Do vortex generators really work.

[Old Koreelah]

...on the low-wing setup on the Jodel...it may be necessary to position the stall strips slightly differently one side to the other, to get it to stall simultaneously; and power may alter that a bit.

Thanks for that detailed response, Dafydd. You may have seen the Jodel plans drawn by the late Frank Rogers. He recommended stall strips attached about 200mm out from the root. I didn't intall them but his design influenced the changes I described. Your fence and stall strips makes more sense so I'll try that. Preventing a wing drop is more important to me than a knot or two less airspeed.

 

I had hoped the wing changes and flaps would have delivered a greater reduction in stall speed. The offset of the flap hinges was pretty much guesswork based on what I'd seen and read, but maybe the airgap is not wide enough to energise the airflow over the flap. Wool tufts might tell me that my flaps are not doing their best.

 

The aircraft is performing pretty close to what I want, but I'm keen to make any improvments I can. Limited elevator authority has kept me out of trouble and I'm lucky to have a pretty reliable AoA meter: there is little to worry about until the long flat engine cowl gets in the way of the horizon and the stick is hard back.

 

 

[Dafydd Llewellyn]

Thanks for that detailed response, Dafydd. You may have seen the Jodel plans drawn by the late Frank Rogers. He recommended stall strips attached about 200mm out from the root. I didn't intall them but his design influenced the changes I described. Your fence and stall strips makes more sense so I'll try that. Preventing a wing drop is more important to me than a knot or two less airspeed.I had hoped the wing changes and flaps would have delivered a greater reduction in stall speed. The offset of the flap hinges was pretty much guesswork based on what I'd seen and read, but maybe the airgap is not wide enough to energise the airflow over the flap. Wool tufts might tell me that my flaps are not doing their best.

 

The aircraft is performing pretty close to what I want, but I'm keen to make any improvments I can. Limited elevator authority has kept me out of trouble and I'm lucky to have a pretty reliable AoA meter: there is little to worry about until the long flat engine cowl gets in the way of the horizon and the stick is hard back.

Your stall speed is probably being limited by elevator power, rather than by wing lift, except under the condition that gives the wing drop. This was the case with the Seeker, too; we had to fit VGs to the underside of the tailplane to get the necessary increase in elevator power to get a reduction in stall speed. But the fences & stall strips are much more powerful than just stall strips alone.

 

 

[Old Koreelah]

I have VGs under the tailplane; they may have helped a bit, and I also increased elevator area by about 5% by filling in the overly-wide gap next to the rudder. That's as much elevator authority I am going to get. It probably explains why the stall is lower and more controlled at 10 degrees flap than 20.

 

 

[Dafydd Llewellyn]

I have VGs under the tailplane; they may have helped a bit, and I also increased elevator area by about 5% by filling in the overly-wide gap next to the rudder. That's as much elevator authority I am going to get. It probably explains why the stall is lower and more controlled at 10 degrees flap than 20.

OK. Please let me know how you get on.

 

 

[djpacro]

See http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19720005341.pdf

Yes, a classic report however NASA did a lot of flight and spin-tunnel tests specifically on GA aircraft since that was published so there is much more information around (on specific configurations and effects of configuration changes so that summary is still valid) if anyone has that interest. The later series of tests were in flight, spin-tunnel, RC models plus the newish rotary-balance with computer analysis of spin modes.

 

The question is why do some spins go flat and other aircraft don't. .....

Dafydd's reference to Jim's report hopefully answered that. "Cliff-edge" effects are often encountered when doing mods that have an effect on spin characteristics. i.e. a mod on one aircraft type might have no effect yet have a dramatic effect on another type OR a mod may have nil effect but extending the mod a little bit can have a dramatic effect on the spin. So, need to be wary about predicting the effect of mods on spinning.I even surprised Jim one day in the spin-tunnel with my prediction of the effect of some mods - I was right (mainly because I had some data that he was unaware of) and he was wrong (because he had a lot of data on other mods to that type of aircraft but not the little bit of data that I had just seen) - so much cross-checking and re-measuring stuff before he accepted it.

 

 

[jetjr]

Any info on VG on std Jabiru short wing?

 

Plenty of anecdotal happy users.

 

Looking to reduce stall 3kts without upsetting things too much.

 

Ideas

 

Full set wing tip to root

 

Just infront of ailerons

 

Under elevator in both options I assume

 

 

[Dafydd Llewellyn]

I would advise you very strongly NOT to use VGs over the full span; read what I said in post #21 on this thread.

 

 

[Dafydd Llewellyn]

Yes, a classic report however NASA did a lot of flight and spin-tunnel tests specifically on GA aircraft since that was published so there is much more information around (on specific configurations and effects of configuration changes so that summary is still valid) if anyone has that interest. The later series of tests were in flight, spin-tunnel, RC models plus the newish rotary-balance with computer analysis of spin modes.Dafydd's reference to Jim's report hopefully answered that. "Cliff-edge" effects are often encountered when doing mods that have an effect on spin characteristics. i.e. a mod on one aircraft type might have no effect yet have a dramatic effect on another type OR a mod may have nil effect but extending the mod a little bit can have a dramatic effect on the spin. So, need to be wary about predicting the effect of mods on spinning.

 

I even surprised Jim one day in the spin-tunnel with my prediction of the effect of some mods - I was right (mainly because I had some data that he was unaware of) and he was wrong (because he had a lot of data on other mods to that type of aircraft but not the little bit of data that I had just seen) - so much cross-checking and re-measuring stuff before he accepted it.

Yes, DJ, I'd be most interested in links to later NASA reports in this area.

 

Yes; spin behaviour is one thing that does NOT reliably alter in a progressive manner as the critical parameters are changed. It can easily be "knife-edged". We have learned quite a bit about spin test techniques in Australia; we made a lot of mistakes, and were very lucky not to have killed test pilots and lost more prototypes than the GA-8. If anybody wishes to know about those mistakes and what we learned from them, please contact me; there is no point in repeating these mistakes - go find a fresh lot yourself.

 

 

[Old Koreelah]

OK. Please let me know how you get on.

Wilco.

 

 

[jetjr]

I would advise you very strongly NOT to use VGs over the full span; read what I said in post #21 on this thread.

The maker does recommend different spacing inside aileron span but definitely says do the lot. Few users have done so with "gentle stall characteristics"

Someone has done testing and have approval from CAA (UK???)for the mod...... Not sure what this mens or whats involved.

 

See http://www.stolspeed.com/id/96

 

Some interesting test videos on the site too comparing full wing and half installations- not sure the assessment or my understanding is up to it though

 

 

[fallowdeer]

Savannahs have full span VGs and seem by all accounts to have acceptable stall characteristics? Mine has had VGs added on the elevator.

 

 

[Dafydd Llewellyn]

I suspect that despite the VGs on the underside of the elevator, there is insufficient elevator power to really stall the wing. If you put VGs on the underside of the tailplane, well ahead of the elevator hinge line, the stall characteristics may give you one Hell of a fright. This is the sort of knife-edge thing DJ was talking about. You're playing with dynamite, here.

 

 

[Dafydd Llewellyn]

The maker does recommend different spacing inside aileron span but definitely says do the lot. Few users have done so with "gentle stall characteristics"Someone has done testing and have approval from CAA (UK???)for the mod...... Not sure what this mens or whats involved.

See http://www.stolspeed.com/id/96

 

Some interesting test videos on the site too comparing full wing and half installations- not sure the assessment or my understanding is up to it though

Do you want my advice, or do you just want me to agree with you? Putting VGs on the wing effectively alters the stall characteristics of the airfoil from one that stalls progressively from the trailing edge, to one that stalls abruptly from the leading edge. The Jabiru wing has no washout. If you have sufficient elevator power to get to the increased stall angle, the whole wing will likely stall instantaneously - the effect is much as though the wing has fallen off.

If there's an approved VG mod, one would presume that they tested it and sorted out any such issues. I can only speak from my direct personal experience; I would be extremely wary of any full-span installation.

 

 

[djpacro]

Quick updates on NASA's GA stall/spin flight test program.

 

https://www.youtube.com/watch?v=Kgzn9zILw6Q

 

http://crgis.ndc.nasa.gov/historic/Stall/Spin_Research

 

http://crgis.ndc.nasa.gov/historic/20-Foot_Spin_Tunnel_(645)_Models_and_Tests_A-M

 

http://crgis.ndc.nasa.gov/historic/20-Foot_Spin_Tunnel_(645)_Models_and_Tests_N-Z

 

http://whycirrus.com/engineering/NASA Stall Spin Paper from 1970s (WhyCirrus).pdf - Dafydd may find page 10 interesting

 

http://www.yourepeat.com/watch/?v=3cJkjk0hOU8

 

http://www.bihrle.com/bibliography.html

 

Then search the NASA tech report server for further details.

 

eg http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19870010807.pdf from my project

 

 

[Dafydd Llewellyn]

Many thanks, DJ.

 

 

[Gnarly Gnu]

Hey that temporary tail chute is neat, haven't heard of that before.

 

 

[Old Koreelah]

Often used in testing. Look at the early pix of the Supermarine Spitfire. It's beautiful lines were spoiled by an ugly big tube sticking out the back.

 

 

[Dafydd Llewellyn]

Called a "Spin recovery parachute" (What a surprise). Essential kit for a test pilot. Ever think about how you'd bale out of a spinning aeroplane? This is one bit of gear that really MUST work when you need it. That NASA one was obviously made before the days of low-porosity parachute fabric; one like that nowadays would oscillate violently. We've had to go to the cruciform style (like a dragster braking parachute) to get a canopy that will not oscillate. This is one of the things the Australian flight test fraternity had to learn the hard way. The canopy size has to be correct and the distance it is behind the aircraft is also critical. Notice that the one on the Seeker and the NASA one are very similar in both these regards. You either get this right or you're likely to be dead. Also, the size of 'chute required is too big to allow the aircraft to be landed with it deployed; the jettison mechanism is equally as important as the deploy mechanism.

 

It's a bit of equipment that you hope you'll not need; that NASA test pilot was intentionally taking the aeroplane to an unrecoverable flat spin, relying solely on the spin 'chute.

 

Now, how far do you really want to go in experimenting with a VG installation?

 

 

 

 

[Old Koreelah]

...how far do you really want to go in experimenting with a VG installation?

Not very far, Dafydd. I am a complete wimp when it comes to my aeroplane performing unnatural acts, so I'm being very careful. The Jodel's outer wing and tail geometry is pretty forgiving, but the aspect ratio is now even tighter since I increased the inner chord. That makes a wing drop so much faster, and my feet and hands are a long way from my CPU. Add geriosis to the mix and you'll see why I am so risk-averse!

 

 

[Dafydd Llewellyn]

Many thanks, DJ.

I was, of course, familiar with the work on drooped outboard leading-edges with a vortex-generating notch ("dog-tooth") at their inboard end; the leading-edge cuff on the Sentinel shows that Bill Whitney was, too - I think that one went the rounds of the industry, when it came out. Most of us were aware of that sort of thing as it was found necessary to control the stall behaviour of swept wings. I doubt it occurred to many designers that it was also applicable to unswept wings, prior to that NASA work. The leading-edge cuff is another partially-effective tool; it's not the complete answer to spin-proofing by itself - but if you combine it with other devices, the total result can be surprisingly effective. However, the basic rules for proper empennage design and adequate yaw damping to prevent an unrecoverable spin must be observed also.

There's a recent patent on another form of leading-edge device that works in this general way, too.

 

What comes out of all this is that there is considerable scope to virtually eliminate stall-spin accidents, if people are of a mind to do so. It won't happen until the buyers stop being impressed by the styling of the aircraft, and demand some real practical value; I despise the whole school of thought that leads to "supersonic styling" of light aeroplanes - what I call the "windswept t**d" look; but it shows how much uninformed market demand dominates the industry. If you read that NASA work, and look critically at the aircraft that are offered on the market, you cannot fail to be struck by what a large percentage of them were designed by people who had never read those reports, or who couldn't give a damn - the C 162 being a prime example, but very far from the only one. The sad thing about it, is that spin-proofing an aeroplane need not detract from its performance*. So we suffer a completely unnecessary accident rate from stall/spin occurrences, simply because such a large proportion of people are bloody fools. I cannot put it in more polite terms.

 

* The stall strips on the Fokker 100 series are a case in point - they are the size and shape of a safety match stick.

 

The "Sunbird" I had seen before, too, but not in this context. Cessna also played with a pusher equivalent of the C150, in the '60s, (If my memory serves me, they called it the "XMC") but didn't proceed with it; I suspect they decided buyers would shy away from it, because it didn't have either "traditional" - i.e. Piper Cub - or "supersonic" styling. It's a layout that gets rediscovered every so often; e.g. the Nardi Riviera amphibian.

 

 

[djpacro]

.. If you read that NASA work, and look critically at the aircraft that are offered on the market, you cannot fail to be struck by what a large percentage of them were designed by people who had never read those reports, or who couldn't give a damn - the C 162 being a prime example, but very far from the only one....

My opinion is that Cessna intended the 162 sometime in the future to have approval for intentional spins for training. On my last visit to the States I had some discussion with some involved in the design of the 162 wrt spinning - happy to discuss privately but not in public.On a recent visit to Aus by Sean Roberts (from National Test Pilot School at the time) he was telling us about one aircraft designed for spin resistance with some of those features. All good but, from memory, very scary behaviour when they did manage to make it depart post-stall.

 

 

[Dafydd Llewellyn]

Haven't seen Sean for quite a while. I have thought, for a long time, that one of the things that drives a spin, is the un-stalling of the up-going wing tip. To my mind, this marks the end of the incipient spin phase; and in the DHC-1 and the L-13 at least, it is very recognisable from the increase in the rate of rotation that occurs abruptly after about 3/4 to one turn of the spin, in those aircraft. It would follow that aerodynamic devices that extend the stalling angle of the outer wing - which are, of course, what is needed for control of the spanwise spread of stall - i.e. spin-resistance - are likely to be adverse for recovery from a developed spin, because they increase the tendency for the rising wing to become un-stalled from the tip inboard.

 

So to a considerable degree, spin-resistance design is at cross-purposes with design for spin recovery, at least in regard to leading-edge slots, VGs etc. It was for this reason that I sought something that would inhibit the authority of the elevators, once the inboard wing has stalled.

 

It was also in recognition of this, that we re-did the Seeker spin-testing despite the remarkable spin-resistance provided by the airflow kit. It turned out that the considerable vertical tail areas provided sufficient yaw damping to prevent real auto-rotation from starting, so no adverse result arose.

 

I think the message is clear - the aeroplane needs to have adequate un-blanketed vertical tail surfaces BEFORE you start trying to make it spin-resistant; the NASA work in 1972 is still highly relevant - in fact, even more so - as a prerequisite. The vertical tail cannot be designed by eye, or used as a form of "trademark", as it was half a century ago.

 

 

[Head in the clouds]

... The outboard leading-edge cuffs were originally tried on the Seeker's predecessor, the Sentinel, in an attempt to compensate for the absence of washout; they went into the rafters when the wings were re-built with washout; but I had them resurrected in the course of the testing of the airflow kit, in order to give the third-stage control of the lateral spread of stall, which they did very well - so they are now an integral part of the airflow kit ...

So has the current Seeker reverted to a wing without washout?

 

 

[Dafydd Llewellyn]

No, it still has the washout - plus everything else. I doubt the washout does anything useful, with the rest of the kit installed. I'd rather, in general, do without washout; it does not help the top-end performance (not that that's an issue for the Seeker).

 

 

[Dafydd Llewellyn]

The maker does recommend different spacing inside aileron span but definitely says do the lot. Few users have done so with "gentle stall characteristics"Someone has done testing and have approval from CAA (UK???)for the mod...... Not sure what this mens or whats involved.

See http://www.stolspeed.com/id/96

 

Some interesting test videos on the site too comparing full wing and half installations- not sure the assessment or my understanding is up to it though

I'd take those performance figures with more than a grain of salt; a lot of the marketing of VG kits relies on the increase in angle of attack causing an increased pitot-head error. He's not saying whether the speeds are IAS or CAS - so you can bet they are IAS. To get REAL speed values, the testing would need to be done using a certification - type airspeed calibration rig. The VG kit on the Seeker was originally supplied by Micro-Aerodynamics http://www.microaero.com/ - but of course they did not have an STC for the installation, so Seabird got a shoe-box full of VGs and some general advice over location, spacing etc. So Seeker had to do their own experimentation, as I have described. It WAS done using a certification airspeed calibration rig, so the speeds were real.

 

What it showed, was that at the full aft CG, where the aeroplane was not elevator-limited, the full-span VGs gave an increase in the maximum lift coefficient of 16% - but with totally unacceptable stall characteristics. By the time the stall behaviour had been tamed, the increase in maximum lift coefficient was 13%; and since the speed change is proportional to the square root of the lift coefficient, that gave a reduction in the real stall speed of just under six percent. If the full-span installation had given acceptable stall behaviour, the stall speed reduction would have been just over 7 percent. That's quite a bit less than Stolspeed are claiming, which suggests to me that about a third of their claim is in fact due to the increase in pitot-static system error at the higher angle of attack.

 

Their claims re cruise speed and rate of climb also do not fit with my experience; there are well-defined methods to measure those things, and they are not as simple as you might imagine. Climb performance as required for certification is usually the result of a set of twelve climbs, half of them on reciprocal headings to eliminate any wind-shear effect, corrected for aircraft weight and density altitude, and averaged. The test conditions must not have any temperature inversion.