Lowering stall speed

[Nev25]

Wondering if stall speed can be lowered

 

And how it is calculated

 

I'm looking at scratch building a KR2-s for Ra-Aus rego that has a stall speed of 52MPH (according to the specs)

 

This equates to 45.1868 Just over

 

Would they allow this

 

 

[Patrick Normoyle]

Here is my silly answer, maybe a different wing profile, or a slatted wing or wing vortex generators, so if it's 19 reg, you might be able to get the stall speed down thus way.

 

 

[Mick]

If it is that close maybe a small increase in wingspan and thus wing area will result in a lower wing loading. Would that result in the reduction of stall speed you require?

 

 

[Bruce Robbins]

Reduce the MTOW by a few kilograms.

 

 

[Old Koreelah]

Nev I have been dabbling in this area. As Patrick says, VGs help to delay the stall (mine came down 3 knots) but when she does finally let go it's a different animal; the stall may be sudden and dangerous.

 

Bruce is right; the advice of Jodel designer Jean Délémontez is worth listening to: "add lightness". Everything you add increases the stall speed. This includes adding wing area. You end up in a merry-go-round of redesigns until you get to your goal. Perhaps researching different wing profiles is your easiest option.

 

 

[Camel]

Wondering if stall speed can be loweredAnd how it is calculated

I'm looking at scratch building a KR2-s for Ra-Aus rego that has a stall speed of 52MPH (according to the specs)

 

This equates to 45.1868 Just over

 

Would they allow this

I'm no KR2 expert but am aware that the design of Morgan Sierra and Cougar share similarity in the design, the cougar and the Sierra have low stall speeds, I would suggest talking to Gary, I believe he put Sierra wings on a KR2 some time ago, and does a lot of repairs and mods to KR2's. Gary is very knowledgeable, experienced, helpful and a friend. I have over 60 hours in the Sierra and Cougar and like them very much.

http://www.morganaeroworks.com.au

 

 

[djpacro]

Question is - is that quoted stall speed IAS or CAS?

 

My rough estimate (5 minutes on the back of envelope) is 50 kts CAS stall speed flaps up and around 45-46 with decent flaps.

 

 

[facthunter]

It has to be ACTUAL speed to mean anything. More effective flaps (if they are already fitted) with any plane would be the easiest way to go There would be some airframe stressing issues but not hard to overcome..Nev

 

 

[spacesailor]

Larger flaps/flaperons, I want to design my Hummelbird like the Foxbat's flaperons. Looks easier than building a new wing.

 

spacesailor

 

 

[facthunter]

Flapperons may give you some control difficulties. I've gone off them . Nev

 

 

[djpacro]

There would be some airframe stressing issues but not hard to overcome..Nev

Nothing money can't fix.

[facthunter]

I would suggest fitting Fowler type flaps but the hinges location ( the simplest way of doing it) tends to tangle in long grass with low wing aircraft, which can be very embarrassing ( even Dangerous).. Nev

 

 

[Marty_d]

Flapperons may give you some control difficulties. I've gone off them . Nev

In what way, Nev? The 701 I'm building has them - obviously designed for them, just wondering what difficulties they can cause.

 

 

[facthunter]

IF your aileron never goes into the UP position you get more adverse aileron effect. Normally there is more up than down in a differential aileron set-up. Once you use the aileron(s) as a flap , this effect is not there. Nev

 

 

[Marty_d]

Would the adverse yaw effect be diminished when the flaperon is mounted below the trailing edge?

 

 

[facthunter]

The advantage of that is the air is not disturbed, so it has advantages at the stall. The adverse effect is there even with wing warping. Any increase in lift is obtained at the expense of a poorer lift/drag situation, on the side where extra lift is desired, which is the opposite of what is wanted. Spoilers work better in this respect, but you still lose performance. Nev

 

 

[Marty_d]

Thanks Nev. That makes sense.

 

 

[Bob Llewellyn]

Wondering if stall speed can be loweredAnd how it is calculated

I'm looking at scratch building a KR2-s for Ra-Aus rego that has a stall speed of 52MPH (according to the specs)

 

This equates to 45.1868 Just over

 

Would they allow this

Stall speed must be measured, normally using a pivotting pitot on a stalk out in front of the aeroplane, a trailing cone static, and a calibrated ASI. With regard to knocking a few knots off, as djpacro said, money will do it... for starters, how does it stall? Does it stop being able to be held wings-level with conventional use of controls, or does the stick reach the back stop, or is there an "uncontrollable pitch-down"? If option one, VGs in front of the ailerons should do the trick. If option two, VGs on the bottom of the tailplane may well drop the stall speed. In either case, the stall behaviour may change radically, though with VGs ahead of the ailerons it's unlikely to kill you. If the stall is an uncontrollable pitch down, it's worth wool-tufting the wings to see where it's initially separating. If it's a trailing edge stalling airfoil - such as anything laminar / 6-digit, or Clark y / USAF 35 / 4 digit, judicious placement of VGs on the wing should drop the stall speed and keep the handling acceptable.

This kind of approach is acceptable for homebuilt / experimental or 95:10, where you don't have to go off and do a full spin matrix to prove that whatever you've done has made the spin recovery no worse. It's expensive if you do... mind you, there is no reason that a cautious and thoughtful approach should be expected to make spin recovery any harder!

 

I would strenuously advise against extending the span of the wings; I was asked to analyse such an approach once (actually, I was asked to ok it!); but the increase in bending moment from even a small tip extension is very large, and the KR-2 centre section is not easy to beef up without increasing the carrythrough spar depth.

 

 

[pog5757]

I would suggest fitting Fowler type flaps but the hinges location ( the simplest way of doing it) tends to tangle in long grass with low wing aircraft, which can be very embarrassing ( even Dangerous).. Nev

If you're in grass that tall, you're well past embarassment and into the terror phase.

 

 

[bexrbetter]

Here is my silly answer, maybe a different wing profile, or a slatted wing or wing vortex generators, so if it's 19 reg, you might be able to get the stall speed down thus way.

I was just reading about Vans and the RV12 the other day - they did all of that for no good result to achieve decent controlled stall speed for LSA regs.

 

Might be worth reading up on what eventually they did do to get there.

 

 

[Head in the clouds]

IF your aileron never goes into the UP position you get more adverse aileron effect. Normally there is more up than down in a differential aileron set-up. Once you use the aileron(s) as a flap , this effect is not there. Nev

It's not the up- or down-going aileron that causes the "adverse aileron effect", it's the difference between the induced drag generated by each wing and that's not dependent on one aileron being up or not, simply the difference between them. This issue of "more adverse aileron effect" with flapperons needn't necessarily be the case, it depends on how the flapperon mixer geometry is set up.

 

For the less initiated - if your ailerons normally go up 25 degrees and down 15 degrees, that's called aileron differential and the designer achieves it by using Ackerman effect i.e. the aileron bellcrank arms are not at 90 degrees to each other so that the linear motion produced in one direction is greater than that in the other direction because of the angular offset of the control arm, simply put it's just like the steering on your car where the wheel on the inside of the direction of turn turns sharper than the wheel on the outside of the turn.

 

So - back to the flapperons - if your flapperon mixer simply pulls both ailerons down by, say, 20 degrees at the max setting then the plane will fly very nicely when the ailerons are centred and the stall speed will be a fair bit lower due to the increased CL of the flapped airfoil section, but when full aileron is applied one way or the other, then, as Nev said, the adverse yaw can be quite extreme because as we know, the drag increases exponentially as the flap angle increases. And in this simple example, at full aileron deflection, the down aileron would be down by its normal 15 degrees plus the 20 degrees flap angle so it's down at 35 degrees (a fairly high drag angle) and the upward aileron is up by 25 degrees minus the 20 degrees flap setting, so it's up by 5 degrees which is a quite low drag angle, and so we have a large adverse aileron effect due to the large difference in induced drag between each wing.

 

BUT - this 'problem' with flapperons doesn't have to exist. All you do, at the design stage, is to work out the geometry carefully so that your mixer also makes use of Ackerman effect. What then happens is that the 'flaps' part of the flapperon control surface movement is provided with differential too, so that when the flaps are fully deflected the geometry is such that there is no further downward movement of the control surface with aileron input and the only movement is the opposite flapperon moving upward. In other words when the flaps are fully deployed only one flapperon can move and that movement is upwards. With that kind of setup the lift increase benefit of flapperons can be maximised and the adverse yaw can be minimised.

 

 

[Bob Llewellyn]

It's not the up- or down-going aileron that causes the "adverse aileron effect", it's the difference between the induced drag generated by each wing and that's not dependent on one aileron being up or not, simply the difference between them. This issue of "more adverse aileron effect" with flapperons needn't necessarily be the case, it depends on how the flapperon mixer geometry is set up.For the less initiated - if your ailerons normally go up 25 degrees and down 15 degrees, that's called aileron differential and the designer achieves it by using Ackerman effect i.e. the aileron bellcrank arms are not at 90 degrees to each other so that the linear motion produced in one direction is greater than that in the other direction because of the angular offset of the control arm, simply put it's just like the steering on your car where the wheel on the inside of the direction of turn turns sharper than the wheel on the outside of the turn.

 

So - back to the flapperons - if your flapperon mixer simply pulls both ailerons down by, say, 20 degrees at the max setting then the plane will fly very nicely when the ailerons are centred and the stall speed will be a fair bit lower due to the increased CL of the flapped airfoil section, but when full aileron is applied one way or the other, then, as Nev said, the adverse yaw can be quite extreme because as we know, the drag increases exponentially as the flap angle increases. And in this simple example, at full aileron deflection, the down aileron would be down by its normal 15 degrees plus the 20 degrees flap angle so it's down at 35 degrees (a fairly high drag angle) and the upward aileron is up by 25 degrees minus the 20 degrees flap setting, so it's up by 5 degrees which is a quite low drag angle, and so we have a large adverse aileron effect due to the large difference in induced drag between each wing.

 

BUT - this 'problem' with flapperons doesn't have to exist. All you do, at the design stage, is to work out the geometry carefully so that your mixer also makes use of Ackerman effect. What then happens is that the 'flaps' part of the flapperon control surface movement is provided with differential too, so that when the flaps are fully deflected the geometry is such that there is no further downward movement of the control surface with aileron input and the only movement is the opposite flapperon moving upward. In other words when the flaps are fully deployed only one flapperon can move and that movement is upwards. With that kind of setup the lift increase benefit of flapperons can be maximised and the adverse yaw can be minimised.

...more or less. A plain, sealed or extremely small gap aileron may be expected to separate from the top surface at not more than 15 degrees downward deflection (and from the lower surface at 20~25 degrees upwards). If your ailerons normally work in the range 10deg down / 20 deg up (differential), then adding a droop of even 10 deg to both will substantially change the adverse yaw effect, in a worse direction when you grab a lot of aileron (eg mechanical turbulence...). If you change the drive geometry, you can largely eliminate this change; but it's likely the force/deflection curve for the controls will reverse near neutral, either with the ailerons drooped or up. Not a simple exercise, unless you don't give a hoot for Certifiable control behaviour.

Also, the benefit is small. An analysis of an LSA pre-prototype design, with plain flaps from 13% to 60% span (from C-line), and plain ailerons fron 60% to 90% span, showed that deflecting a 20% chord flap down 15 degrees gave a 10.5% reduction in stall speed; subsequently drooping the (20% chord) ailerons by 15 degrees gave an additional 1.7% reduction in stall speed - hardly value for money.

 

This is because the spanwise lift distribution approaches elliptical* at high lift, even if the wing does not, so the outer bits are not doing much lifting - mainly reducing drag by holding the tip vortices apart! The wing analysed was treated as almost constant chord (constant but with cropped tips), of aspect ratio just over 6, and no twist. A tapered wing would get marginally less benefit from drooping ailerons.

 

*Ignoring NACA TN606, which everybody else does anyway...

 

 

[Dafydd Llewellyn]

For the methods used to measure stall speed, see http://www.casa.gov.au/wcmswr/_assets/main/rules/1998casr/021/021c40.pdf

 

The stall speed that is relevant to aircraft registration category limits is generally the result of the methods described in the AC, at maximum take-off weight and most froward CG - so determining it required accurate weight & balance. Most aircraft are elevator-limited at forward CG, usually as a means of improving the stall handling. If you increase the elevator power, to the point where it becomes capable of stalling the wing at the forward CG limit, the stalling behaviour is very likely to become considerably less benign. VGs, if intelligently applied, can give about 6% reduction in the REAL stall speed; but unless you know what you are doing with them, they can make the stall extremely vicious. Any change in stall behaviour normally requires spin testing - and that's NOT something to get involved in unless you have extensive knowledge in that area.

 

Generally, if you need a significant reduction in stall speed, it's cheaper to sell the aircraft and buy one that has the stall speed you need.

 

 

[rgmwa]

Bob, what's your view on full span flaperons? Nev has said he has `gone off them' as they can present some control difficulties. The RV-12 and others have them. What are the issues?

 

rgmwa

 

 

[Bob Llewellyn]

Bob, what's your view on full span flaperons? Nev has said he has `gone off them' as they can present some control difficulties. The RV-12 and others have them. What are the issues?rgmwa

This is not a short-answer question! But here goes:

It depends on whether the suction side of the aileron is effective - or not. Most are not. The Thrusters (and Drifters), which have a very effective "hydraulic break" to trigger separation over the suction side of the (full span) ailerons, have strong adverse yaw with any use of ailerons. The few Thrusters in the world that have tried to droop the ailerons as flaperons, have found that the adverse yaw is not noticeably worse than standard. Mind you, at 20~30 kts on the runway*, it is possible to steer the standard aeroplanes - in the anti-roll direction - with secondary effects of aileron! They also have the glide angle of a well-fired brick....

 

*Mostly noticed on bitumen; the response lag on grass is large enough that the control deflections have generally been changed before a swing is noticed.

 

The Cessna 402, which - quite deliberately I suspect - does not achieve suction on either aileron surface, does not have enough aileron power in the engine-out situation; hence Microdynamics VGs, to restore the suction on top of the downgoing aileron.

 

IF the control surface (flaileron?) has a semi-circular leading edge, AND the diameter is slightly greater than the depth of the wing adjoining it - that is, the top & bottom of the aileron (undeflected) stand a little "proud" of the wing surface lines, AND the gap is sealed or VERY small, then the suction side will be effective at up to ~15 degrees downwards deflection. For part-span ailerons, this ties in very well with the 2:1 differential Fred Weick's NACA research program found, ie upwards movement should be twice downwards movement for maximum effectiveness / minimum adverse yaw (with plain ailerons). As mentioned in previous post, droop them very far and they go through separation at some point, which changes the force/deflection* curve, control power, and adverse yaw - for the worse.

 

*The pilot input force vs control deflection - force has to increase with deflection at close to constant rate at any given speed, for certification.

 

The NACA Wildcat (F4-F I think, not FM-2 or their Buffalo) fitted with full-span slotted flap with an aileron in the outer flap, achieved excellent controllability and a quite worthwhile reduction in stall speed. This would not be a minor modification to most aeroplanes...

 

From what I have seen of Van's aerodynamics, and Ed (Kreuger?)'s, I expect that they are using a suction-side-ineffective control surface, which avoids a change with part-deflection separation (since it's always separated!), at the expenses of less effectiveness* and slightly greater cruise drag. It also makes aileron flutter in cruise far less likely, which is why Cessna have stuck rigidly to the formula since pre-WW2.

 

*The change in lift coefficient / change in drag coefficient with control deflection is greater for a suction effective surface; I am not saying that the RV-12 will have inadequate aileron authority, but it could have the authority with less drag!

 

So - simple plain flaperons can be made to work - without huge R&D - if they're full-span and suction-ineffective; but they don't increase the Clmax / drop the stall speed much (10% can be had). Slotted flaps and plain, undrooped, ailerons will give a lower stall speed (up to 20% typ.) and better L/D, if the flap shroud is properly shaped; and, if the aileron is suction-effective & differential, less adverse yaw and lower cruise drag.

 

Slotted ailerons with anti-balance tabs (or pro-balance, as the force-deflection curve requires), and Fowler flaps give a better low-speed L/D, higher Clmax (lower stall speed - as much as 40%) than either of the above, with admirable force-deflection curves.

 

BoeingBus do not fit all those fancy control surfaces to their wings for no reason. RVs traditionally have bags of power, and so the Cessna-style ailerons are just fine for them. Given the running cost of most recreational aeroplanes is governed by the engine, I personally favour the slightly more sophisticated (complex, costly, versatile, efficient) solution of slotted flaps and suction-effective differential ailerons.