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).

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.

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.

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?

Many thanks, DJ.

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.

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.

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.

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

OK. Please let me know how you get on.

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.

Yes, I did all the work for the airflow kit - fences, VGs, stall strips etc.; the reason for that was that it was originally certificated to FAR 23 at amendment 34. Due to the passage of time, in order to get an FAA TC for it, to allow it to be marketed in the U.S.A., it needed - in the days before Australia had a bilateral airworthiness agreement with the FAA that really WAS bilateral - to comply with FAR 23 at Amendment 42, which brought in the requirement for dynamic seat testing - the aeronautical equivalent to the barrier crash test for cars. That DID raise a problem at the time, because energy-absorbing seats need the whole cockpit designed to cater for them, and of course that wasn't on the agenda when Bill Whitney was involved. However there was, at the time, a loophole for a fixed undercarriage aircraft if its stall speed was less than 49 kts CAS. The Seeker as it was then stalled at 52 KCAS, so Seabird decided to try VGs to see if they could squeeze the extra three knots by their use. No, I wouldn't say the Seeker was a "problem child", apart from one minor issue that is common to all pusher aircraft having a single engine and a single vertical tail - e.g. the Sea Bee, the Trident Trigull etc - which is that in a power-off dive to the design diving speed - 111% of Vne - the windmilling propeller blankets the vertical tail; so what happens when you take your feet off the rudder pedals at that speed, is that the aeroplane gently yaws one way or the other, until the vertical tail comes out from behind the propeller. It's neither dangerous nor even exciting, but it's a technical non-compliance, and the finlets, & ventral fin were added to try to fix that. It finally took all those plus the geared vane under the fuselage, to tick that particular box. As a by-product of all that vertical tail area, the Seeker won't spin; it merely spirals. The high-wing pusher layout had two very good handling features - the Seeker has zero trim change with flap deployment, and also zero trim change with power. The wing airfoil is an NACA 64 series - I seem to recall, 642A215 - which was selected by Bill Whitney for reasons only Bill could tell you, but a low pitching moment was one of them, in order to reduce the download on the slender rear fuselage. It's an airfoil that stalls progressively from the trailing edge, which all the textbooks will tell you is a Good Thing; but what they do not mention, is that it makes the ailerons completely ineffective by the time the aircraft has slowed to about five knots above stall speed. In its original configuration, without the airflow kit, but with the washout, the situation with the ailerons was disguised, because what seems to happen - and this has been the case with the Skyfox and the Jabiru LSA 55 also, is that a "bubble" of separation forms in the centre of the wing; and if one wing starts to fall a bit, the resultant sideslip blows the bubble towards the opposite side, so the thing tends to recover - most of the time. The pilot reports that the lateral control is sloppy - but in reality, it is non-existent, apart from the secondary effect of rudder acting on the separation bubble. That was the reason for the somewhat disappointing (but not out-of-the-ordinary) stall handling with which the Seeker was originally certificated. I'd never have discovered all this if Seabird hadn't asked me to see what could be done to lower the stall speed, by the use of the VGs; I'd previously discovered the aileron issue, by a test with enormous stall strips that covered the whole wing leading edge for a distance equal to the tailplane span; that removed the "drifting stall bubble" and revealed that the ailerons were actually reversed by 5 knots above stall speed. However, it wasn't a solution that helped certification at the time, so merely remained as an item of knowledge - until Seabird decided to look into the possibilities of VGs. 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, and all Seekers were retrofitted with the airflow kit, because whilst it did not quite provide the desired reduction in stall speed, it radically improved the stall handling, to what I suspect may be the most docile single-engine aircraft in existence. The Seeker is probably the most relaxing aircraft to fly I have ever experienced. The bulge on the top of the fin is there to house the rudder mass-balance arm. Overall, the pusher layout has its own set of compromises, which are a bit different to those of a tractor layout. They mainly affect the propeller - firstly, the propeller diameter is restricted, which limits its efficiency at low speed. Secondly, they tend to be noisy, because the propeller works in dirty air - so it's more difficult to achieve noise certification than with a tractor layout. Thirdly, the introduction into FAR 23 of a requirement for pusher propellers to be able to withstand chunks of ice falling into them from the airframe in front, poses a major obstacle to IFR certification. On the good side, you do not get propeller ground strike problems; and if some ham-handed twit manages to stand the aircraft on its nose, all you get is some grass-stains on the chin (I saw this happen with the prototype Seeker, with an overseas General in the pilot seat - the ground crew simply hauled the tail back down and told him to keep the stick back, and sent him on his way.)

On a high-wing aircraft such as the Seeker or the Aeronca, the effect of the fences and stall strips is to stall the whole centre part of the wing simultaneously - i.e. the stall seems to spread across the top of the fuselage as though the wing were continuous. You may find things a bit different 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. The effect on the Seeker is that you cannot get it to drop a wing, no matter what you do short of performing aerobatics in a non-aerobatic aeroplane. With the full-span VGs, it dropped a wing with extreme violence, such that I lost 600 feet in the recovery, despite being fully alert for the wing drop. With the fences & stall strips, and the VGs removed inboard of the fences, it became a pussycat; the change was astounding. The Aeronca was much the same, I understand, though I have not flown it myself. Ian McPhee rode in it for the testing, about eight years ago, he can tell you about it. Whether you will get a perceptible buffet is determined by the relative position of the wake of the stalled wing centre-section and the tailplane, and that's something the designer either got right or not; in any case, it's almost impossible to get it right for all flap positions, if the aircraft has flaps. The Jodel tailplane being where it is, it has some unshielded rudder in a spin, and that takes precedence over locating it to get pre-stall buffet. Whether that was by accident or design, I cannot say; but the overall aerodynamic design is very clever, so it was probably no accident. Note that the mechanism is NOT the same as a limited elevator power that prevents the wing from ever reaching its stalling angle of attack; that form of limitation is strongly affected by CG position. Instead, what I have described will work even if the elevator does have the power to take the wing to its stall A of A, because it uses the initial stall to inhibit any further elevator power. So the situation is not one of the aircraft "mushing" and not being able to use the full lifting capability of the wing; however the further aft the CG, the more powerful the elevator becomes - so you must set the stall strips for the aft limit situation. In the case of the Seeker, it was possible to set the CG range such that the elevator had just sufficient authority to stall the wing at the forward CG - and the effect of the full airflow kit was just sufficient to still control the situation at the full aft CG; and having thus set the CG limits, the rest of the handling and stability testing, and spin testing, was completed at those limits. You do not have that freedom except in a basic aircraft certification situation, so set it up for the aft CG condition and put up with a bit of mushing at forward CG if necessary; you will be getting the best performance the aircraft is capable of delivering, consistent with safety. In effect, the setup I have described gives the wing a two-stage stall; and provided you set it up so you cannot reach the second stage stall, it will not drop a wing. To get this control over a large CG range seems to need a three-stage stall setup, which is a bit more complicated; this is what the Seeker has. Stall/spin accidents are entirely the result of inadequate control of the lateral spread of stall along the wing. The conventional approaches of planform and washout help, but are not sufficiently powerful to prevent assymetric stalling, leading to a spin entry, with crossed controls. The correct use of VGs and fences and stall strips is a much more effective means of control, from what I have seen of it so far. It's so effective that one would not want it on a trainer, because it would make proper training impossible.

I can't guarantee this will work on any aeroplane - however it did work a treat on the two that I have been able to try so far. I'm talking here in the context of somebody developing their experimental homebuilt: First, remove the VGs on the wing from the wing root, to just past the same spanwise station as the tailplane tips. Leave them on the wing, outboard of that point. Next, fit small leading-edge "fences" to the wing leading edge, in line with the tailplane tips - i.e. a couple of inches inboard of the first VG. See photo. The fence need only go to about 20% of the wing chord, on the top surface, and it need only be around 60 mm high. It can probably be stuck on with double-sided tape, for the initial test. Then: Fit small stall-strips on the INBOARD side of the fence, as shown in the photo. Make sure you put them at the same height on each wing. The stall strips need to be about the size of a piece of triangular file, about 8mm wide on each side, and about 50 mm long. Butt the outer ends of them hard against the side of the fence. Now, load your aircraft about 3/4 of the way to its most aft CG limit, go to a safe height (at least 3000 ft AGL) and try a gentle power-off stall, keeping the ball in the middle. If you get a wing-drop, go home and raise the location of the stall strips by 3 mm, and try again; repeat until you do not get a wing drop. If you do NOT get a wing-drop, go home and lower the location of the stall strips until you do get a drop, then raise them one increment. Do NOT allow the aircraft to start an incipient spin; if the fence comes off on one side, it could get exciting. Once you have found the correct location for the stall strips, progressively move the CG aft to the rear limit. If you get a wing drop in a straight power-off stall, raise the stall strips another increment, and try again. Once you have the stall strips located in this way, you can put them on for keeps. What this does, is to cause the central part of the wing - the bit between the fences - to stall just before the outer part, where the VGs are. That does three things: Firstly, it creates a pair of powerful trailing vortices that turn the opposite way to the wingtip vortices. These vortices in effect magnify the effect of the fences so that the stall cannot spread outboard past the fence. The vortices are not there until the inboard part of the wing stalls, so there is no additional drag in normal flight. Secondly, it reduces the wing downwash on the tailplane - which prevents the elevator from being able to pull the aeroplane any further into the stall. Thirdly, it produces a strong "centring" effect on the vertical tail, which acts to prevent the aircraft from yawing. Whether these effects will be sufficiently strong to overcome the effect of full power or full crossed controls, will vary from one aircraft to another. You just have to suck it and see. If the setup works on your aircraft as well as it did on the Seeker and the Aeronca Champ, your aircraft will now be extremely reluctant to enter a spin. However, that is no guarantee that, if you do manage to get it into one, it will be recoverable. This is REAL test-pilot stuff, i.e. hazardous. Spin testing is a separate issue.

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

OK, do your VGs run all the way in to the wing root? If they do, I may be able to suggest some things that may help.

Try Airport Metals file:///C:/Users/Toshiba/Downloads/AIRPORT%20METALS%20Price%20List%20March%202012.pdf

Nev, installing a significant number of VGs is a major modification, and would nowadays need to be handled via a Supplemental Type Certificate (or under CASR 21 Subpart D, if the mod. is being done by the holder of the TC). It's absolutely something that has to be properly approved, for any certificated aircraft. This would require full stall and spin handling tests at the very least. What's a significant number? Up to somebody's judgement, I suppose, but could be as low as two. There are a number of STC packages available for GA aircraft. My warning was intended for homebuilders of -19 aircraft or VH Experimental aircraft.

Nev, I used to see VGs as merely a "band-aid" too; but after developing the airflow kit for the Seabird Seeker, I've come to the conclusion that they are rather more than that; they're worth considering as part of the basic design of the aircraft - as Boeing obviously know. By judicious use of VGs and some other devices, it would be possible to virtually eliminate stall-spin accidents; I defy anybody to get the Seeker to drop a wing more than 15 degrees, if at all.

Be careful about statement like "600 Kg is now there for all". For those for whom this is not obvious, the MTOW of the aircraft is what it was certified for by the manufacturer, for LSA; or what it was certificated to, for a certificated aircraft. It's NOT the category limit. The problems I see - or perhaps, forsee - with LSA are these: Firstly, it's a recipe for "regulation by litigation". A Type Certificate issued by a national Airworthiness Authority is, to some degree, protection for the manufacturer against product liability litigation in regard to design fault. LSA certification by the manufacturer is a much more porous umbrella. LSA aircraft have not really been around long enough for the manufacturers (except the likes of Cessna, who appear to have seen the light) to be sufficiently wealthy to be attractive targets for litigation, but in the fullness of time . . . Secondly, modifications to LSA aircraft can only be approved by the manufacturer. Almost any repair is a modification. So when the manufacturer goes belly-up, a damaged LSA aircraft cannot be repaired without dropping to experimental status. That means the insurance situation for these aircraft has a big question mark hanging over it; and the re-sale value is likely to be low. Thirdly, there are too many LSA manufacturers for the market. That means there is inevitably going to be a shake-out. Add this to the first two considerations, and you can see where it's headed. All this being the case, building an LSA kit as an experimental aircraft makes a great deal of sense. Buying one as a factory built, well . . .

This has been covered in other threads. VGs need to be used with care; if they are mis-applied, they can result in an extremely vicious stall behaviour. So if you use them, do so ONLY in accordance with a set of instructions that have been developed by testing for YOUR aircraft model. Be especially wary of VG installations that run all the way in from the wingtips to the wing roots. If they are correctly used, they can greatly improve the stall handling. They are quite powerful devices. On a certificated aircraft, an approved VG modification would normally require a re-run of the spin testing. They can, correctly applied, reduce the stall speed approximately 6% - but ONLY if there is sufficient elevator power available at the forward centre of gravity limit, to take advantage of them. That is normally not the case; most aircraft are elevator-limited (i.e. the elevator does not have sufficient authority to take the wing all the way to its stalling angle of attack) at their forward CG limit. If that's the case, it may need VGs on the underside of the tailplane, before VGs on the wing will show any benefit. The do not normally make a measurable difference to cruise speed or climb rate. They do make the aircraft more difficult to wash.

Handy to know; thanks. However, as far as I can discover, they supply only commercial-grade material. That doesn't mean it's not good; but what it DOES mean, is that to use it in any aircraft structural application, the user may need to apply his own quality-assurance test. If you buy aircraft-grade material, you are also buying the quality-assurance process. For example, 6061 is a commercial (SAE) alloy composition specification. T651 is a heat-treatment specification for cold-finished bar or plate. To buy 6061 T651 to an aviation quality specification, you need to purchase it with a release note certifying that it meets QQ-A-225/8 or AMS 4218. (This information can be found in Mil-Handbook-5, which can be downloaded.) You do not normally get that from any Australian aluminium manufacturer. Particular care is necessary with aluminium extrusions, which can have a surprising range of defects; these mainly arise as a consequence of slag inclusions. The billet - called a "log" - of material from which extrusions are pressed, is made by a continuous vertical casting process, in which the slag continually rises to the top, and is - hopefully - discarded. However if the supplier crops the log just a bit too short - and they naturally do not want to waste good metal - some slag can find its way into the material that is being forced through the extrusion die, and processes such as the QQ-A specifications usually require a "back-end etch" test of the cut-off piece to verify that there are no slag inclusions. Another source of extrusion defects can occur when the log runs out in the middle of a run of extrusion, and another log is fed into the machine behind it - the two are supposed to weld together in the process of passing through the extrusion die. However, you do NOT want the piece of extrusion for something like a lift-strut to contain such a weld. For this reason, lift-struts made for Australian - certificated aircraft are normally required to be subjected to 100% proof-testing. One can do this for a lift-strut, because it is designed by the compression buckling case, and thus has excess tensile strength, so the proof-test need not subject it to loads that would damage it. Not so easy for most other components.

Wrong photo, I think

Since when did hypersonic and stealth go together?

Just as a point of historical accuracy, that was Leonard Cheshire VC, not Bennett. Not that Bennett was any mean pilot himself.