Leading edge slots

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skier

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Does anyone know how/why slots increase the lift of a wing/airfoil? I realize that it allows the wing to stall at a higher angle of attack, but what is happening to the flow that allows this to occur? I have read the section in Abbott and Von Doenhoff about slots, but that didn't give much information.

As a related note, how much do they increase drag for a given Cl (such as in cruise)?
 

Aircar

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Actually a good question -- I think that probably you could make the analogy to a small biplane at the leading edge --the first 10 % of the wing makes 90% of the lift (as a general rule) which is why they balance around the 25% chord point (less the basic Cm) --the speed of flow around the very LE is very large and has accelerated from zero in the space of the stagnation point just below --or at least much below the free stream --putting a second "leading edge" in the region of accelerated flow will give much more lift than any geometric area increase (from a slat say ) so why not double slotted leading edges on the same basis as double slotted TE flaps ? --has been tested, does work ..... (the common misconception -still taught to pilots as far as I know is that the narrowing slot on a fowler flap 'squeezes the air' to much higher velocities and thereby gives the very high Cls --local Cls of five --this is false since the only velocity that CAN be produced in a free stream is the original velocity --slowing the air down to create pressure cannot somehow create a higher velocity afterwards --the effect is to remove the 'dead' air at the back of the airfoil and the new freestream velocity acts on the flaps . This is not the case for leading edge devices and just flow turning is also not an explanation (a vane would simply create drag and the narrowing slot effect once again cannot 'supercharge' the wing in the same way that rear flaps are mistakenly assumed to. For further reading check out the controversy of how a jib works on a sailing boat and note the same misexplanations at work . Max Munk'c biplane theorems apparently explain it all (but the math is way beyond me now ) --this interaction thesis is what makes the Synergy and John Mc Ginnis work so interesting ....
 

SVSUSteve

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this interaction thesis is what makes the Synergy and John Mc Ginnis work so interesting
...and makes most of us look at John like he's a freaking genius, mad scientist and magician rolled into one neatly mustachioed package. There are few people that I occasionally cross paths with that make me feel like I have a tendency to drool uncontrollably on myself by comparison and John is one of them. Thank G-d he's so freaking patient, kind and humble as to help teach the rest of us.
 

wsimpso1

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Actually, TOWS does talk about it a little and they do have a big table showing effects of various slotting schemes.

Basic scheme is this - at high AOA, high energy air is given a path from the bottom onto the top, and helps keep the air attached all the way back to the trailing edge. Slots are usually applied when you need to raise the stall AOA a couple degrees. If your ailerons become ineffective in the flare, it was a decent fix along the outer portion of the wing, and cheaper than scrapping your production tooling when you go to a wing design that might work better. Or if your all moving tail needs just a little bit more AOA for forward CG landings. VG's are now more commonly applied and a lot easier to adjust.

Slots do have one good thing going for them - at low AOA and Low Cl, there is little dP across the slot, so it changes things only a little and adds little drag there, while becoming more effective as AOA is increased. VG's are making drag all of the time...

Billski
 

Norman

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"Slot effect" is a misnomer. All shaping the slot does is minimize the drag that the air flowing through the slot produces. Slots only add drag it's the interference between the pressure fields of the two or more airfoils that can make extra lift and delay stall. Read "aerodynamics of sail interaction" on ArvelGentry.com

and before anybody argues that he doesn't know what he's talking about they should read his resume
 

orion

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Slots in of themselves do not create extra lift - in that sense the interaction of two overlapping sails is applicable to the discussion, as is the lack of that configuration's functionality. However that is not the purpose of the slot, nor is it the mechanism through which it works.

As alluded to earlier, the slot's function is to energize and stabilize the boundary layer, thus allowing the wing to reach a higher angle of attack before the boundary layer destabilizes and separates. The increased lift is due for the most part to the wing's ability to achieve a higher angle of attack - in other words, the presence of the slot extends the wing's linear part of the CL/alpha curve, causing later separation. The stall progression is still nearly the same, except extending over higher angles and higher lift coefficients. But the increased lift is strictly due to boundary layer control, not any lift generating mechanism.

One way you can look at what's going on is to examine the stream lines as they pass the wing profile. Ideally, the stream lines want to stay flat and horizontal. Introduce the wing section and the flow moves over and under the wing and as the angle of attack increases, the flow has to change more and more to go around the physical body. But the flow is acted upon not only be the wing but also by the air that surrounds the disturbed volume. Go far enough away and the streamlines are again horizontal.

In looking at this wing and the condition where we reach stall, we have two actions at play. First, we have the flow next to the wing skin which, due to friction, is generating a boundary layer of slow and turbulent air that is getting thicker as it nears the trailing edge. Second, we have the surrounding air that want's to flow straight and level and thus it acts on the air closer to the wing, entraining it back to level flow (this is not really a good explanation but for the purpose of visualization, it works). The entrainment of the deflected air acts to pull it away from the wing's surface, thus causing separation (stall). Flow along a level flat plate for instance does create an ever thickening boundary layer but the mass movement never really separates from the flat plate. Angle that plate and the mechanism is different due to the entrainment action of the distant streamlines.

Now we take the same wing and introduce the slot. First though, not just any hole or slot will do. Just like the most efficient conventional slotted flap is one that sits at just the right position and attitude in relation to the upper skin (and creates a proper slot geometry), a proper leading edge slot must be formed so that the flow it ejects is nearly parallel to the upper skin geometry. Ideally, the flow should also be accelerated (this is usually limited due to the geometric constraints of most slot geometries) through a converging nozzle. Again, this is not to generate more lift but more so to introduce a high energy stream of air that will flow with a minimal boundary layer as far aft as possible. It is this mechanism that delays the stall to higher angles of attack.

The slot's flow is taken off the bottom of the wing foil, just aft of the leading edge at or near the stagnation point, at the high angles of attack. The slot is usually a point design since it is less effective at lower and intermediate angles of attack, and therein lies its penalty. Due the pressure differential between the lower and upper skin, the slot is continually leaking a small amount of flow, which is usually of much lower energy than the upper from with which it combines. The net effect is drag.

I haven't seen any detailed papers of slotted leading edge characteristics for some time but from what I recall, on one airplane they were used (Globe Swift), it was estimated that the slots decreased cruise speed by at least five knots, if not more. As such, the use of slots has historically been mainly to fix other problems such as tip stall or poor roll control near stall. The slot is much easier to incorporate as an afterthought than changing a section or overall wing design.
 

Aircar

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Many thanks for the Arvelgentry link Norman ( I tried to link to it on another thread but it didn't 'highlight' (blue) as it should so just one of my usual computer illiterate efforts.
That is the primary source for interacting sail theory that sometimes rages (in a gentlemanly fashion) in sailing circles --and has a lot to offer for airplanes ......the Wortmann "Sail-plane" paper written after he took up sailing in retirement is another gem.

Orion is correct in the operation of slots ( I think there is some confusion between a fixed slotted wing and a slot created by an external airfoil SLAT --which is almost always spoken of as a 'slotted' wing as well --the humble Tiger Moth and others like the Me 109, F 86 sabre etc feature MOVING LE 'slots' (that are slats in fact ) --the Aerospatiale Rallye is the most notable GA exponent of retractable slats --the Helioplane, Scottish Aviation pioneers (twin and single) and others like the Zaunkoenig,Fiesler Storch etc are more example of large chord leading edge devices (as compared to slots in a fixed wing ) -- the Pioneers are perhaps the greatest area increasing embodiment outside of some airliners. They act to not only extend the basic airfoil lift curve but to increase the basic wing camber and area and effectively create two leading edges .

The actual function of the 'slotted leading edge' was taken as the question -- I think the conundrum that is perceived is 'why does the air not just separate around the front leading edge (radius) -unchanged on the fixed slot wing - as it would otherwise?

The question is a fair one --it could be likened to the scene out of Superman where he flies to the rescue of Lois Lane-- snatching her in mid air from a fall from a tall building -- to his "don't worry I'm holding you up " she replies " you're holding me up but WHAT"S holding you up ? "

The analogy here is that just cutting a slot behind a wing whose leading edge is already too sharp to let the air stay attached should not allow the amount of turning to increase yet more --at least not intuitively (and putting a slot directly through the wing between the points of highest pressure underneath and the point of lowest pressure above sonds lie a good way to destroy lift entirely ) so what is allowing this great increase in turning angle ? (giving much higher centripetal forces on the air trying to make it leave the surface and with the slot seemingly destroying the pressure drop that is the only thing keeping the flow attached ?)

The slats on aircraft like the Tiger Moth are simply bent flat plates and hence they move the same or lower leading edge radius further forward and the passage of the air is not apparently eased by increasing the radius (they are more like flow turning vanes in a wind tunnel for example) -- circulation theory and explanations using Bernoulli don't seem to work on inspection --like the perennial 'how wings work' debate using half venturis or 'equal time' etc 'hypothesis's .

Compare the slot to the drooping leading on the same rationale if the point is not clear .

The 'substituting' of VGs for slots or slats is another case of different flow mechanisms at work and maybe misconceptions again (has the boundary layer really slowed down and grown thick by 15% of the chord ? )
 

wsimpso1

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If some one is so bold as to wade through Smith's article and the incredible amount of circulation theory, etc, you can get his story, which is that by shifting flow from the bottom to the top, you increase the circulation. This is increased velocity on the top flow (closer streamlines, etc, which is more energy), and the opposite on the bottom. More lift.

While theoretical aerodynamicists have loved the superposition of circulation and streamline flow as a mathematically based method for describing all of this, I have always found it too complicated to do much with in designing a real airplane. There is so much more to designing an airplane, so I get all practical, and just use the published data, knowing that it will get me as close as anything else and do it quickly.

Try to have fun.

Billski
 
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orion

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Actually Billski is correct - the additional mass flow over the top should create more lift however the practical is rarely as benign to the theoretical. This is one of those things I remember from back in school - the conclusion to the lecture series of high lift devices was that in practical use, simple leading edge slots provided a very minute amount of additional lift and thus most designers tended to ignore the benefit. The practical application of the configuration is simply as a built-in fix for boundary layer stabilization.

The configuration should however not be confused with or substituted for more effective leading edge devices, especially those that aid in changing the wing profile's camber geometry.
 

Aircar

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Here is a link to one of the best exponents of high lift leading edge device (in GA other than airlines ) http://en.wikipedia.org/wiki/Scottish_Aviation_Pioneer --note the extreme take off climb angle.

Typical high performance homebuilt aircraft are high speed also at 'low' speed and frequently have stall speeds well above certified aircraft with lowered safety margins as a result --good high lift devices can restore the low speed safety and/or allow for yet smaller wings in cruise with better speeds (exactly as airliners NEED super high lift devices to allow practical landing and take off distances and even then by flying at great height they minimize the need to reduce wing area at cruise --typical light aircraft do very little to tailor the wing for optimal performance . The leading edge slat also balances the pitching moment from the rear Fowlers so has more benefits than appears -- loss of laminar flow is not that much of an issue for 23 series airfoils or insoluble otherwise and airliners live with them quite well --- in any event the question of the functionality of slats and slots is quite a subtle one and bears on some significant isues for personal aircraft where they are hardly ever used .
 

wsimpso1

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+1 on Orion.

Actually, I was not trying to say that the slotted wing has higher Cl at any particular AOA. In my earlier post, I was talking about getting the section to stall a couple degrees higher AOA, which buys higher Cl at about 0.11/deg. Way better than how much (how little) the Cl changes at lower AOA.

Have fun.

Billski
 

ARP

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I was interested to see the idea of how a jib and mainsail are mentioned in relation to a wing with slots or slats. This film from the 70's Early Hang Gliding (The Flying Prentices) Part 4 - YouTube shows such an arrangement. The article by Arvel Gentry is interesting as it gives an explaination from an aerodynamicist point of view rather than the generally held view by sailors. All I know is that it worked and very slow flight was possible. The L/D was better than the standard Rogallo's of the day and no step landings were the norm with a pronouced flare possible. The main had no battens to maintain aerofoil profile and the leading edge can be seen collapsing but then reforming further back along the sail. The interaction of the two sails enable a greater AoA to be achieved for the whole wing. In flight the wing can be seen to nod as the jib starts to stall but recovery is soon made with little height loss. Other gliders were built on the same lines and it is hoped to return to this theme in the near future.

If there are any others working on such designs or interested to know more then please contact me direct.

Tony
 

autoreply

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Typical high performance homebuilt aircraft are high speed also at 'low' speed and frequently have stall speeds well above certified aircraft with lowered safety margins as a result --good high lift devices can restore the low speed safety and/or allow for yet smaller wings in cruise with better speeds (exactly as airliners NEED super high lift devices to allow practical landing and take off distances and even then by flying at great height they minimize the need to reduce wing area at cruise --typical light aircraft do very little to tailor the wing for optimal performance . The leading edge slat also balances the pitching moment from the rear Fowlers so has more benefits than appears -- loss of laminar flow is not that much of an issue for 23 series airfoils or insoluble otherwise and airliners live with them quite well --- in any event the question of the functionality of slats and slots is quite a subtle one and bears on some significant isues for personal aircraft where they are hardly ever used .
That's an interesting remark. I don't agree though. I've done some trade-offs for my own design. While Fowler flaps (logically) allowed much higher Cl's, the increased drag of an exposed flap track, or offset hinges completely nullified the smaller wing area, compared to laminar airfoils. A bigger wing with a lower Cd deliver the same stall speed and roughly the same drag as a much smaller wing with efficient Fowler-flaps. In terms of complexity or building time/cost, that's an easy choice. Naturally, your dL/dAlpha goes up, so a rougher ride, possibly lower Va (for the same stall speed) and requiring a stronger wing for the gust case @ VNE. Also your drag in landing can be too low.

I would think the above would be even worse for LE devices on a small aircraft?

For airliners I guess those are rarely concerns. They have LE devices anyway (de-ice, truck damage tolerant parts on the LE), laminar flow is outside their Re territory and their penalty for complexity is lower as for most homebuilders or GA manufacturers.

If one is going for LE devices anyway, wouldn't a Krueger flap be an interesting alternative? Seems a lot simpler from the mechanical perspective to me?
 

skier

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This is one of those things I remember from back in school - the conclusion to the lecture series of high lift devices was that in practical use, simple leading edge slots provided a very minute amount of additional lift and thus most designers tended to ignore the benefit. The practical application of the configuration is simply as a built-in fix for boundary layer stabilization.
Is reality that much different from what is explained in TOWS? It seems from figure 134 that it should be relatively easy to increase the lift with a slot. The table showing the results of a study by Weick and Shortal seems to indicate a 37% increase in Cl_max due to a slot, but an increase in minimum drag of 57% with a configuration as shown below:
ARA-D13.jpg

I found the following image online and was curious as to whether this is a good representation of airflow around a slotted wing at high angles of attack:
slat1.jpg
 

StarJar

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Is reality that much different from what is explained in TOWS? It seems from figure 134 that it should be relatively easy to increase the lift with a slot. The table showing the results of a study by Weick and Shortal seems to indicate a 37% increase in Cl_max due to a slot, but an increase in minimum drag of 57% with a configuration as shown below:
View attachment 17598

I found the following image online and was curious as to whether this is a good representation of airflow around a slotted wing at high angles of attack:
View attachment 17599
I think what Orion was saying, is that the slot does not create the additional lift, it alows a higher angle of attack, which in turn creates the higher lift figures.

I didn't know that they increase the minimum drag by that much, 57%. All the Stork guys won't want to see that.;)
 

Wagy59

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Hmm..I sort of breezed through all this discussion so tell me to shut up if necessary, but my impression is that everyone seems to be over looking the whole point of slots in the first place, which has nothing to do with "increasing" lift...
just me....old, grumpy., and practical...(sort of)..
 

Norman

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Hmm..I sort of breezed through all this discussion so tell me to shut up if necessary, but my impression is that everyone seems to be over looking the whole point of slots in the first place, which has nothing to do with "increasing" lift...
just me....old, grumpy., and practical...(sort of)..
OK I'll bite. The slot itself doesn't do anything. It's just the incidental space between two lifting surfaces. The proximity of the two wings allows them to reach a higher angle of attack, and thus coefficient of lift, than they otherwise could.
 

orion

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OK I'll bite. The slot itself doesn't do anything. It's just the incidental space between two lifting surfaces. The proximity of the two wings allows them to reach a higher angle of attack, and thus coefficient of lift, than they otherwise could.
Exactly
 

Head in the clouds

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With reference to slots with swept wings and vortex lift - l/e sweep of, say, 35 degrees or more to comply with Norman's earlier comment in another thread about 30 degrees plus something for yaw in a crosswind - in what way might the use of a l/e slot, such as shown on Bradyaero's flitter for example, affect the vortex, and hence the lift on the outboard portion of the wing?
 

Norman

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I've seen comments that sweep decreases the effectiveness of slats but I haven't seen any evidence that that has stopped anybody from using them on moderately swept wings. My comments on the vortex lift thread only apply to a permanently attached LEV which you will not get with less than 55 degrees of sweep or some kind on effective vortex flap. Smaller sweep angles will only result in a longer dynamic stall period. Dynamic stall, AKA delayed stall, is a phenomenon that occurs when you pitch up faster than circulation can keep up and the airflow momentarily separates from the leading edge. This separated flow rolls up into a vortex, appropriately called the stall vortex, that separates from the wing when the circulation catches up with the new AoA (of course if the new AoA is beyond the steady state stall angle then you're just stalled). While the stall vortex is on the wing it massively boosts CL which will make the plane balloon a bit but mostly just makes you lose speed. The duration of dynamic stall can be calculated with the Strouhal number. It's a simple formula but don't ask me how to apply it. In any case the period will only be a few seconds. Hang glider pilots are using this phenomenon when they flair for a standing landing but it takes a lot of [skill and luck to] get your feet on the ground just as the stall vortex separates. When you see a hang glider pilot fall an his face it's usually because he didn't get the timing right or a gust changed the conditions.

I haven't seen any papers on how slots or moveable slats affect the LEV but my guess would be that the LEV that forms on the slat would behave normally but that would just contribute to pulling the slat forward more forcefully. The LEV on the the main wing though either wouldn't form at all or would be blown off by the slot flow. Just guessing though
 
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