilot can't push the angle of attack higher, everything is fine. However, when many full-span droop configurations are pushed to stall, the wing drop and incipent spin are even more viscious than the baseline airplane. When the airfoil finally lets go, it kinda falls off the cliff. We saw this many times in our program. So the moral is, use a full-span droop with pitch control capability and limits in mind (a la Helio Courier, etc.) and it works fine. But part-span applications of the NASA droop works much better for spin protection. By the way, we wanted people to stop calling the concept a "droop" because it conjers up the wrong vision in people's minds (as evident here). The leading edge add-on does not droop below the lower surface of the airfoil. We called it the MOLE for Modified Outboard Leading Edge.

Quick comments: Dave G., using a leading-edge droop to effect roll control would not be good for 2 reasons--one, as I said above, the droop deflection will only increase lift near stall angles of attack, whereas a trailing edge flap or aileron will increase lift at all angles of attack up to stall. The second bad thing is that the droop increases drag much more, and causes a BIG bunch of adverse yaw that makes the airplane want to roll in the opposite direction. Roll control will be very squirrelly. BTW, today's researchers are looking at "smart" structures that allow the entire wing section to be continuously deformed, without any trailing or leading edge devices. Now that really opens the possibilities. rparigoris, you're right--the NASA concept is for the outboard portion of the wing, not the inboard. U812, believe it or not, hidden from public view in the "Space Agency" is a band of dedicated, enthusiastic civil servant researchers who represent the remnants of the NACA and fight the rest of the agency for funding to keep the first A --Aeronautics--in NASA alive. Unfortunately, the battle is being lost at an alarming rate as funds are cut for Space Stration, wind tunnels close, and opportunities for breakthroughs in aeronautics can't happen. Look for my new book and read what these guys have done for the nation's current civil and military aircraft, and you'll be impressed. And like me, take note as the Europeans pull ahead of us again with a well-funded, focused program in advanced aeronautical technolgies--just as they did in WWII. Enjoy your ride on Airbus products! Scott Winans, thanks for the words. I have an awful lot to thank you forum members for, so anything I can contribute is yours. Maybe one day a quick discussion of how to use RC models with correct scaling laws to replicated full-scale motions and how to instrument them with low-cost approaches would be in order.

Dreamer (Martin), you're still the guilty party whose postings of the Gym-E started me down this wonderful road of e-flight. Now, PLEASE tell me about that B-17 in your photo. Is it a modified Cargo, or what?!

Thanks to all, and keep up your experiments. I love it!!

Sep 26, 2002, 01:53 PM
Leave me alone!
Martin Hunter's Avatar

Re: You're Welcome!

Originally posted by Spinner
Dreamer (Martin), you're still the guilty party whose postings of the Gym-E started me down this wonderful road of e-flight. Now, PLEASE tell me about that B-17 in your photo. Is it a modified Cargo, or what?!
Sorry Joe... I missed the request earlier

Believe it or not, it's a Dave's Aircraft Works TG-3 that I've "slightly" modified. It's another slope combat warbird in my fleet. It's virtually unkillable. Sorry it's not electric

Sep 26, 2002, 06:45 PM
Registered User

Hey Spinner

Hey Spinner (although when you droop you may be not much of a spinner)

Great info. Thx.

Although it appears NASAs intention was to prevent spinning, NASA Droop works real nice as a device on a Flying Wing to increase drag without pitching too much!

It still prevents stalling of the tips first even at 30 plus degree droop 20% chord, but drags up alot. Pretty hard thing to simply drag up a flying wing and decrease stall speed at the same time. I am a little over 31 oz per square foot on a 2.55 square foot 47 inch span wing, and although spoilers would increase drag it would also decrease lift, not a desirable thing. Split elevons or doing crow, would drag up, but not necessarily decrease stall. Droop seems a pretty good, fairly easy resolution to drag, slower fly, faster fly.

Do you know if NASA ever investigated use of droop for this purpose on a flying wing?

Having the LE articulate, I can even go negative a bit, set right now to 5 degrees. i am thinking it may create less drag at high speed on my Slope Slayer as the wing has 3 degrees washout and this may negate it? Also i am thinking it will make in negative droop a good upside down flier.

Any comments?

Ron Parigoris
Sep 26, 2002, 06:51 PM
Registered User

Good Stuff!

Ron, I'm not aware of that particular application. As I used to tell my "Space Agency" bosses, there's still a lot to be learned in aeronautics!!

Good luck, and keep up the interest.

Sep 26, 2002, 10:14 PM
RPV builder & operator
Pierre Audette's Avatar
Joe, was there any connection between your work and the Avro Arrow? I understand that a number of Avro engineers ended up at NASA when the Arrow got scrubbed.
Sep 27, 2002, 12:11 AM
Registered User



No relationship there. I'm glad you brought up the topic of the use of notches and snags on swept wings (Avro, F-4, F-8, etc.). I think most everyone is familiar with the fact that those swept-wing aircraft use such devices to prevent spanwise flow building up at the tips due to sweepback, and that the mechanism is a vortex shed at the snag acting like a physical fence to prevent the outflow to the span.

What is surprizing about the NASA droop is the fact that it works on a wing with no sweep. Many, many people were surprized to see the amount of spanwise flow on the unswept rectangular wings near stall (note this phenomenon in my posted wind tunnel photos of the baseline wing at 30 degrees angle of attack). Goes to prove that stall is really a 3 dimensional issue, not just 2-D airfoil stuff!

Thanks to everyone for the interest and comments!!
Sep 14, 2011, 06:24 PM
Cascade Mtns Sloper

Vortilon versus vortex from droop discontinuity?

Originally Posted by Spinner
we found that the secret was to begin the droop at the wingtip and abruptly end the droop at a distance of about 40 percent inboard toward the fuselage.

Now, the secret to the concept is the discontinuity... we found that when we faired the droop and removed the discontinuity, each of the 3 airplanes entered spins easier and spun flatter! The real surprize to the engineering community was that the rectangular wings on these airplanes had extensive spanwise flow to the tips near stall, and that the vortex acted like a fence.

So Joe, how much spin resistance is given by the tip droop, and how much by the vortex from the discontinuity? Would a vortilon at 40% from tips do durn near as much?


Sep 15, 2011, 03:03 PM
Registered User
Originally Posted by pcaffeldt
I kind of understand the concept, but putting it into practice eludes me. Ron P. did a good job of describing it but still not quite getting it.

I found this article and it helps some. Ron, you got any pictures or diagrams ?
RcSuperPowers (Dave Powers) knows a large amount of information on this. They can help you alot. Look at their youtube video. You can also E-mail Dave directly and he can explain a lot more.

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