B Vial

I have a few questions on Laminar Flow:

What is Laminar Flow?
What makes a wing a laminar flow wing? Are there specific plot points one can use and scale to desired cord?
Do aircraft that have a laminar flow wing also employ a laminar flow stab?

Sorry I am ignorant on this subject and would like to get educated in how to make 'fast' wings for racing (giant scale/AT-6/ETC)

Thanks for any help one can give to me on this subject.

BMatthews

Did a google.com search on airfoil laminar flow and got lots of hits that explain and illustrate it very well. Three of the better ones that came up in the first two pages of links are....

http://www.aviation-history.com/theory/lam-flow.htm
http://mb-soft.com/public/lowdrag.html
http://quest.arc.nasa.gov/people/jou...uque/tran.html

At model speeds all you can do is delay the laminar separation and the transition to turbulent flow. But for racing any delay results in a good drop in overall drag. You only need to win by a few 1/10's after all.

For what you want a slippery symetrical or very low camber value is probably best. Something like some of the slope glider racing airfoils in the 8% thick region would work great if that thin an airfoil is allowed by the rules.

HighPlains

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Ahh, the Holy Grail – Laminar Flow

There are few experts outside of the professional aeronautical engineers, but a lot of informed amateurs.

To understand laminar flow, you have to study fluids. Think of the airflow as multiple layers of air sliding across your wing surface. Right at the surface of the wing, the lay may not even be moving at all, but the next layer is sliding past, but not at the speed of the airplane. Each successive layer is also sliding over the one below, and at increasing speeds. Eventually the air is at the free stream speed of the aircraft. All is well unless the layers start mixing with each other. At this point, the air quits being laminar, and become turbulent. This mixing takes a lot of energy, primarily due to the increased moment of the air, and so the drag is higher.

So the Holy Grail is preventing this mixing from happening for as long as possible. This is done on a wing by controlling the change in pressure gradients. Generally, laminar airfoils move the high point of the wing toward the trailing edge so to preserve the laminar flow as long as possible. However, these sections are very critical of surface waviness and surface finish. It takes very little to trip the boundary layer, and so a wing may need to have waves in the order of one part in one thousand in order to maintain the flow. Difficult, but not impossible.

Selecting the correct airfoil for your application depends on the size of the airplane’s wing sections and the speed that it flies at. You need to determine the Reynolds number that it will be working at when at racing speed, and determine the best airfoil from that information.

LouW

Your desire to make “fast” wings for racing involves a lot more than a laminar flow airfoil. The laminar flow (six series) airfoils do in fact have exceptional low drag coefficients at relatively low angles of attack. However at larger angles, they are not any better (and sometimes worse) than more conventional shapes. Such varied airplanes as the P-51 and the low wing piper aircraft use a laminar flow airfoil. In both cases low drag in cruise is an important design parameter, the P-51 as a long range bomber escort, and the Piper as a traveling machine. For flat out straight away speed where the angle of attack can be kept in the drag “bucket” they can be a good choice. However for something like pylon racing, any gain in the straight away will likely be lost in the turns where operation at high angles increases drag significantly.

Radio control models are constantly turning to keep the aircraft in the confines of the airfield with little time spent in straight flight. For such a flight profile, there is little to be gained with the laminar flow airfoils, especially since a slightly cambered airfoil with the maximum thickness a little farther forward usually gives better overall performance in turning flight. Since you mentioned giant scale, you can’t go far wrong using the scale airfoil. The AT-6 uses a slightly cambered airfoil that should work as good as any for a model.

To go fast (assuming the engine size is a given) keeping the weight as low as practical will be more effective than almost anything else you can do. Light wing loading and high power loading are the two most important ingredients for speed. Some areas of possible drag reduction include engine cowling, wing/fuselage interference, landing gear, etc. You must evaluate the drag reduction vs the added weight. These modifications may be limited in a scale model.

As to tail surfaces, it’s hard to beat the conventional slab design. It’s simple light and pretty low drag. That being said, if you use an airfoil section for the tail surfaces, a modified laminar flow with the maximum thickness at about 50% makes some sense as the tail surfaces operate at quit low angles of attack. I say modified because the “cusp” trailing edge typical of the laminar flow series airfoils is not particularly suitable for control surfaces. Since the drag of the tail surfaces is a small part of the airplanes overall drag, don’t expect too much improvement.

Tall Paul

What Lou says!

daven

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Quoted by LouW

However for something like pylon racing, any gain in the straight away will likely be lost in the turns where operation at high angles increases drag significantly.

Thats interesting, considering the last 5 Quickee 500 Nats were won by a plane with a Laminar Airfoil (Naca 66-012) High point rough 60% back from the LE. Any ideas of an airfoil for quickee that would be faster through the turns? 1 3/16" thickness with a 9 3/4" chord, and 500 square inches of area.

Not questioning you, just curious.

banktoturn

Lou & Dave,

There are several airfoils which operate within the drag bucket for both straights and turns. Dave, I think your 66-012 is one of them.

banktoturn

LouW

I only said “likely”. The only way to tell for sure would be to compare two airplanes identical in every way other than the airfoil. As for a suggestion for improving the quickee, a slight camber would move the drag bucket to a higher lift coefficient that should result in less drag during turns without affecting the straightaway portion of the flight. Possibly a 66-412 or 66-612 airfoil would do the job. (Other than flying upside down, a symetrical airfoil is seldom the best choice)

The point of my earlier response is that for R/C models, laminar flow airfoils generally offer no particular advantage with the possible exception of pylon racing.

Tall Paul

I'm surrpised the benefits of camber haven't been exploited... or, has it, but not spoken of?

B Vial

This is an interesting discussion going here. For my purposes in giant scale AT-6 racing many of the design elements are fixed although I am not up to date with what they are I am only going with my memory here from about 7 years ago with the defunct GASARA rules and I am not sure if any changed with the USRA rules. Weight has a min limit (25 pounds), the LE of the airfoils has a min radius, the wings and stabs have a min thickness values, min wing span, and there are limits to go with the fuselage that I do not recall.

I do believe that the idea was good in the rules so that no one had to have a certain manufacturer to compete (IE you could have a scratch Ziroli, a Barons kit, etc). This however has led the competition to just what they were avoiding because if you didn't have a plane that was built with mins and racing in mind you most likely was not going to be competitive enough. The 'kits' that are winning are not cheap by any means and it looks as if AT-6 is far from the entry level giant scale racing it was intended to be.

I was able to locate another article on the laminar flow design of all places from [link=http://www.aerosport.org/robert/article.html]Aero Sport[/link]. They also have some coordinates but that would be little use I think for me since it is for Formula 1 racing wings.

Once again thanks for everyone's input. It is nice to hear the discussion on benefits of laminar flow wings in our Reynolds numbers and if they are negated in the turns or even in the straight aways.

banktoturn

I think that part of the reason that camber has not been identified as a really important airfoil characteristic to optimize for pylon planes is that zero camber is so close to the ideal value. If none of the symmetric airfoils were able to operate in the drag bucket during turns, then camber would be more of a hot button. If you look at the specially designed pylon airfoils, I think you would find that they all have small, but non-zero, camber.

banktoturn

LouW

As noted in the drag curves in my response above, it doesn’t take much camber to put the drag bucket in a most favorable position to cover both straight flight and steep turns. Everything else being equal, in positive “g” flight, a little camber always results in a more efficient wing. I think the prevalence of symmetrical sections (if that is the case) is a carryover from aerobatic design where inverted flight is a given. That is really the only place a symmetrical airfoil is the best choice.

(Whether some existing symmetrical airfoils operate in the drag bucket during steep turns is largely a matter of speculation. Moving the bucket to a higher lift coeficient would certainly increase the margin and might allow a little tighter turn. Every little bit helps.)

acropilot_ty

The Nemesis was a formula 1 racer that used a 14% thick laminar flow airfoil, and it was unbeatable in racing for several years. Because the laminar drag bucket is so narrow... maybe 4 degrees angle of attack with a thick leading edge... The pilot had to fly in a different profile then the others. Instead of flying straight, then making high aoa turns he would stay in a constant turn and make a much greater radius around the pylons, he would also stay a bit higher then the other planes to stay out of the turbulence from their wakes, and the ground itself. Here is a link to the nemesis web site http://www.nemesisnxt.com/

Ty

destructiveTester

Martin Hepperle has designed a number of pylon racing airfoils.

Check out his pylon racing page, which includes some sections:-

http://www.mh-aerotools.de/airfoils/...ylonracing.htm

MH16, MH17 look like "laminar" sections - though I am guessing here (http://www.mh-aerotools.de/airfoils/mh16koo.htm http://www.mh-aerotools.de/airfoils/mh17koo.htm)

You are going to have a vacuum bagged and glassed wing or equivalent to preserve laminar flow in all likelihood.

Look at the note on MH24 (http://www.mh-aerotools.de/airfoils/mh24koo.htm): - "Needs perfect construction methods (machined moulds recommended)"

banktoturn

Lou,

I don't think you need to speculate about whether any symmetric airfoils operate in the drag bucket during turns. It's not that hard to estimate the CL required in the bank, and look at the polar. Some do, and some don't.

Using a more cambered airfoil may help, and may not. If you add more pitching moment at the same time, so the stabilizer needs to generate more lift, you could end up with more drag. I do tend to agree, as I said before, that the optimum camber would not likely be zero, but it would be close to it.

banktoturn

LouW

If you check the specifically designed pylon airfoils in the post in destructiveTesters post you will find each and every one is slightly cambered. They also have a somewhat reflexed trailing edge to address the moment problem. I was not trying to define a specific airfoil design but only voice a general principal. (with which serious designers seem to concur). It is seldom the case that a symmetrical airfoil is the absolute optimum choice except for flying upside down.

There are so many factors affecting the overall performance during a race, it is unlikely that a small difference in airfoil will usually be the deciding factor. As in most competition, the rules are written so that a major factor will be pilot skill and technique. In other words, one pilot may win with a merely satisfactory aircraft, and another may not, even though his airplane is better. This isn’t to say the designer shouldn’t strive for an optimum machine, it’s only that that alone seldom determines the outcome.

destructiveTester

totally agree LouW, so many factors affecting who wins. Even disregarding pilot skill, engine performance/tuning/porting will probably be just as or more important than wing section aerodynamics.

banktoturn

Lou,

If you look at my posts, I think you will find that we are in vociferous agreement about whether zero camber is optimal. My only disagreement is your comment that it is only speculation that symmetric airfoils can operate in their drag buckets during pylon turns. That's so close to agreeing that we could call off the debate.

banktoturn

LouW

Roger that vociferous agreement.