# Turbulent flow question

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##### Well-Known Member
Generally I feel that almost every attempt at doing a low wing has been done by designing the structure first and then trying to figure out the aerodynamics after. Most times in small planes all the fattest parts line up and create a sort of mess where the bulk of the low pressure peaks are. Sacrificing or deleting pressure gradients in one area to pull hard to evacuate another area longitudinally has not ever really been examined. I think there is room to follow behind Mike Arnold and to try to understand such phenomena.
I don't entirely agree. On sailplanes there's been done a lot of research on the same subject. Low-pressure flow, even more so the flow on the top of a wing is very sensitive. The basics are known. Sure, we can improve the details, but the big picture will always be draggy with a low-wing, since you can't fix the big issue (having most "sensitive" flow in a disturbed area).
And so it goes. There are only two choices of shapes to look at and move around.... convex and concave. Most simple concave blends have not worked. So that leaves the other or combinations of the two changing along a longitudinal line. The other one is to shed off into the free flow. If separated flow starts but doesn't have any real wetted area to follow then it is minimized. How much of the large tailboom of some of the competition sailplanes are in the turbulent zone? For what distance after separation? 3-4 meters?
Sure. But the extra momentum around that tail boom (=extra drag) is tiny because it's so small. That's what planes like the SB14 are about. Once flow gets turbulent, minimize wetted area.

But way better is to maximize laminar flow given the almost order of magnitude difference in drag.

Admittedly that's terribly hard to do though..

#### Jay Kempf

##### Curmudgeon in Training (CIT)
I don't entirely agree. On sailplanes there's been done a lot of research on the same subject. Low-pressure flow, even more so the flow on the top of a wing is very sensitive. The basics are known. Sure, we can improve the details, but the big picture will always be draggy with a low-wing, since you can't fix the big issue (having most "sensitive" flow in a disturbed area).

Sure. But the extra momentum around that tail boom (=extra drag) is tiny because it's so small. That's what planes like the SB14 are about. Once flow gets turbulent, minimize wetted area.

But way better is to maximize laminar flow given the almost order of magnitude difference in drag.

Admittedly that's terribly hard to do though..
If the MU31 uses the premise that you can shed the entire trailing edge into the free stream with a blend on the other side what's to say you can't reverse it. Put the wing way low and shed the entire trailing edge into the free stream with a blend on the other side. The blend is still tough to come up with and manage at thermaling speeds. At low AOA and so low constructive interference from induced drag not as much of an issue. At very low AOA there is near as large and unstable a low pressure region on the bottom as there is on the top. So it boils down to thermaling speed optimization. A motorglider is going to be too heavy to slow down and compete at being a floater with super high aspect ratio and low speeds making high LD in that region. Everything is a compromise. With a motorglider and some fuel onboard making a bad decision based on LD and altitude available is not an issue especially if engine start is a simple act that doesn't have a lag and a huge drag rise.

##### Well-Known Member
If the MU31 uses the premise that you can shed the entire trailing edge into the free stream with a blend on the other side what's to say you can't reverse it.
The premise is not to shed the wake, but to minimize "dirty" flow on the fuselage/wing. This way you obtain more laminar flow on the fuselage (especially the lower side) and you have considerably less dirty flow on the inside of the wing.

#### Jay Kempf

##### Curmudgeon in Training (CIT)
The premise is not to shed the wake, but to minimize "dirty" flow on the fuselage/wing. This way you obtain more laminar flow on the fuselage (especially the lower side) and you have considerably less dirty flow on the inside of the wing.
I understand. If the wing only has a small footprint on the fuselage side and laminar flow continues way back on top and under the fuselage and wing leaving only one short small area that is turbulent doesn't that do the same thing?

#### bmcj

##### Well-Known Member
HBA Supporter
In a nutshell, we're taking fusion energy from the Sun that fell on Earth millions of years ago, passing it through extinct plants and dinosaurs, and combusting reconfigured organic matter in which they stored that energy, so that we can push our airplane forward - and spin up and therefore heat the air through which it passes. Go team.
Rube Goldberg would be proud. :gig:

#### don january

##### Well-Known Member
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I can't seem to count the crashes at Reno in the years but I do now it's one heck of a good airshow if you get the 4x4 out north abit. also watched a race will doing a roofing job, could almost reach up and touch the planes: :speechles

#### mcrae0104

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Competitive in the sense that the designs are competitive and thus small changes are significant. If you look at the Reno racers, the difference between sports gold winners and 5th place is like a 40-60% higher power/drag ratio. For what I consider competitive aircraft racing (sailplanes and the red bull air races), we're talking about 1-2% in drag and sometimes way less than that between the number 1 and 5.
I see where you're coming from. But just because some of the competitors can't compete doesn't make it non-competitive. And small changes make big differences in Reno too. It's just that money makes bigger changes.

I really like the bring-what-you-got-and-race spirit of Reno. Some people prefer to even things out more (in racing as in politics). To each his own. Perhaps if more even competition is desired then the classes could be further divided, but there's plenty of competition within smaller groups of each class.

If somebody would just tell those backwards cowboys to put their wings on TOP of the plane, then we'd have a real race!

##### Well-Known Member
I see where you're coming from. But just because some of the competitors can't compete doesn't make it non-competitive. And small changes make big differences in Reno too. It's just that money makes bigger changes.
Don't agree about that (small changes make big differences). I've been following Reno for a decade and I haven't seen any spectacular improvements. Make the winner race at 50% power and he'll still be in place 5 or 6.
I really like the bring-what-you-got-and-race spirit of Reno. Some people prefer to even things out more (in racing as in politics). To each his own. Perhaps if more even competition is desired then the classes could be further divided, but there's plenty of competition within smaller groups of each class.

If somebody would just tell those backwards cowboys to put their wings on TOP of the plane, then we'd have a real race!
Sports class does great. I don't see however anybody who's even seriously trying to do any major break throughts. Much of it (save the engine electronics I guess) is 60's technology at best. Nothing wrong with that, but seeing someone lapping every other competitor with something radical like the Vmax Probe or comparable projects would be a proper step into the 21st century.

I do however understand why nobody does that. Neither the economics, nor the monumental task would be worth it for anyone with the abilities, money and time to pull this off.

If anybody has a million to spare?

#### WonderousMountain

##### Well-Known Member
Jay, I consider your approach to be correct. Adjusting shape to local flow conditions to avoid separation drag.

I'll present an example that I'm lofting up later.

LuPi

#### mcrae0104

##### Well-Known Member
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Don't agree about that (small changes make big differences). I've been following Reno for a decade and I haven't seen any spectacular improvements.
Tom Aberle's Elippse prop made a pretty big difference for him.

You should come and walk the pits and talk with the pilots and crews; I think it might moderate your view.

Much of it (save the engine electronics I guess) is 60's technology at best. Nothing wrong with that...
Yes, much of it is. But many of the planes in the Sport class would be pretty foreign if they had showed up at the races in the '60s. (Composite airframes, cleaner aerodynamics, ignition, engine monitoring & data analysis, etc.) Please come sometime if you can!

#### henryk

##### Well-Known Member
Any energy

A hypothetical "no-parasite-drag" airplane would leave completely undisturbed air in its wake, and have perfect laminar flow over every square inch of the airframe. Even then, the work necessary to displace the air occupied by the airframe itself

and then bring it back together again

is energy that's sucked from the thrust of the powerplant, too.

.
"then bring it back together again"

-the air molecules have own kinetic \thermic \ energy and is mooving with greate speed,circa 500 m/sec \1800 km/h\ !

-thay are "selfmooving" into lower pressure volume,then continue it by inertia forces...

=the main "work" is doing by this energy,generated by Sun...

=iff NO Sun,NO aerodynamic forces !!!

BTW.=KASPERWING UL with low AR \circa 7\ have L/D>15 ...
http://www.kasperwing.com/Kasperwing images/kasperwing2.jpg

-probably this explains this fenomenon=
http://www.homebuiltairplanes.com/forums/attachments/light-stuff-area/13817d1315258527-witold-kasper-kaspersketch.jpg

##### Well-Known Member
Tom Aberle's Elippse prop made a pretty big difference for him.
Sure. Something like 8 percentpoint improvement is massive. But it's still half an order of magnitude less than the difference between mid-gold and a gold winner.
You should come and walk the pits and talk with the pilots and crews; I think it might moderate your view.
I've talked to various designers, builders and pilots in depth. It reinforced, not moderated my view, especially compared to the sailplane world and (though much less exposed) to Red Bull airraces.
Yes, much of it is. But many of the planes in the Sport class would be pretty foreign if they had showed up at the races in the '60s. (Composite airframes, cleaner aerodynamics, ignition, engine monitoring & data analysis, etc.) Please come sometime if you can!
This is a picture of me a decade ago. This plane was almost 40 years old by then and certified half a century ago, in those very 60's. Reno is still there, save "digital engines".

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#### mcrae0104

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...especially compared to the sailplane world
As a glider pilot, chances are good that you're aware that they do not have engines. As such,

1) they are more evenly matched to each other, hence a tighter spread;
2) a glider pilot cannot outgun the competition with horsepower;
3) they are less expensive to own and operate, so it's no wonder that newer ones find their way into competition more frequently.

I will grant that sailplanes are superior to powered aircraft in every imaginable respect (and may well end poverty, child hunger, and bring about world peace) but none of the above facts (nor the age of the airplanes racing) makes Reno non-competitive. If it's not a sailplane, it's crap; we got it. :tired:

##### Well-Known Member
As a glider pilot, chances are good that you're aware that they do not have engines. As such,

1) they are more evenly matched to each other, hence a tighter spread;
2) a glider pilot cannot outgun the competition with horsepower;
3) they are less expensive to own and operate, so it's no wonder that newer ones find their way into competition more frequently.

I will grant that sailplanes are superior to powered aircraft in every imaginable respect (and may well end poverty, child hunger, and bring about world peace) but none of the above facts (nor the age of the airplanes racing) makes Reno non-competitive. If it's not a sailplane, it's crap; we got it. :tired:
Not sure why you act so offended? It's about Reno...

If sailplanes put your panties in a knot, please re-read. I also mentioned Red Bull Airraces. Look at how they evolve.

Any sport I would consider competitive (F1, Nascar, Superbike, le Dakar) the differences between the first places are orders of magnitude smaller than in Reno. Reno, not even sports class is even remotely competitive and save the engines, it's 1960's technology at best. A nice show with old warbirds.

Especially if we're talking about advanced aerodynamics Reno is one of the least relevant examples.

#### clanon

##### Well-Known Member
There must be something else to choose from ; besides 300Hp monsters or Sleek beauties...onder:
YMMV

#### Topaz

##### Super Moderator
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There must be something else to choose from ; besides 300Hp monsters or Sleek beauties...onder:
YMMV
Yes, there is. 300Hp sleek beauties.

I see a fundamental break in the aviation community between "engine people" and "drag people", very similar to the "muscle car"/"sports car" break that used to (and still does, to an extent) exist in the automotive world.

In aviation, the former tend to cluster around events like Reno, and the latter around things like soaring. Very rarely do those worlds overlap, although - as we're seeing in this thread and have seen in several others - they very often collide. I have yet to see an "engine person" really understand what's possible in drag reduction, and very rarely see a "drag person" properly understand advances in engine and propeller development. Both poles tend to dismiss the other - the drag people dismiss the engine folk as Luddites, and the engine folk like to claim that "all practical drag reduction has already been accomplished." Both positions are horrifically wide of the mark, and the debate goes nowhere.

The "something else to choose from" would be a properly drag-reduced design that takes advantage of modern engine and propeller technology. But that solution is hard, because there's so much more knowledge that must be mastered in order to get it all right, and the compromises to be worked through make regular airplane design look like a cake-walk.

#### mcrae0104

##### Well-Known Member
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Yes, there is. 300Hp sleek beauties.

I see a fundamental break in the aviation community between "engine people" and "drag people", very similar to the "muscle car"/"sports car" break that used to (and still does, to an extent) exist in the automotive world.
Topaz, I generally agree with your analysis. I don't think you have to be one or the other (I don't consider myself an "engine guy," and AR did mention Red Bull).

There are two things going on here.

Thing one:

AR's definition of competitive seems to be a tight race. That's fine as a working definition for this discussion. The reason that Reno racers are not competitive by this definition is not that they have engines, or that they have poor aerodynamics (it's a mixed bag), but that the field of performance capabilities is much wider, even within individual classes and races. Here's the hitch: just because it's a wide field does not mean that small improvements do not make a difference, or that the guys there do not take drag seriously (Look at Nemesis, the AR-5, Wasabi, or Phantom, just to point out a few). "Poorly informed" is the nicest thing I can think to say when Reno is dismissed as...

A nice show with old warbirds.
Thing two:

Remember that kid in your fifth grade class who just went on and on about how much better his "thing" was (whatever it was--a soccer ball, interest in this activity or that)? Surely every class had one. He may have even been right, but it just wears thin after a while. Remember the whole roadable-is-king theme, guys? I like hearing about sailplanes and drag reduction. But at a certain point (the point when other, arguably non-comparable things are put down or dismissed) it really comes across as with an air of superiority which I believe is probably not intended. Hopefully.

The "something else to choose from" would be a properly drag-reduced design that takes advantage of modern engine and propeller technology. But that solution is hard, because there's so much more knowledge that must be mastered in order to get it all right, and the compromises to be worked through make regular airplane design look like a cake-walk.
I could not agree more, and I would like to see more of this. That's a part of the reason I was so disappointed not to get to see the GP-5 in the races last year.

...if we're talking about advanced aerodynamics Reno is one of the least relevant examples.
Then why did you bring it up, for Pete's sake? If you wanted to, you could have used it as an example where rigorous attention to drag reduction by some of the competitors has paid off.

#### BJC

##### Well-Known Member
HBA Supporter
Seems to me that the AJ-2, Nemesis and the NXT were pretty serious drag reduction efforts. Also pretty neat what Bruce Hammer has done with his Glasair. There are are others too.

Not all is bleak, it's just that more of us are talkers rather than doers.

BJC

##### Well-Known Member
Then why did you bring it up, for Pete's sake? If you wanted to, you could have used it as an example where rigorous attention to drag reduction by some of the competitors has paid off.
It was on the table long before this thread. Same discussion, same people. Reno (with all participants being low-wingers) is all too often brought up of an "example" that low wings are fast/low drag.

Seems to me that the AJ-2, Nemesis and the NXT were pretty serious drag reduction efforts. Also pretty neat what Bruce Hammer has done with his Glasair. There are are others too.
Kind of the essence of what I'm saying: Plenty roomy 2-seat production planes that have way lower drag than every single of those planes, both with and without an engine...

#### mcrae0104

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Reno (with all participants being low-wingers)
A significant number of participants are not low-wing. You're generalizing too much, and your conclusions do not follow from your generalized premises.