Discussion on high lift devices, contributed by Bill Harris
This text has been edited to remove individual names & email addresses, and to correct speiling.
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I recently crashed a P-51 due to a tip stall and was told I needed to add washout to reduce the possibility of tip stalling. I was further told that I could do this by giving both ailerons a slightly "up" attitude. What I need to know is how much "up". Can anyone comment on this? I don't want to cartwheel it down the runway again!! Does this happen because the landing speed is too slow?
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Yes, partly. As the plane slows down, the wings' angle of attack (AOA) must increase to produce the lift required to maintain altitude or limit the sink rate. When AOA reaches a point where flow over the top surface separates and is no longer laminar (this point varies depending on the particular airfoil) it will stall. Not necessarily a bad thing - a minimum roll take-off or landing has the wing on the verge of a stall. The big problem occurs when the stall begins at the tip of a wing, while the other wing tip is still lifting. The large unbalanced moment results in a rapid (snap) roll. You can not correct it by applying opposite aileron, as this would only increase the stalled wing's AOA, making matters worse if any effect at all - so if close to the ground it's a guaranteed crash. The situation can be prevented by designing the wing such that it stalls first near the root, rather than the tip. The large rolling moment is prevented and the plane tends to stall straight ahead. This can be done either by causing the wing root to stall earlier, or the tip to stall later. The advice given to Ted was to add washout to the tip by turning the ailerons up (I presume his P51 had outboard 'barn door' ailerons), which has the same effect as twisting the trailing edge up at the tip: the AOA is reduced relative to the rest of the wing, so it stalls later. The other approach, often seen as a 'stall proofing' retrofit on 1:1 scale light planes, is to add an angled strip to the leading edge of the wing near the root. The sharper leading edge causes the wing section to stall at a lower AOA. Probably the most common way to correct for tip stalling tendencies in model aircraft is to simply twist the wing trailing edge up at the tip and re shrink the covering material to hold that twist.
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When you drop flaps, does your plane nose up? Especially under power? Dropping flaps increases the camber and the incidence in the flapped portion of your wing increasing the lift (up to a certain flap angle, then it just creates drag). You trim out the nose up either by reducing power and reaching a new trim speed for the new airfoil shape or you push forward on the stick. Either of these lowers the angle of attack on both the flapped and non-flapped sections of the wing. The wing is producing more lift for a given angle of attack. You can now fly slower at the same angle of attack that you had before you dropped the flaps. By lowering the flaps, you have also washed out the non-flapped portion of the wing. When you drop 15 degrees of flap on your 152, would you not have to trim nose down to maintain altitude at the same speed? This lowers the AOA on the wing, so the non-flapped portion of the wing is flying at less AOA than the inboard flapped section. Now pull the nose up to hit your stall speed. The stall speed is lower but the wing is also at a lower AOA than if it was non-flapped because of the increased camber and incidence in the flapped section of the wing. The non-flapped portion doesn't tip-stall because it is nowhere near the AOA/speed combination it has to reach to produce a stall (It has wash-out!) Next time you are out bumping around in your 152, do a power off stall with no flaps and with 15 degrees of flaps. The stall with no flaps should occur at a higher speed but also at a higher angle of attack.
Kind of hard to describe without drawing a picture or using my hands, but generally dropping flaps increases wing incidence at a given angle of attack and creates wash-out in the non flapped sections. Even the STOL mods on planes that droop the ailerons only droop them a fraction of the flaps. This maintains the wash-out effect at the tips.
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You are sort of right. Dropping the flaps WILL increase the lift generated by that portion of the wing that the flaps cover. However, it also increases the AOA of that portion of the wing, thus raising the speed at which that portion of the wing ceases generating lift if all other things remain constant. Do you notice the tendency of the 152 to 'balloon' when you apply your flaps? what is happening is the plane tries to lower it's nose, thus reducing the AOA of the entire wing. This is really caused by the extra drag under the wing from the flaps, but it actually helps to prevent the plans from stalling. Have you never wondered why flaps are always in the center part of the wing rather than at the tips? Stalling the center portion of a wing is much more easily controllable than stalling the tip...
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When I fly a full size Cessna 152, The stall speed with flaps is 10 KIAS less than with 15 degrees flaps? This would seem to indicate that lowering flaps lowers the stall speed. So, if flaps reduce stall speed, wouldn't the stall speed of the wing section with the ailerons (dipped as flaps) have a lower stall speed then the inboard section of the wing (sans aileron effect)? Therefore the inboard section of the wing should stall at a higher speed, thus preventing tip stall? Or does the resultant higher AOA cancel this reduced stall speed (and then some)?
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Yes, flaps lower the stall speed, but the wing will still stall at much the same Angle of Attack as before. There is a common misconception that flaps increase lift by increasing the AOA - they do, but that does not reduce the stalling speed! Imagine a plane in level flight, lift = weight. Putting the flaps down increases the AOA which increases the lift so the plane starts climbing, in order to re-establish level flight you have to push the nose down to reduce the AOA of the wing back down again until lift = weight, in which case it would be pointless as far as reducing stall speed is concerned. Lowering the flaps changes the camber of the wing, and that increase in camber means an increase in the lift coefficient of the wing section, so at the same AOA and same speed the wing will develop more lift. Now, since lift > weight you can reduce the AOA until lift = weight, and now the wing is at a lower AOA than it was before using flap. Thus if 2 identical aircraft are flying at the same speed, the one with flaps down will actually be at a lower AOA because its wing has a higher coefficient of lift. Both aircraft can reduce speed and maintain straight line flight by raising the nose until the stalling AOA is reached. Since the plane with flaps down is at a lower AOA than the one with flaps up, the plane with flaps down will still have a few degrees of AOA left when the unflapped plane reaches the stall angle. Thus the flapped plane can slow down further yet and keep lifting the nose until it too reaches the stall angle at much the same angle but at a lower speed than the unflapped plane.
Remember that AOA and nothing else determines the stall. Each type of wing section and planform has a critical AOA at which it will stall, irrespective of speed. In 1G flight the plane will stall at its quoted speed, in a turn with 90 degrees of the bank the stall speed is at infinity, in a zero G pushover the stall speed is zero knots.
Putting both the ailerons down increases the camber and therefore the lift coefficient of the wing in that area. It also increases lift due to the increased AOA. To maintain straight line flight the whole wing must reduce its AOA until lift=weight again. But all the time the part with ailerons down will be at a higher AOA than the rest of the wing. The increased camber due the to ailerons down will increase the mean lift coefficient of the wing thereby lowering the stall speed of the wing, but the first bit to stall will be the highest AOA which is the drooped ailerons. If the ailerons are turned up, the camber is reduced, the mean lift coefficient is reduced and the stall speed will rise very slightly, but the part where the ailerons are will be the last part to stall. The change in stall speed will be very slight and any pilot will be more than glad to give up a few knots of stall speed in return for a straight forward stall instead of a tip stall. In a model it is doubtful that you could tell the difference in stall speed but you will certainly tell the difference in stalling behavior.
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Tip stall means that the tips of the wing get to the stalling angle of attack before (or just before) the rest of the wing does. Since in a stall the airflow is not attached to the surface, you do not have any control authority from the control surfaces. So, what results is that the tips of the wing stall, and one side (perhaps the heavier side) might start to fall, but you have no aileron power to raise it.
To avoid tip stalls, you have two choices; either decrease the angle of attack(A0A) at the wing tips (so that the tips are still at a low enough AOA, with the flow attached for positive control), even when the root of the wing enters the stall. This way, you will get an indication that a stall is starting, and will have a chance (and control) to get out of it. The second choice is to make the root stall quicker than the tip (same thing said backwards), by for example making the leading edge into a triangle rather than round. This of course is not really preferred, because you are deliberately making one part of the wing easy to stall, and perhaps a little unpredictable.
So, taking the first approach, washout is the most general method used to avoid tip stall. It simply sets the tip airfoil at a less AOA than the root.
The use of ailerons however, also has an effect on the AOA of the airfoil; I usually imagine the imaginary line passing from the leading edge to the trailing edge of the airfoil to be its angle (which is not true, but good enough for coffe-table-imaginary-wind-tunnel-tests!). So, if you lower one aileron, the AOA of that part of the wing increases, and it becomes closer to stall. If you raise an aileron, the AOA decreases, so a stall is less likely.
Therefore, what you were told is to apply up aileron to decrease the tip AOA of the wing, and avoid tip stall. Although this might work, it will deform the usual airfoil, and give you an inefficient wing (that might be OK though, considering how difficult it is to build washout into a completed wing). A few degrees should be enough. The best way is to try in one degree increments, take it up, and stall it there to see.
BTW, using too much aileron during landing or takeoff when the airspeed is marginal, is not recommended for the above reason. You might give too much aileron input, exceed the AOA limit on the down aileron side, and stall that wing. Use rudder to turn away from the dropped wing to recover in such cases.
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I have seen a lot of accidents due to THTF... most of them are broken gear and firewalls that occur when the plane is forced down. This is better then the few too slow too low accidents I've seen where a spin into the ground totals the plane, but either way it'd be better to be able to put it exactly where you want it every time. I think flaps are the best way because they increase drag a lot as well as lift which means you can come in slower... One other plus is the fact that a wing with inboard flaps stalls at the root way before the tips so tip stalls are HARD to do. Split flaps increase drag real well, but aren't the best for lift... Plain flaps are OK at both drag and lift but not especially good at either and they cause that nasty pitch up when lowered. Slotted flaps are the best way to go. They increase lift a lot as well as drag and don't cause as much pitch up when lowered. They aren't hard to make... just make the back of the wing at a 45 degree angle sloping up and back from the bottom to top and maybe extent the top TE sheet out a 1/8-1/4 inch overhang... then sand it all rounded and smooth and cover it. I'd make the flap longer chord (shorten the ribs) and use a piece of triangle stock to make the 45 at the back. The flap could be made out of regular TE stock sanded to an airfoil shape or built up. Shape the flap so it fits in nice and neat under the overhang and is flush with the bottom of the wing. There will be a big gap between the LE of flap and TE of wing on the bottom but as long as the top is close it will be ok (check out full size Cessnas... same thing). To make the flap deploy back and down you need the hinge point a little below the leading edge of the flap. You can use two blocks of balsa one under the wing and one under the flap to put the hinges in... If you want to get fancy you can even sand them into an airfoil shape. I draw it out to scale on a piece of paper then move the hinge point around till I find the point that makes the flap move into the location I want (about a 1/8inch gap with leading edge of flap about 1/8inch under the overhang). A pushrod can push/pull directly on the leading edge of the flap to lower and raise the flaps.
Another option is Junkers flaps that are small wings/flaps that are placed below the TE of the wing (about 1/8-1/4 below and leading edge flush with TE of wing). The wing is normal with a thin TE. All you have to do is make a good airfoil for the flaps, then put blocks on the bottom of the wing to hinge to and set them up normally from there. The slot between the wing and flap acts like a slotted flap... I set up one plane with full span junkers flaperons... worked pretty good.
As far as landing you will find that a plane with big flaps can be brought in high every time and still slow down and land... and you can make a normal approach with power on to have very precise control of speed. When you get down to the runway reduce power and land (with big flaps or high deflections you will need extra up elevator to flare)... gotta have a reliable engine to do that though.
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A quick tip on use of flaps for flying is don't use more then 20 degrees for takeoff and slow flight... any more then that and the flap is usually stalled and not making extra lift (more harm then good). Use as much as you want for landing, but remember when the flap reaches the stalled deflection point, about 20-25 degrees, the drag will increase rapidly and the lift will stay about the same or drop slightly. Make sure you have enough elevator to flare... the biggest problem with flaps is running out of elevator on landing... if you can't get enough elevator you should land at a higher idle so the prop wash helps you flare better.
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I'd say 20-25% would be about right. There are a few planes that use 30% and if you want as much lift/drag as possible you can go that high. So between 2.75 and 4 inches should work. For a wing of that size I'd use a gap around 1/4 inch. The hinge point will probably end up being below and back a little from the leading edge of the flap. for a 3.5 inch flap I drew up a 14% thick flat bottom airfoil with a 3.5 inch flap on it. To get the gap right the hinge point had to be 1 inch down and .75 inch back from the leading edge of the flap. The leading edge of the flap is fairly tight and very low almost at the bottom skin. The main wing airfoil at the leading edge of the flap is 1 inch thick and I used a 45 degree line up from the bottom surface to the top starting at 3.5 in forward of the TE. The overhang was 1/2 inch. The maximum thickness of the flap is .75 inch at 1.25 inches back from the flaps leading edge (directly below the bottom of the overhang sheet. If you use these dimensions you should come up with something about right.
While looking at the drawing I noticed that with a little filler between the triangle stock and the overhang sheet the inside of the slot could be cleaned up pretty good, maybe another small piece of sheeting could be used. Another thing that I forgot to mention is that a slotted flap can be deflected to around 30 degrees for takeoff and slow flight because the slot energizes the boundary layer and keeps the flap from stalling until higher deflections then plain flaps.
About the hinges etc.. I have seen a few complicated ways to do it... but I prefer to keep things simple so I just cut out a piece of 1/4-3/8 thick balsa that goes about an inch back from the hinge point on the flap and about an inch forward of the leading edge of the flap on the wing... and down about 3/8 inch below the hinge point. Then I draw a line from the hinge point to the leading edge of the flap. Cut the block along the line and glue the back half to the flap and the front half to the wing... being very careful to place it properly... you could probably just glue one on to begin with and then get the hinges in and the flap lined up just right before gluing the other one on.. that would ensure perfect alignment. I use either great planes or Robart hinges that are plastic spikes that stick into a small hole... they are the only ones narrow enough to work.
The only thing left is to cut a V. out of the blocks below the hinge so the flap can be lowered... if you want a stop at a specific deflection use this V. to do that. If you want to save some drag sand a nice airfoil into the whole thing... you can put a swept leading and trailing edge on for style.. taper it... whatever you want as long as it functions... For pushing and pulling on the flap I cut a small block out of the leading edge of the flap... then put a small metal rod across where the block was taken out... cut up the block so you can secure it in etc.... this way the linkage runs straight out the TE of wing and nothing is exposed. You can also put a control horn on top... or extend one of the hinge blocks connected to the flap down and push pull on it...
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The slats will give provide better slow flight and shorter landings, but won't shorten takeoff run or increase payload unless the plane is able to rotate to very high AOA's on the ground... The way a slat/slot works is it doesn't change the lift of the wing until the normal wing would stall... then the wing with the slat keeps on increasing in lift until it's stalling AOA which is very high... if you were looking at a lift to AOA graph you would have one line going up at an angle like normal with a curve for the first stall (no slot) the line for the slotted airfoil would be the same up till the normal stall but keep going at the same angle until higher AOA and CL. The biggest problem with this is that to take advantage of the extra lift you need to fly at high AOA. A conventional vertical stab gets blocked out pretty bad at high AOA's so twin tails work best... or T-tails... I usually add small fins about 2/3 out on each side of the horizontal stab, they extend about two inches forward, one inch above, and two inches below... the area below and in front of the horizontal stab is most important because it is in "fresh" air.
You got the idea of the junkers flaps, they are just small wings that trail below and behind the main wing.. They act on the same principle of slotted flaps allowing high energy air to flow over top of them which allows them to be deflected to higher angles and produce more lift. Junkers flaps are great for two reasons. First they are easy to set up, and second they can be added to a plane with no major modification to the wing... just glue on small tabs to hinge to and find some way to push/pull on them.
The spoilerons for my Telemaster are a slip mechanism... they have to be pushed to go up, but when pulled on the linkage has to slip. I designed a simple slip mechanism that would fit onto the pushrod using a steel tube, a couple wheel collars and a small spring. The spoilerons allow for full span flaps... but best of all they have no adverse yaw which means you don't have to mess with mixing in rudder at slow speeds.
When you get into "payload" planes instead of STOL There is a whole different way to do it. To get max payload a good undercambered wing with out any slats or flaps is usually the best way to go. The Gottingen 525, Selig 1223, and Wortman fx63-137 are three excellent airfoils to use on model planes... the 525 can be built from balsa... the other two almost have to have foam cores. There is also a different set of devices for payload planes. Leading edge droop is a good one... the leading edge basically hinges and can be drooped down to "make" an undercambered airfoil. Fowler flaps that increase area/lift but don't make much drag are good. Retractable slats are good and can be set up so they hang way down when deployed which increases lift even when at low angles of attack. You can do the same thing with a fixed slat/slot. If you attach the slat so that it's leading edge is below the bottom of the wing and it increases the camber in will increase your payload and shorten takeoff... when at high AOA's the air flows through the slot and it acts like a normal slot from there on. Krueger flaps are great.. they fold out from below the leading edge forward... Basically the bottom skin of the wing from the leading edge back an inch or two would fold forward so you had a slab pointed down from the leading edge of the wing forward about 45 degrees.. this "flap" basically grabs tons of air and directs it over the wing which increases it's camber/lift (used on 707 and 747)
The web page below is loaded with cool aerodynamic stuff.. scroll down the right side until you find "high lift systems" Click on that and you'll see lots of high lift devices.
http://www.desktopaero.com/appliedaero/appliedaero.html
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Your argument for wing loading doesn't hold up here. One of the main points for doing all this stuff is so you can carry up cameras, bombs, or whatever else you can think of... no matter how light the wing loading of the model alone it will increase when the payload is added and you need to have enough lift to take the increased wing loading. You could build a plane without any of these devices that relied on a normal airfoil and light wing loading but it would be about twice as big which means more hassle and more money. I know quite a bit about low Reynolds number aerodynamics (I am an aerospace engineering student and chief designer for my schools RC heavy lift plane), energizing the boundary layer with a leading edge slot will increase your stalling AOA much more then a trip strip or even vortex generators ever would... and it's not too hard... make a slat out of anything you want, someone from RCO used a PVC pipe, and glue it to small tabs on the leading edge of the wing.. pretty easy. The slotted flaps aren't too hard either... the only reason my post on how to set them up is so long is because I went through the steps very clearly and gave an example... that way it is even easier to make them. One big reason for there being a demand for this plane is it's novelty.
If there were two planes at a flying field, one was a fun fly with a high powered engine and a light wing loading, and the other was a STOL plane with Slats and slotted flaps... which do you think would get more attention. Fun fly's are great, pattern planes are great, helis are cool, scale is fun too... all STOL is another type of plane to add to the list. It has advantages and disadvantages.. weigh them and decide if it's for you... just like all other planes.
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The lift coefficient with leading edge slats and slotted flaps would probably double... I haven't got data on the lift coefficient changes at these low Reynolds numbers with slats and flaps. But my Stick's stall speed decreased by about 50% when the slats were added so that shows a significant increase in max CL. We are talking about two completely different ways to do the same thing when you talk about STOL. Both ways will give you good slow stall speeds, both will give good climb rates and climb angles, The plane I'm talking about will be able to descend much steeper with the flaps though.. and my plane will be able to carry a payload as well as fly slow.. that gives my plane an advantage over the light wing loaded plane you are talking about in two areas. I don't see why you are so against this idea... could you fill me in on your reasons to dislike this type of plane so much?
If you look at the posts preceding this one you find that most of the people liked the idea and think it would be fun to try out. FUN is the keyword.. We are all in this hobby to have fun and flying around at 5mph at a 30 degree AOA with the elevator back and 1/4 throttle is fun!! Landing at 5mph with the nose pointed way up, touching tail first with the mains a foot off the ground is fun!! Doing loops and rolls and inventing weird maneuvers all at high AOA's and low to the ground without having to worry about stalling or spinning is fun!! that is what this is about. I have done all of this with my planes with slots and that is the proof that it works. If you don't believe me then oh well... no skin off my back.
Now why don't we get this thread directed back to answering questions about making a plane that can do this fun stuff... I'm making up a set of small diagrams and instructions to send out to those of you that have asked for them and they'll be out in a couple days or so.. I will need snail mail addresses though.. I'm not set up to do it via email.
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Wing tips are a good point... The air under the wing is at a higher pressure then the air on top of the wing so at the tip the air goes from the bottom around the tip to the top.. by the time it gets around, the wing has already moved forward and your left with a vortex that constantly trails each wing tip. You loose a little lift at the tips because of the lowered pressure on bottom and increased pressure on top... but mainly you increase drag a bunch... A good wing tip will lower stall speed, increase cruise speed, and increase stability. The easiest wing tip is the tip plate.
Just make a plate that attaches to the tip and extends above and below the tip.. this disrupts the flow around the tip, it doesn't completely stop the vortex but it helps. There are several variations of the swept tip or crescent tip including horner tips (droop tips). The horner tip sweeps back so that the trailing edge of the tip is farther out then the leading edge... it also curls down and has a sharp edge to it... the idea is that the flow has a harder time getting around the swept lower tip and the sharp edge acts like a ramp so the vortex "jumps" out farther away from the wing before it forms. If you do the opposite with the wing curling up you have a crescent tip which is most popular these days. The idea of a crescent tip is that it is shaped to exactly block the flow as it tries to go around the tip... so the air flows out at an angle that is equal to the angle of sweep of the tip.. the tip curls up at exactly the angle that the air would flow around the tip.. and at the end the tip flares out a little to push what little vortex does form out away from the wing. The next tip is the winglet which is best for STOL planes. It is a wing that goes up and out from the tip.. the angle is usually about 70 degrees. The winglet is also angled so that it pushes the air out away from the wing (lift vector is pointed IN toward the fuse) this angle is usually around 3-5 degrees. The winglet helps block the flow of the air around the tip and pushes the vortex out away from the wing... it also uses the energy of the vortex to produce lift in a forward direction which is actually thrust... it's ironic that it uses the drag causing tip vortex to make "anti-drag" The winglet also acts like dihedral increasing roll stability and acts like an automatic drag rudder increasing yaw stability.
If you want info on wing tips let me know...and I'll include it in with the high lift stuff.
Andy Lennon was mentioned earlier and I'm glad that happened. Andy Lennon is a great aeronautical engineer that has worked with Burt Rutan in the past. He has designed some incredible RC planes using high lift devices... He uses modern technology on RC planes to get maximum performance which proves that this stuff does apply to models... If you want a good STOL plane but don't want to modify a plane or design one yourself look at his STOL CROW. It uses spoilerons, full span slotted flaps and full span leading edge slot/slats. The plans are sold through MAN I think. The plane isn't strictly STOL it really moves out and does good aerobatics as well.. it just uses these devices to get good STOL performance as well as good speed etc.... I still think a kit STOL plane that is designed for STOL would be a good selling kit.
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The wing span is 72in, the chord is 10in that gives us a wing area of about 720 in^2 when you count in the area of the slats as well the area jumps to 825 in^2. According to my calculations that gives a max CL of 1.89 which is right on the money for a wing with a slot. There are no HARD numbers already established for high lift devices at low Reynolds numbers as far as I know. I am thinking about building a plane with slots here at school, all my other ones are in Alaska, so I can do some more research. I will also build a test section with a slot and one with out and put it in the wind tunnel... I can get Reynolds numbers down pretty low in our small wind tunnel so I should get good results. It will take a while to do all this because I have a lot of other stuff going on. Classes for one, Designing the SAE heavy lift plane for another, and designing my first "full size" plane which I will be building next year. I don't like to boast but I do have a very good understanding of aerodynamics. I have read two books on low Reynolds number aerodynamics. Three on laminar flow and laminar flow control. Several on general aerodynamics.. I have read books on preliminary design, stability and control etc.... Let's put it this way, in my school the seniors design a plane the first semester and build a wind tunnel model to test the second semester. Some of the seniors that I know come to me for advice on designing their planes.. and I am a sophomore. I am sharing information on this string to help people that are interested in this stuff understand it better because I like to help other people understand aerodynamics etc.... not because I am full of myself and think I know everything.
Burt Rutan used to put out a news letter for all of the builders of his planes. He would report everything he could think of that might help the builders out in building and flying their planes safely. He even reported all of the accidents in order to help the builders avoid making the same mistakes as the person that crashed... on a few occasions design flaws were found and he quickly told all the builders about them not even thinking about what it would reflect on him the designer. That is what I am doing.
I'd like to thank all of you that have stuck up for me here... it's nice to know that people appreciate what I am trying to do. I have almost completed the paper on slats, slotted flaps, and winglets.. it is a little sloppy but should help out a lot. Again if you want a copy give me your address and I'll send it to you!
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I made a mistake on the calculation for CL in the previous post. The CL needed to get a 12mph stall speed would be 2.72 which is higher then could be achieved at our RN's without major devices. I would estimate that the airfoil with the slot has a max CL of 2.0 which would give a stall speed of about 14.5 mph. I would estimate the max CL of an airfoil with both a leading edge slot and slotted flap to be around 2.3-2.45.
Since it is a triple element airfoil the effects of low Reynolds numbers on this airfoil will be minor. The Selig 1223 airfoil is the only one I have data for at low RN's and it's max CL at 100,000 is about 1.7, at 200,000 it is 2.17. If you add vortex generators to it the max CL at 100,000 is about 1.95 due to the increased energy of the boundary layer keeping laminar separation bubbles from forming. The slot does the same thing which means it makes a big improvement on performance at RN's below 200,000 or so.
You can get high CL's at low RN's... not nearly as high as at high RN's, but still high. Some airliners have max CL's in the 4.0 range using slats and slotted flaps... they can get such high numbers because of the high RN's that they operate at... We don't get numbers nearly that high at our speeds and wing chords, but we can get well over 2 though.
Andy Lennon's STOL CROW has a stall speed of 14 mph, a wing area of around 650 in^2 and a weight of about 5.5 lb. That would mean it has a max CL of about 2.43 using full span LE slots and full span slotted flaps. There is the proof.
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Rutan's been one of my heroes since his Vari Viggen days (did you see it in Deathrace 2000?). Rutan is an "idea" guy; he likes to build working prototypes, but has little interest in developing them beyond that stage.
He's driven by the NEXT idea, not the last one. Nothing wrong with that in my book. (Sounds a lot like Ty!) I like Andy Lennon's work, too.
It's hard to understand the cynicism from Ron and Dannyboy. I'll take Ty's enthusiasm any day. Here's a student, probably not rolling in dough, who's learning about aerodynamics and applying what he's learned to so he can see it work in the real world, not just on the pages of his books.
Right now, his real world subjects are model airplanes and we should be glad that he's sharing his enthusiasm and experiences with us. Eventually, he'll move on to the full-scale world. I don't sense any exaggerations in his posts and look forward to seeing his creations in Popular Science or Sport Aviation or wherever.
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I tracked down the MAN issue that had the CROW construction article (August 1996). The specifications are:
Wingspan = 57.5 in.
Wing area = 500 sq. in.
Weight = 88 oz. (gross)
Wing loading = 25.4 oz. per sq. ft.
Horizontal tail area = 112 sq. in.
The text lists the wing's maximum lift coefficient as 2.31 and a flaps down stall speed of 16mph.