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Why Lifting Stab?

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Why Lifting Stab?

Old 10-20-2005, 09:04 AM
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skylane42
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Default Why Lifting Stab?

In the 1/2A forum someone asked what the advantage or reason for the lifting/symetrical airfoil horizontal stab on alot of the older designs like the goldberg falcon and skylane models was. I thought it had something to do with the planes being designed for single channel or freeflight but an not sure. What do you think?
Old 10-20-2005, 09:24 AM
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Dr1Driver
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Default RE: Why Lifting Stab?

It's not just for old planes. I use one on the fun fly/sport plane I designed only a few years ago.

It keeps the tail lifting, even after the wing stalls, so the nose drops to regain control. It's also more effective at low speeds.

Dr.1
Old 10-20-2005, 09:26 AM
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skylane42
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Default RE: Why Lifting Stab?

Thanks DR.
Old 10-20-2005, 09:54 AM
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Ed_Moorman
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Default RE: Why Lifting Stab?

Your question is really asks 2 things. A thick, symmetrical section for the stab as is used on most pattern planes has more and better control than does a flat plate section.

A lifting airfoil on the tail is completely different. On older free flights, the CG was way to the rear, 70% on the MAC in some cases, so the stab was in effect a rear wing, lifting upwards. Under power, the plane would climb, but would glide flat.

On a normal RC plane with the usual CG location, the stab force is downward. Have the stab come off in flight and see what happens. You get immediate pitch nose down. Use of positive incidence or a lifting section for the stab is an attempt to maintain level flight under changing power without changing trim. On a trainer, for example, you climb like mad with full power on a .46. The Telemaster series of planes has a flat bottom, lifting aorfoil on the stab. As power and speed increase, the plane would normally tend to climb. The lifting airfoil on the stab also generates more lift as speed increases, lifting the stab and negating the climb tendency.

Telemasters do fly around with their noses down and tails up in the air, looking stupid to my way of thinking. In my opinion, this is a poor design feature and adjusting the wing incidence does a better job and does not result in the nose down flight attitude.
Old 10-20-2005, 11:09 AM
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Rodney
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Default RE: Why Lifting Stab?

I agree with the above comment, on any modern design the tail lift DOWN, not up. By having the airfoil as the Telemaster does, any increase in speed dectreases that down force on the tail and keeps the plane from zooming with increased speed. Even on the Telemaster, the tail is lifting down in spite of the airfoil shape.
Old 10-20-2005, 11:22 AM
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skylane42
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Default RE: Why Lifting Stab?

So wouldn`t putting a small degree of positive incidence on a flat stab accomplish pretty much the same thing??
Old 10-20-2005, 02:44 PM
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Default RE: Why Lifting Stab?


ORIGINAL: skylane42

So wouldn`t putting a small degree of positive incidence on a flat stab accomplish pretty much the same thing??
No, it would just act like down trim.

You need to remember that even if the tail incidence angle is less than the wing that this does not mean that it automatically flies at a negative angle of attack. Lift at the tail is related to the direction the air hits it from. If the wing is flying at a high postive angle then the tail is often flying at a slightly lower positive angle and generating positive lift. Push the nose down to pick up speed and then level and trim for the new high speed flight and the wing's angle of attack will be low and the tail's angle of attack will be an even lower positive angle, zero or even negative. So you see that the lift off the tail changes as the wing's lift changes depending on the flight mode. So the tail's lift changes from positive to negative during a typical sport flight even for stable flight. So why doesn't the model constantly react to all this? Because the tail's lift is harmonized with the wing's lift such that the overall model acts as one unit to react to speed changes and trim changes in a predictable and consistent manner.

So just put the tail where it should be and trim as required.

Many model airplanes, not just free flight models, have tails that provide neutral or positive lift during level flight depending on speed. As soon as you put the CG behind the 25% mark you set up this mode where the tail has to lift from negatively to neutrally to positively in order to control the wing's pitching action at various flight speeds in level flight. This applies to trainers, pattern models and funfly designs.
Old 10-20-2005, 09:35 PM
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Jim Thomerson
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Default RE: Why Lifting Stab?

Some of the older free flight designs, particularly gliders, have the stab airfoil upside down. I never understood that.
Old 10-21-2005, 04:09 AM
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Bill Teller
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Default RE: Why Lifting Stab?

I think the first lifting stabs I saw were on freeflight or maybe PAAload contest models that had a wing area rule & it was a rule beater idea.
Old 10-21-2005, 06:07 AM
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rmh
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Default RE: Why Lifting Stab?

Most of this mystery is easily understood if you build a few flat airfoil foamies. really light ones .
You can duplicate the old free flight "straight up and then flat glide" (tho much more of a rapid descent)
Just run the cg way far aft, to make the total trim for level flight ,power off 0-0 .
You should find that the relative AOA (whatever the airfoil -the true ZERO AOA) wing to stab needs to be tiny -and the CG- waaaay back. the wing and stab are sharing the job of providing lift.
The nice thing about using some flat foam is that if you screw up, a little quick CA and it is fixed for next test.
When I was a kid -we also threw FLAT wing, chuck gliders and folding wing AJ Walker gliders - best performance was aft cg /very low wing to stab incidence .
Old 10-21-2005, 12:53 PM
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Default RE: Why Lifting Stab?

...or for the old fashioned folks among us you can just use balsa....

But Dick is right. Simple gliders like this are a great way to play with aerodynmics. A flat airfoil acts the same as a thicker symetrical airfoil. You can also simulate cambered airfoils by just curving in some camber into the wing and stab so now the wing and stab are lifting. A long stick for a fuselage with sliding wing and tail will let you play with tail moment arms to see how lengthening the tail increases the tail volume coefficient and allows an even further rear CG but still retains positive pitch stability. Make the taill long enough and you can actually balance the model at or behind the trailing edge but still have positive pitch stability.
Old 10-22-2005, 08:53 PM
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B.L.E.
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Default RE: Why Lifting Stab?

One of the main problems with a lifting tail is that the tail is in the wing's downwash, this means the tail's induced drag is way out of proportion to the lift it generates. A conventional negatively lifting tail, on the other hand, is surfing on the downwash of the main wing and thus it's induced drag is low. Instead of giving air that has already been accelerated downward even more energy, it can recover some of that energy by decelerating that air. If you want an efficient lifting tail, it would have to be outboard of the wingspan somehow. There is a reason that migrating geese don't fly right behind each other in a single file.
Old 10-23-2005, 12:58 PM
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Default RE: Why Lifting Stab?

Hi, I 'm building a CAP232 2.2 m now and I'm introducing a lifting stab with it, here's the reason why.
I know that the tradition in acroplanes is to put everything at 0 referred to the fuse neutral line.
All acro's I build that way in the past(cap232,extra 260, sukhoi26, katana s) had to be trimmed down to fly straight. So I think that the geometrical neutral line of fuse is not the aerodynamical neutral line of fuse because a symmetrical airfoil at 0 degrees cannot lift.
So all these planes must have been flying a bit tail down.
So on this plane I've set the wing at the calculated angle of attack it should have to lift the 79N is't gonna weight and to compensate the fuse willing to inclinate out of the geometrical neutral line I've put the stab at the same angle of attack.
It hasn't flown yet???? but I will report when it has. Everything can be changed of course because I must admit not to be very confident in my theory.
Let's wait and see.

cuwaert
Old 10-23-2005, 02:30 PM
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Default RE: Why Lifting Stab?

You guys are reading far too much into all this I think. Setting the wing and tail at 0-0 is still the best option for a pure stunting design. It ensures that the model will fly the same way both upright and inverted. There's nothing wrong with the fuselage flying at a degree or three positive angle. The real key is how it responds in a dive test. For something like your Cap you want to be able to push it into a vertical dive at about 1/4 throttle and have it stay in the vertical or only very slightly try to pull up. Then play with more or less power and adjust the downthrust so it works the same way from idle to full bore. Now THAT will be a truly neutral handling model. At that point there should not be much elevator trim at all.

I'll leave it to the pattern and TOC style flyers to tell you if it's the desireable setup or not.

Also, do not confuse a lifting stab with stab lift or a stab with a symetrical airfoil as being a lifting stab. A lifting stab has a cambered airfoil but any stab can lift if it has an angle of attack to the oncoming air. An airfoiled stab is not always a lifting stab.

Confused yet?
Old 10-23-2005, 08:14 PM
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B.L.E.
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Default RE: Why Lifting Stab?


ORIGINAL: BMatthews
Also, do not confuse a lifting stab with stab lift or a stab with a symetrical airfoil as being a lifting stab. A lifting stab has a cambered airfoil but any stab can lift if it has an angle of attack to the oncoming air. An airfoiled stab is not always a lifting stab.

Confused yet?
I think I know what you mean, once while I was flying my G.P. Cub 20, an onlooker came over and asked me how it flew. I explained to him with a straight face "Well, with that flat bottom lifting airfoil on the wing, this plane can't be flown upside down because the wing won't lift that way" whilst flying the pattern inverted the whole time I was talking to him. He got the joke BTW.

On aerobatic planes, I like to set the elevator trim for a neutral downline. This makes the plane equally out of trim upright and inverted. I just live with that out of trim condition and actually find that it helps give me a little feel for the plane's airspeed when landing.

It's the CG location that ultimately determines whether the stab lifts, is neutral, or has to make down-lifting force to trim the plane, not the shape of the airfoil.
Old 10-24-2005, 06:10 AM
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Default RE: Why Lifting Stab?

B.L.E.,

You are quite right, the CG determines the stab lift/neutral/down-force. The incidence of the stab determines whether this is done with the elevator in line with the stab or in up or down trim. If you get the stab incidence right everything is in line and efficient.

Bedford
Old 10-24-2005, 07:33 AM
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Default RE: Why Lifting Stab?

Regarding the lifting stab - the idea behind it was to the best of my knowledge not quiet well understood sometimes.
First of all, in order for the aircraft to be stable you need the stab to "lift" downward.
But sometimes, especially in free-flight type models, the change in that lift force as a function of speed was somewhat upsetting the flight path as there was no possibility to adjust the trim in flight.
Some RC models also feature this "lifting" stab (e.g. the famous Lazy Ace), and although there you could adjust the trim, you would rather not have to do so.

So, the idea of the "lifting" stab was really to provide some kind of "automatic" trim change with speed change, and the way it worked was like this:

The model flew along in straight and level at a certain speed, with the stab producing some downward "lift" as a result of being trimmed for S&L flight, despite the stab's "upright" airfoil. Now as speed increased for some reason for example, the wing's lift would increase and so this could cause the model to zoom upwards. However, since speed also increased over the stab's "upright" airfoil, the stab's downward acting aerodynamic force would now decrease (since the airfoil would start to produce a little more upward aerodynamic force) and this would allow in turn for the tail to rise slightly, lowering the wing's angle of attack, and therefore the wing's lift, and therefore decrease the zooming up tendency usually associated with speed increase. A kind of automatic pitch trim change as a function of speed.
Old 10-24-2005, 10:52 AM
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Tall Paul
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Default RE: Why Lifting Stab?

Yes!
Free-flights are single speed airplanes. The lifting stab provided that automatic return to trimmed gliding flight.
Old 10-24-2005, 03:20 PM
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Default RE: Why Lifting Stab?


ORIGINAL: EF

....First of all, in order for the aircraft to be stable you need the stab to "lift" downward.....
This is where the misunderstanding comes in. Stabs do not have to life downwards to provide stable flight. The idea that they do is largely from full sized aircraft where the designed pitch stability margin is so high that the aircraft run CG positions in the 25% and forward region and thus mostly operate in a negative stab lift situation. But most model designs are set up for far more relaxed pitch stability factors and positive lifting stabs with our models are much more common even in sport flying RC designs. Positive stab lift is not just for free flight designs.

Mark Drela posted up a lovely chart a while back that shows the relationship of the CG position and "stabilizer" lift for a continuous transition from designs with small stabilizers where the lift must be negative to large stabilizers where the lift is positive and on to tandem wings and finally ending with canards where both surfaces are lifting positively (obviously). Only the one extreme end where the stabilizer is small is the stabilizer lift negative.

Of course this is all based on if you choose to operate any given model at a point close to the neutral stability point. If you operate any of the "conventional" setups with a CG at or ahead of the 25% point then you ensure that the stab must operate with negative lift.

Carrying on.... Any aircraft with positive pitch stability is a single speed design. Any displacement from S&L will cause the speed to change and the balance between wing and stab lift components acting against the CG will change in such a way as to return the model to it's trim speed and S&L flight.

I've read the bit about the airfoil on the stabs supposedly providing an "autopilot" like function but in reality this doesn't happen. What IS at play is the balance between the wing's lift,leverage and pitching moment and the stabilizer's lift and leverage all in a balance. However to avoid large speed related changes in the flight path the free flight models are trimmed to operate VERY close to the neutral stability point. However they also use rather extreme airfoils and it's not uncommon for the various non linearities in lift and structural flex to gang up and cause a tuck and final death dive if the model wanders out of an often rather narrow envelope where the forces are in a positive stability balance. This can happen where the lift of the tail rises faster than the lift of the wing and the lift lines hit a crossover point and the tail lifts the model into a dive rather than the wing lifting it out of the dive. Then it's lawn dart time....

Contrary to what the old time designers may try to tell you the successful designs came about mostly through trial and error... a LOT of error. I know because I've also had my own share of errors when trying to walk the tightrope of this aspect.

The most recent one was a P30 rubber model design where I used a highly cambered airfoil for the wing and a fairly highly cambered one for the stab. If it was just climb and glide then all was well. But if the nose went down due to some turbulence it was a guess if it would pull out or if it was lawn dart time. I fixed this by making a new stabilizer that had a lower camber airfoil. But as a result I was able to operate with an even further back CG than with the higher cambered stab airfoil. Re-read this bold bit again if it isn't clear. It's a large part of the secret of understanding this whole issue.


Old 10-24-2005, 04:39 PM
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rmh
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Default RE: Why Lifting Stab?

Playing with the cheap flat foam models - -the comments made by Matthews -are very easy to prove.
We (I have a mouse in my pocket) have tried all kinds of cg shifts -just to see what happens . This experimenting is what led me to my original flight theories -- one being
"if the thing is light enough- the cg does not matter ."
And I can prove that by doing it. (minor radio trim inputs corrected the problems)
Absurd -yeh- but whilst doing these experiments -I also saw a lot of other cherished fables go right down the dumper.
"Thick airfoils make more lift"--
Phooey-they can't-if you keep getting thicker - you soon have a vertical plate!
All of the careful streamlining could be shown to have little or no value on the lightly loaded, slow speed flyers.
The real puzzler is the old fable about how a flat bottomed wing makes a better trainer.
It does make a good free flight tho. My Seniorita can be instantly trimmed for a level wide circle -shut off the radio and it stays.
as any dive creates speed and lift and it simply self corrects . P. oscillations
I can do same thing with the flat wing foamies-- just a bit trickier to get the cg just right so trim is speed sensitive
Old 10-25-2005, 11:47 AM
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Default RE: Why Lifting Stab?

Chuck Cunningham from RCM was a proponent of lifting stabs. His designs; Druine Turbulent, Lazy Ace, etc. all used this idea. I remember chuck stating he felt lifting stabs helped with take-off roll control on tail draggers. I agree with him. Most of his lifting stabs consisted of a flat sheet of balsa with a spar acting as the airfoil high point and a couple triangular ribs on the front and back. So, you actually end up with a triangular airfoil and this seemed to work very well.
Old 10-28-2005, 01:00 PM
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Default RE: Why Lifting Stab?

ORIGINAL: B.L.E.

One of the main problems with a lifting tail is that the tail is in the wing's down-wash, this means the tail's induced drag is way out of proportion to the lift it generates. A conventional negatively lifting tail, on the other hand, is surfing on the down-wash of the main wing and thus it's induced drag is low. Instead of giving air that has already been accelerated downward even more energy, it can recover some of that energy by decelerating that air. If you want an efficient lifting tail, it would have to be outboard of the wingspan somehow. There is a reason that migrating geese don't fly right behind each other in a single file.
Careful now, you don't want to confuse anything.

Let's understand a few things about some designs. The Telemaster was designed for a specific purpose (pulling a load). Do some research, and you will find out why the horz. stab was a flying/lifting stab. To answer the first question here: Symmetrical stabs are useful for predicable flight. A slab style tale on a sport plane is a contradiction. A symmetrical foil is used to prevent laminar separation. A flat stab cranked over hard induces tremendous drag as airflow separates from the blanked out side of the stab. A symmetrical foil will allow air to !QUOT!hug!QUOT! the foil when the stab is outside of a zero AOA to itself. Not every plane has down-wash issues - take a look at any T-tail design.

BTW, geese fly in a !QUOT!V!QUOT! so that their bodies are in the wash of the wing tips ahead of them (drafting) - energy efficient.

If you really want an inverse lifting stab, do what my friend Andy Lennon does: Build in an inverted leading edge slat on the stab. Be sure that the foil is symmetrical, and use an all moving stab. As the stab defects, the inverted slat will dig in and give you lift when you need it. But give you nothing when the AOA is zero.
Old 11-01-2005, 10:15 PM
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B.L.E.
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Default RE: Why Lifting Stab?


ORIGINAL: Drexus

Not every plane has down-wash issues - take a look at any T-tail design.
While a T-tail can get the horizontal stab out of the propwash, I think it would have to be awfully tall to escape the wing's downwash. The low pressure area over the wing doesn't just lift the wing up, it also pulls the air above the wing down resulting in downwash. You would have to get that tail nearly a half wingspan above the wing to be clear of the downwash.

Any one who has raced sailboats is familiar with the "safe leeward position", a boat that's ahead of you and within a mastheight down wind (leeward) can outpoint you because you are in his sail's zone of downwash, well sidewash in this case. Sailors call it backwind or "dirty air". BTW, I took first place in the Hobie 14 Nationals in Oklahoma City in 1989.
Old 11-03-2005, 05:51 PM
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Default RE: Why Lifting Stab?

Just to get my two bits worth in, any stable airplane must have its CG ahead of its overall center of aerodynamic pressure. The center of aerodynamic pressure on the wing is at about 25% of the mean chord behind the leading edge. If, as is nearly universal, the CG is aft of 25% of mean wing chord, you have to have lift from the stab, or the model would pitch up violently, requiring corrective a stabilizing upward force from the stab. That is all there is to it. It's just simple physics.

A lifting stab does not have to have camber - i.e. - flat bottomed; a symmetrical section will do just fine, and produce less drag too boot, since stabs seldom develop a very high lift coefficient, and can't take much advantage of a cambered section.

I used to build free-flight duration models with 50% stabs that had their CG close to the trailing edge of the wing. These very clearly had lifting stabs, or they would have been radically unstable in pitch. Having rather long tail moments, their stabs developed at least half the lift coefficient of the wing, and the large stab helped to control high power in the rapid climb.
Old 11-04-2005, 03:28 PM
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Drexus
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Default RE: Why Lifting Stab?

ORIGINAL: B.L.E.
ORIGINAL: Drexus

Not every plane has down-wash issues - take a look at any T-tail design.
While a T-tail can get the horizontal stab out of the propwash, I think it would have to be awfully tall to escape the wing's downwash. The low pressure area over the wing doesn't just lift the wing up, it also pulls the air above the wing down resulting in downwash. You would have to get that tail nearly a half wingspan above the wing to be clear of the downwash.

Any one who has raced sailboats is familiar with the "safe leeward position", a boat that's ahead of you and within a mastheight down wind (leeward) can outpoint you because you are in his sail's zone of downwash, well sidewash in this case. Sailors call it backwind or "dirty air". BTW, I took first place in the Hobie 14 Nationals in Oklahoma City in 1989.
OK, on that point... let's not forget the change in pressure around the entire plane as it flies. This pressure envelope is measured many times the distance of the span. We don't really care about it because its so insignificant, that the effort to stay clear of it is pointless. Sure a T-tail (and by that I'm obviously referring to sailplanes - unless you have a DC-9 kit) will encounter down-wash from the air blanket higher up, but it's effect is so minimal, it's pointless to worry about. My point was not to say "there is absolute clean air just a little higher up", but that "cleaner" air is.

A sailplane lacks one dirty feature found in many well known wing designs. That being a flat bottom design. The dirty flat-bottom wing goes through air like a blade on a lawnmower. The aggressive nature of it's design begs the surrounding air to succumb to it's will at the cost of drag and loss of power. Then everyone using it wonders why they experience swelling in a turn. The foil on a sail plane is thin and sleek - almost symmetrical at points, making only nominal lift in comparison to a flat bottom foil. The down-wash from this wing is very minimal. The needed lift is generated from it's immense span, gaining it the efficiency it needs.

With a wing like that, putting a tradition tail behind it would only experience down-wash at a tiny fraction found otherwise. The blanket of air flowing over a foil with such a minimal cord - in conjunction with the tail being so very far back from the wing, would make most people question if there was any down-wash at all. But pilots like my friend Peter Schober doesn't stop there... no, they make use of a T-tail on top of all that. The level of down-wash your pointing out is like saying your receiver antenna is the cause of poor performance due to drag. So I still maintain "Not every plane has down-wash issues". If you consider the amount of down-wash on a sailplane an "Issue".

BTW, Peter is veteran sailplane champion 6 times over (1/3 scale).

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