Van's Air Force
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Getting to know my airplane. Wing washout?
- Thread starter AirHound
- Start date
ZERO. Washout is not needed with a "square" wing planform.
KatanaPilot
Well Known Member
Mel's statement can be confirmed by the blueprints of any wing that Van's Aircraft offers for sale.
Really?
Try rigging a cub wings without washout and then go stall it. Not a very pleasant experience.
What he said! Sorry for the "abruptness".
KatanaPilot
Well Known Member
I wasn't questioning the statement regarding the RV-12 wing having zero washout. What I was asking about was the blanket statement about "square" (rectangular) wing planforms not needing/desiring washout. No argument about washout adding complexity to the construction of the wing. Certainly the designer has to decide on the relative merits of each.
I didn't find the reply to be abrupt, just somewhat incomplete.
I didn't find the reply to be abrupt, just somewhat incomplete.
scsmith
Well Known Member
why no washout
Hi,
A rectangular wing does not need washout because it naturally stalls first at the inboard regions of the wing. Washout is added to some wings for two reasons. First is to achieve a more nearly elliptical lift distribution, which is more efficient. Second is to prevent the outer wing region from stalling first, which can lead to undesirable stall characteristics.
A rectangular wing does carry too much lift outboard for best efficiency, but with our low-aspect ratio wings, the difference is small. And at cruising speed, the induced drag is so low that it is not worth doing anything about it. An exception would be for flying at very high altitude, but again, the low aspect ratio is such a dominant feature that it is not worth worrying about the deviation from elliptical loading.
A rectangular wing stalls in the inboard-most regions first because the strong tip vortex shedding produces more downwash on the outboard regions, reducing the effective angle of attack on the outboard region. The inboard region sees a higher effective angle of attack and starts a gradual stall progression from inboard to outboard. This makes for a very safe, manageable stall characteristic. Even if manufacturing asymmetry makes one wing stall before the other, the ailerons are unaffected and recovery is easy.
A highly tapered wing has a more nearly elliptical lift distribution, which means that some of the lifting vortex is shed more inboard. This tends to produce less downwash on the outer region compared to the inboard region. So the outboard region of the wing is producing more lift in proportion to the shorter chord lengths near the tip. Since the short chord airfoils near the tip are working harder, the wing tends to stall on the outboard region first. Slight manufacturing variation, or slight slip/skid angle can cause one tip to stall before the other, leading to abrupt wing drop. To make matters worse, the stalled region includes the ailerons, so trying to use aileron to correct for the wing drop may actually make the stall worse, and can start the initial entry phases of a spin. This is why rudder should be used to level the wings from a wing-drop during a stall. Yawing the airplane with the rudder allows the dihedral to produce the rolling moment to recover the wing drop.
As a point of extra interest, not related to RVs, adding wing sweep further aggravates the tip stall problem. A swept wing, if twisted to produce nearly elliptical lift distribution at a cruise condition, experiences very little downwash, or actually even upwash, on the outer region of the wing, because the portion of the lifting vortex that is shed inboard is longer (extending farther forward because of the wing sweep) and produces more upwash. I recognize this may be a difficult concept to grasp without a background in wing lifting-line theory - I'm sorry I don't know any other way to explain it. The upwash on the outer wing region gets stronger as the angle of attack is increased, and you are guaranteed to stall on the outer region of the wing first.
Anyway, now with this swept wing, you have the potential for a very serious tip-stall problem. In this case, not only is there a tendency to drop one wing, but also, because of the sweep, when the tips stall and inboard portion of the wing is still lifting, the effective center of lift moves forward, causing a nose-up pitching moment -- the last thing you want when you are trying to recover from a stall. This pitch-up can be severe enough to overwhelm the elevator. The worst condition occurs when the deepening stall sheds enough of a wake to reduce the effectiveness of the tail enough to find a new trim point, or balance point that is stable, so the airplane stays at that deep stall attitude, and the controls are too ineffective to pitch the nose down.
To prevent this pitch-up at stall, swept wings have excess washout, to make the lift distribution more triangular than elliptical at the cruise condition. While this leads to some loss in cruise efficiency, it provides extra stall margin on the outer wing region that is needed for safety. And as it turns out, the reduction of structural bending moment from reducing the lift outboard also allows for a greater wingspan, which largely offsets the efficiency loss. In fact, there is an optimum lift distribution for minimum drag at a fixed structural weight that is not elliptical, but rather part way between triangular and elliptical.
Swept wings usually also have vortex generators, or a leading edge fence or Vortilon, to energize the flow on the outer wing region to get more stall margin. This is most important for transonic flight where the shock-boundary layer interactions can cause a high-speed stall that we call buffet.
Hi,
A rectangular wing does not need washout because it naturally stalls first at the inboard regions of the wing. Washout is added to some wings for two reasons. First is to achieve a more nearly elliptical lift distribution, which is more efficient. Second is to prevent the outer wing region from stalling first, which can lead to undesirable stall characteristics.
A rectangular wing does carry too much lift outboard for best efficiency, but with our low-aspect ratio wings, the difference is small. And at cruising speed, the induced drag is so low that it is not worth doing anything about it. An exception would be for flying at very high altitude, but again, the low aspect ratio is such a dominant feature that it is not worth worrying about the deviation from elliptical loading.
A rectangular wing stalls in the inboard-most regions first because the strong tip vortex shedding produces more downwash on the outboard regions, reducing the effective angle of attack on the outboard region. The inboard region sees a higher effective angle of attack and starts a gradual stall progression from inboard to outboard. This makes for a very safe, manageable stall characteristic. Even if manufacturing asymmetry makes one wing stall before the other, the ailerons are unaffected and recovery is easy.
A highly tapered wing has a more nearly elliptical lift distribution, which means that some of the lifting vortex is shed more inboard. This tends to produce less downwash on the outer region compared to the inboard region. So the outboard region of the wing is producing more lift in proportion to the shorter chord lengths near the tip. Since the short chord airfoils near the tip are working harder, the wing tends to stall on the outboard region first. Slight manufacturing variation, or slight slip/skid angle can cause one tip to stall before the other, leading to abrupt wing drop. To make matters worse, the stalled region includes the ailerons, so trying to use aileron to correct for the wing drop may actually make the stall worse, and can start the initial entry phases of a spin. This is why rudder should be used to level the wings from a wing-drop during a stall. Yawing the airplane with the rudder allows the dihedral to produce the rolling moment to recover the wing drop.
As a point of extra interest, not related to RVs, adding wing sweep further aggravates the tip stall problem. A swept wing, if twisted to produce nearly elliptical lift distribution at a cruise condition, experiences very little downwash, or actually even upwash, on the outer region of the wing, because the portion of the lifting vortex that is shed inboard is longer (extending farther forward because of the wing sweep) and produces more upwash. I recognize this may be a difficult concept to grasp without a background in wing lifting-line theory - I'm sorry I don't know any other way to explain it. The upwash on the outer wing region gets stronger as the angle of attack is increased, and you are guaranteed to stall on the outer region of the wing first.
Anyway, now with this swept wing, you have the potential for a very serious tip-stall problem. In this case, not only is there a tendency to drop one wing, but also, because of the sweep, when the tips stall and inboard portion of the wing is still lifting, the effective center of lift moves forward, causing a nose-up pitching moment -- the last thing you want when you are trying to recover from a stall. This pitch-up can be severe enough to overwhelm the elevator. The worst condition occurs when the deepening stall sheds enough of a wake to reduce the effectiveness of the tail enough to find a new trim point, or balance point that is stable, so the airplane stays at that deep stall attitude, and the controls are too ineffective to pitch the nose down.
To prevent this pitch-up at stall, swept wings have excess washout, to make the lift distribution more triangular than elliptical at the cruise condition. While this leads to some loss in cruise efficiency, it provides extra stall margin on the outer wing region that is needed for safety. And as it turns out, the reduction of structural bending moment from reducing the lift outboard also allows for a greater wingspan, which largely offsets the efficiency loss. In fact, there is an optimum lift distribution for minimum drag at a fixed structural weight that is not elliptical, but rather part way between triangular and elliptical.
Swept wings usually also have vortex generators, or a leading edge fence or Vortilon, to energize the flow on the outer wing region to get more stall margin. This is most important for transonic flight where the shock-boundary layer interactions can cause a high-speed stall that we call buffet.
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KatanaPilot
Well Known Member
Very thoughtful and thorough explanation.
If you have ever looked at many Boeing wings, you will notice Krueger flaps on the inboard leading edge and traditional slats further outboard. This was one method of forcing the inboard section of the wing to stall first. When we were working on the 767, the wind tunnel tests were showing a pronounced pitch up moment at stall - which as Steve said - not desirable and in fact, not allowed. There was a lot of effort coming up with aerodynamic fixes for this issue - which turned out not to be an issue at all on the full scale airplane.
If you have ever looked at many Boeing wings, you will notice Krueger flaps on the inboard leading edge and traditional slats further outboard. This was one method of forcing the inboard section of the wing to stall first. When we were working on the 767, the wind tunnel tests were showing a pronounced pitch up moment at stall - which as Steve said - not desirable and in fact, not allowed. There was a lot of effort coming up with aerodynamic fixes for this issue - which turned out not to be an issue at all on the full scale airplane.
scsmith
Well Known Member
Yes, interesting case about Cubs. Champs and Citabrias too. Rectangular wings, yet they need some washout. I think the difference is that with the higher aspect ratio, any slight asymmetry can cause a lot of roll moment (wing drop) at stall. But one would think that the large ailerons would be very effective.
I don't think I fully understand why.
Very good explanation, Thank you.
Just because a wing is rectangular does mean that washout is not needed.
The Piper Cub variants are a good example. Remove the washout from a Cubs wing and hold on Nelly...
KatanaPilot
Well Known Member
Swept wing stall progression starts at the tips, hence the pitch up and loss of aileron control.
I?ve got a bit over 13,000 hours in the 767/757, flew it yesterday stateside from Paris, but I?ve never stalled it, because I don?t trust aeronautical engineers. Flying is magic, and aerodynamic performance is largely derived by microscopic beings called ?Lifties?. They instinctively push on things...
Proving otherwise is impossible.
Two molecules of air meet at the stagnation point on the leading edge...
I don't trust aero types either and I am one.
I'm not sure why either but having rigged the washout out of Cubs purposefully (many years ago) and then doing stalls...The ailerons are completely ineffective and The stall recovery takes many hundreds of feet.
In the seventies in Alaska, it was popular to remove the washout from a Cubs wing to increase the short field performance. After doing the stall series I never rigged another one that way again.
scsmith
Well Known Member
And it is a good point to emphasize that most all swept wings do have some form of leading edge flaps, especially outboard, to make the low-speed stall safe. Those that don't have even more 'bandaids' like fences and VGs.
And a great point about the difference between wind tunnel Reynolds number and full scale. It is a frustrating reality that the pitch-up is much worse at low Reynolds number. Even getting two or three different Reynolds no. runs to see the trend in diminishing pitch-up with increasing Reynolds no. doesn't always lead to a trustworthy extrapolation to flight Reynolds no. Ultimately, it becomes a significant full-scale flight test activity to position some vortex generators on the wing to meet the cert. requirements for stall characteristics.
The NTF tunnel at NASA Langley, and the ETW wind tunnel in France are cryogenic wind tunnels, extremely expensive to build and operate, but the investment is considered important for exactly this reason, to get closer to flight Reynolds no.
In my defense, the question was "How much washout is built into RV-12 kits?"
The first word, "ZERO", was a complete answer. The rest was additional.
KatanaPilot
Well Known Member
scsmith
Well Known Member
Swept wing stall progression starts at the tips, hence the pitch up and loss of aileron control.
I?ve got a bit over 13,000 hours in the 767/757, flew it yesterday stateside from Paris, but I?ve never stalled it, because I don?t trust aeronautical engineers. Flying is magic, and aerodynamic performance is largely derived by microscopic beings called ?Lifties?. They instinctively push on things...
Proving otherwise is impossible.
What makes lifties decide what to push on, and how? Why don't they push on me while climbing a steep road on my bike? Or push on that big bale of hay my wife expects me to throw onto the back of the truck, which gets harder as you get older.
Clearly, the way to corral the lifties into doing what you want is to make something round on the front, sharp on the back, and blow wind over it.
KatanaPilot
Well Known Member
?Lifties?, like any other living creature, are made of molecules. Is this what you are referencing?
Yes, that and the overly simplistic and totally inaccurate explanation I was given during PP ground school in 1973 of how lifties come to be. I had to go to the North Avenue Trade School in downtown Atlanta to get "edumacated" on how lifties are really made.
The way to corral lifties is easy - spend money! Liftie generation is proportional to cash poured into a project.
I know all the text books show stall progression from the root on square wings. The lifting line theory shows that the inboard wing is the most highly loaded part. But I learned quickly that while this is true on the RV wings (in this case a -4), if you ignore the initial buffet and keep pulling, the stall can propagate outboard quickly to where it envelopes much of the wing, and it generally doesn't do it symmetrically. The pilot had me stall from the back seat. Nice stall warning buffet, stop pulling, recover. Lovely. Then he said "do it again, but keep pulling". The sky disappeared and the canopy was filled with a nice clear view of the earth and a reflection of my eyeballs wide open. hmmm. I was glad he showed me that.
So yes, zero wash out, and you don't need any if you pay attention to the airplane talking to you because it will stall inboard first. But the area of separation won't stay there if you keep pulling. Ignore the initial stall warning and keep pulling and it you can roll off
60 deg in a second, which will really get your attention if you aren't prepared, or if you are really low.
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The above description is not typical of all RV's (even RV-4's)
There is a lot of variation from one to another because of many different factors (many of which are related to the fact that the airplanes are after all, amateur built.)
"A little knowledge can be a dangerous thing"... very true statement.
Back then, we thought we were outsmarting old mister piper but some of my fellow aviators payed a steep price for our lack of knowledge.
I think it is related to the fact that with a 4 with a lardass in the back (guilty) the cg is quite far back so the elevator has the power to bring you quite deep into the stall, well past the initial break. My point is that the difference in AOA Between the inboard stall onset and the entire wing being stalled is only 2 or 3 deg. RVs have a lot of control power so they can get you to the AOA region where the entire wing has stalled. If there is any asymmetry, or imbalance or crosswind etc it can roll.
JonJay
Well Known Member
I owned perhaps the fastest 85 HP Champ in the world; the "Rose". Quite famous in the Champ circles, if there is such a thing. 120mph indicated. Crazy fast for a Champ.
Most of the mods where drag reduction efforts, intersection fairings, engine baffling and plenum, etc....
However, the one area the restorer swore had the biggest benefit was removal of the wing washout. On the Champ, that washout is significant.
It made absolutely no difference in stall/spin characteristics. Stall speeds where exact to spec. She spun on command, and stopped when you asked her on any heading you chose.
Don't ask me why, but washout just added drag in this example and had no benefit.
Most of the mods where drag reduction efforts, intersection fairings, engine baffling and plenum, etc....
However, the one area the restorer swore had the biggest benefit was removal of the wing washout. On the Champ, that washout is significant.
It made absolutely no difference in stall/spin characteristics. Stall speeds where exact to spec. She spun on command, and stopped when you asked her on any heading you chose.
Don't ask me why, but washout just added drag in this example and had no benefit.
JonJay
Well Known Member
My experience is there is very little warning as far as a buffet, especially in accelerated stalls, where there is no warning. I can only speak for simple stalls in the models I have flown and accelerated stalls in the 6. I will leave the other questions for others to answer.
scsmith
Well Known Member
I have found quite a variation between models and between different examples of the same model.
Start with the realization that the wing airfoil on the 3/4/6/7/8 is an NACA 23013.5, which has a notoriously terrible stall break. On a high-aspect ratio wing, this would create a very harsh stall. But with the low aspect ratio, rectangular wing, the stall progression outboard is slowed by the large downwash outboard, giving a seemingly gentler stall. For reasons I don't quite understand, accelerated stalls are harsher. Good examples are the T-6 (SNJ) and the T-34B, which both use the same basic airfoil, and both will do a 1/4 snap roll on an accelerated stall if you are just the slightest bit off perfect coordination. The sound is quite impressive too. When I unintentionally snapped a T-34, it felt like I had flown into a bowling ball.
The RV-7 I flew with Mike Seager had a pretty harsh stall I thought, and usually dropped a wing a little bit. Aileron control was good and it is tempting to develop a habit of using it, which I resist, because it is pretty deeply ingrained in me to use the rudder from glider flying.
My RV-8 has a complete mush stall. I can get the stick all the way back without the nose dropping. It just sinks like a stone with its nose held high. As for warning, the -8s in particular are quite variable because of variations in the shape and size of the landing gear intersection fairing, which interacts dramatically with the wing leading edge at high angle of attack. Mine gives no buffet, but does make some interesting swishing sounds that I have never heard in any other airplane. One time while practicing short-field landings I got a little slow at about 30 ft, and I heard that swishing sound. It helped- I added some power and lowered the nose a little.
I have no experience with the RV-9. The wing is higher aspect ratio, and the Ronz airfoil is going to be different.
On the RV-10 and -14, the airfoil of which I designed, I worked pretty hard to try to get a more docile stall than the NACA 230xx, and I think I succeeded. Would love to get to fly one sometime.
The RV-12 has an NACA 4-digit airfoil, which is noted for having a more gentle stall, with a smooth progression of separation from trailing edge fwd.
You might find almost as many descriptions of stall behavior from RV pilots as there are RVs. So many variables. Not just build variation, but c.g. position, paint, pilot technique, and more. But I think it is totally safe to say that any and all of them have stall behavior that is manageable and 'safe'.
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I've been very fortunate to fly (and test) a very large number of aircraft, a majority of them experimental, and every model of RV - and in some cases, a significant number of examples of particular models. In general, I can't think of any of the RV's that have poor stall characteristics, and all of them gave me plenty of warning before giving up. Yes, the break might be sharper than a lot of airplanes that simply mush along nose high and start a sink....but in all of the RV's, you break the stall by unloading (or it breaks the stall itself by unloading), and you're flying again. No big deal.
A lot of experimental have very poor stall characteristics because the designer stopped improving the model when they had a prototype that simply flew. Some have no warning, some have a tendency to want to spin right away - but the RV's fly just fine.
If all of your experience is in Cessna and Pipers, yes, the stall in an RV will be a little more exciting - but always recoverable (unless you stall at too low an altitude off course).
I am a big believer in AoA, as many know - why not have an accurate indication of where the wing is flying? No mental math related to bank angle. And the delta cost to add one these days is a minimal percentage of an aircraft's cost.
Paul
The three has an airfoil unlike the 4,6,7 & 8.
I've built the Kitfox models II, IV, and Seven over about a 20 year spread. They are definitely a square wing and we build washout in those wings, as per the plans.
Vic
Vic
KatanaPilot
Well Known Member
You designed the airfoil and with all of the RV-10's flying, no one has offered you a chance to fly one? With Van's is just up the road from you - that is just wrong.
Mark Cooper (VAF - "CharlieWaffles") has the nicest RV-10 I've seen and is near Portland. Maybe he will read this and offer to come fly with you
Even though the -3 has a shorter chord than the 4, 6, 7, & 8, I'm pretty sure that the airfoil is the same.
Correct.
It is also the NACA 23000 series. I think the thickness profile is the only difference... If I remember correctly it is 23012 instead of 23013.5 (with the last digits being the thickness to cord width ratio so the 23012 means it is just a little bit thinner than the wing on the rest of the RV's that use it).
BTW, the RV-12 also uses the 23000 series.
The original proof of concept prototype used a different airfoil which in flight testing proved to have CL values that came up a bit short of what all of the technical data said it would be so a change was made to the 23000.
The only RV models that don't use the NACA23000 airfoil are the RV-9 (John Ronz custom design) and the RV-10 and 14 (Steve Smith custom design).
KatanaPilot
Well Known Member
Blunt trailing edge
With all of this talk about airfoils and stall characteristics - I hope our resident aerodynamicist can answer this.
I've noticed the RV-14 has blunt (divergent) trailing edges on the ailerons (and I assume the flaps). My very limited knowledge of the aerodynamics associated with this feature led me to believe the benefits in drag reduction were only apparent at higher mach numbers (like 0.5 and above).
Is there some other benefit besides drag reduction or was this done primarily to simplify the construction?
(Sorry, I know this was an RV-12 thread originally)
With all of this talk about airfoils and stall characteristics - I hope our resident aerodynamicist can answer this.
I've noticed the RV-14 has blunt (divergent) trailing edges on the ailerons (and I assume the flaps). My very limited knowledge of the aerodynamics associated with this feature led me to believe the benefits in drag reduction were only apparent at higher mach numbers (like 0.5 and above).
Is there some other benefit besides drag reduction or was this done primarily to simplify the construction?
(Sorry, I know this was an RV-12 thread originally)
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The riveted trailing edges on control surfaces (on the RV models that have them) are actually sharper / less blunt than the traditionally used bent trailing edges.
The primary purpose for them is to simplify construction and result in more uniformity in the finish shape than the bent trailing edge has provided.
The largest construction variation in the entire RV fleet is the control surface shape dependent on how accurately the builder finished the trailing edge bend. It is an import detail... it can have a major impact on the handling qualitys of the airplane.
scsmith
Well Known Member
You designed the airfoil and with all of the RV-10's flying, no one has offered you a chance to fly one? With Van's is just up the road from you - that is just wrong.
Mark Cooper (VAF - "CharlieWaffles") has the nicest RV-10 I've seen and is near Portland. Maybe he will read this and offer to come fly with you
I was at Van's a while back, but the weather was not flyable. On a previous visit, I opted to fly the -12 with Van. That is a sweet-flying little plane. I think I could teach my mom to fly in a -12.
But yeah, if someone wants to drop in at Ashland in an RV-10 for a great burger at Caldera, I would love to get a quick hop.
I was unaware that when the stall speed on the -12 was a bit too high, they switched back to the NACA 230xx. I thought they just made the wing a little bit bigger. Thanks Scott for the info.
Have you tried that with a guy in the back? What you are describing is a scenario where you are elevator limited at fwd CG. With Bubba in the back seat chances are you will be able to stall to the point of a pitch break with full aft stick. CG makes a huge difference.
