NLF 0414(f) on a lancair

Discussion in 'Aircraft Design / Aerodynamics / New Technology' started by gschuld, Nov 7, 2009.

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  1. Nov 20, 2009 #41

    orion

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    Just to add fuel to the fire, here are a few numbers for one of our current developments. This is a twin engine, unlimited class racing configuration (all numbers confirmed through CFD analysis as conducted by Analytical Methods, Inc.):

    CD0total = .018 (clean configuration, assuming only minimal leakage drag)
    CD0fuse = .0027 (without canopy)
    CD0emp = .0026 (empennage contribution, no trim)
    CD0nac = .0031 (engine nacelles, no cooling)
    CD0canopy = .00065 (possibly a bit optimistic but in ballpark)
    CD0cool = .005 (cooling drag for both engines - established as a function of total power output)
    CD0wing = .0037 (profile drag only)

    As such, in this particular case the wing represents about 21% of the total profile drag count.
     
  2. Nov 20, 2009 #42

    autoreply

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    Our hooligan :ban:

    Just to clarify it a bit, how are the engines configured, conventional tractor engines (and thus 6 ft of turbulent wing?)
     
  3. Nov 20, 2009 #43

    orion

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    Yes, the engines are conventional tractor. Initially we planned to use a modified Aerostar wing however we couldn't find an undamaged one. The primary Aerostar accident is a hard landing, which usually destroys the wing and the main gear, so in the end we had to do a custom one of our own. Much more expensive but we can eek out a bit more performance that way.
     
  4. Nov 20, 2009 #44

    Topaz

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    Although I agree that Denis' profile drag numbers are somewhat high, you need to be a little careful with your own profile drag numbers, too. Wind-tunnel and even glider-mount test panels (supposedly 'real world') are far from a real wing and indicative only of the potential performance of an airfoil. Built as part of an actual wing structure, subject to distortions from real-world manufacturing tolerances, distortion due to the wings being loaded (profile accuracy is usually checked with the wings unloaded), dirt, bugs, etc., all contribute to the "0.0025" airfoil generating more drag than that on a real airplane. You also have to remember that sportplanes do not have their wings built to the same tolerances as competition sailplanes, even if they purportedly have laminar flow wings. They tend to warp coming out of the mold. I've read accounts of kit-built pre-molded wing halves having to be distorted by the builder as much as a quarter-inch to bond together properly. Admittedly an extreme example, but it demonstrates the point. So the drag number you see from a wind tunnel or test panel is not what the drag will be in the real world.
     
  5. Nov 20, 2009 #45

    autoreply

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    Having read the Dick Johnson reports for years and done some actual testing myself I can completely second your statement. With some effort - and especially; an excellent structural design - however they can be achieved. This is also done in major contests for example where all aircraft achieve optimal laminar flow, also after 5 hours of flying through bugs, rain and dust.

    In wind tunnels tests are also performed in real-life conditions. Elastic skin deformation is actually barely an issue (gelcoat is flexible enough), but especially bugs and dust are and are also simulated for.

    There are by the way rumors that the first suction wings can go to 0.0015/0.0012 or lower which is definitely not a real-world achievement...yet :)
     
  6. Nov 20, 2009 #46

    Denis

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    As can you see, I've analyzed the drag of the Diana at 138kt, what is slightly more than the best L/D speed. This point is valuable for comparison with aeroplanes, because ligt nonpressurized planes with powerful engines fly close to this speed range in real world. The design CL range of interest for them is spread from about the best L/D point to 1/3 or 1/4th of this CL. But individual non-induced drag components vary within this range to say nothin about the higher CL, where the fuselage drag rises together with the wing profile drag due to the beginning of the flow separation on the tail section. But at the low AOA one can estimate the balance between the wing profile and fuselage on the basis of their external surfaces, particular Re numbers and boundary layer structure. the 93sq. ft. of the Diana wing area means 186sq. ft. of its two surfaces minus a small portion intersected with fuselage. This is vary close to the external surface of a fuselage of a Cub. Obviously the 15m class sailplane fuselage is smaller and one can estimate its area of about half of this number. The major part of the fuselage surface is the pod, which has well - streamlined shape and smooth pressure distribution along its length. The pod is close to a body of revolution. Such conditions are favourable for low drag.

    But this situation changes with increase in angle of attack and the fuselage drag at higher Cl will be obviously higher. Because the low total drag is cruical around the best L/D point, the sailplane airfoils are optimized for reduced profile drag at quite high Cl about 0.6 and higher. The wing incidence cannot be chosen to lay the fuselage strictly along the flight direction at this AOA, because the performance at the rest of the speed range above the best L/D speed will be compromised. Therefore the 50/50 drag structure may be true near the best L/D region, but not over the whole usable AOA range.
     
  7. Nov 20, 2009 #47

    Topaz

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    Okay, but again we're talking sailplanes, and competition sailplanes at that. If the average Lancair's actual airfoil is anything even remotely 'close enough' to an NLF-0215(f)mod to get that airfoil's CDmin, I'd be amazed. The sportplane community just doesn't build to sailplane-style tolerances for the wing profile.
     
  8. Nov 20, 2009 #48

    Denis

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    The only achievement in airfoil development against the bugs, dust and moisture is the preservation of the CL , but not the low drag. The turbulent transition point moves forward towards the leading edge covering larger part of the chord. The lower drag with contaminated surface is possible with thinner airfoil and now the main benefit of high-strength materials in sailpalne wings is understood as a measure facilitating the reduction of the airfoil thickness.
     
  9. Nov 20, 2009 #49

    autoreply

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    @ Topaz, most don't. If you look at the finish of some homebuilts there definitely are people capable of achieving such a finish, typically those who buy a high-end homebuilt. That a majority does not doesn't mean it's unrealistic to achieve.

    In theory that's true, in reality not, around 12% is optimal, thinner was tried and failed. Bug-wipers are what you need, not a thin airfoil :)
     
  10. Nov 20, 2009 #50

    Topaz

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    You're splitting hairs mighty fine there, my friend. ;) You know as well as I do that a smooth finish is not the same as maintaining the correct profile. A smooth finish is neccessary, but not sufficient, to getting the kind of laminar flow you're talking about.

    And if most people don't do something, isn't the fact that it can be done rather a moot point when you're talking about the use on a particular design in practice? I'm sure most homebuilts that use composite wings could be profiled (eventually, and with much work) to the correct airfoil. I've heard of a very, very few that actually are.

    And again, on a sportplane, even a major profile drag reduction just isn't going to have as big an effect as it does on a sailplane. In my opinion, it'd probably be limited to a single-digit percentage improvement at best, on a real-world sportplane built by a real-world homebuilder.
     
  11. Nov 21, 2009 #51

    Denis

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    It is not optiumal to use thickness above 12%. Further improvement in Clmax/Cdmin ratio possible with 8-10%. Such airfolis can have very low Cd with mostly turbulent boundary layer and this result is very stable and less sensitive to surface roughness, Re and shape accuracy. Tht jnly problem here is spar satrenght and stiffness, which can be solved with struts. A brilliant example of thin airfoil use is Wittman Tailwind.
     
  12. Dec 17, 2010 #52

    Mac790

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    Orion one small question in relation to this wing, I don't expect very accurate answer (I understand that you can't say everything), so I'll try to ask a general question, a.) did you use already available airfoil without mod, b.) did you (David, etc) modify existing airfoil, c.) did you (David, etc) custom design airfoil?

    I understand that you are talking about sailplane airfoils, what about powered aircrafts?, I mean sailplane airfoils might work for some powered airplanes as well, if I'm right DA40 has sailplane airfoil FX 63-137, but generally those airfoils has high cl even at 0 AoA around 0.5, which is probably not the best idea for a powered aircraft, I mean high cl means smaller wings, which means higher wing loading.

    I was wondering do you have coordinates for this airfoil, I couldn't find it.

    Seb
     
  13. Dec 17, 2010 #53

    autoreply

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    Well, higher wingloading with the same stall is always beneficial (less affected by turbulence because dCl/dAlpha is constant with a smaller wing area).
    I guess (but that's just that, a guess) that they used the FX63 for two reasons. It has fairly low drag at high aoa and thus climbs much better, especially with the underpowered aircraft Diamond has. Secondly, they probably just stuck to an airfoil that they knew (Dimona) and that "did the job". The successor of the FX63 (FX79 and later the 81) are far superior though, especially in microturbulence and when contaminated. Even when almost black of the bugs, they drop only 10-15% in performance, compared to a quarter or more for even modest contamination on their predecessors.
    Airfoils can be found here:
    UIUC Airfoil Data Site

    And CTRL+F for Wortmann
    Me neither, unfortunately enough.
    I'm examining the FX-79 (used on the Nimbus 3 amongst others) which is past halfway the FX63 and 81) in terms of contamination and looks very similar:
    fx79k144.jpg
    Airfoil Investigation Database - Showing WORTMANN FX 79-K-144/17

    [​IMG]
    0,0175...
     
  14. Dec 17, 2010 #54

    orion

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    If your question is referring to the subject matter of this thread, we did not use the NLF section - the wing on the race plane consists of two modified Riblett sections: a 37A1.7-13 at the root and a 35A215 section at the tip.
     
  15. Dec 18, 2010 #55

    Mac790

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    Thanks Orion for your response, I didn't expect so accurate answer, thanks again, I would expecting something more let's say exotic like Naca 66 series. For whatever reason those foils aren't so successful like for example 63/64 and 65 series. I've found very few planes with 66 series airfoils, like for example Polish PZL M17 WSK-PZL-MIELEC M-17, seems that the most successive design with 66 series was/is White Lighting. Was my question related to the subject matter of this thread, well I was refering to your earlier post.

    I was looking at it either, I'm familiar with those web pages you listed, but I must say that I'm surprised that you don't have access to earlier DU airfoils as a student. I understand that they won't publish coordinates for their latest airfoils, but what about those a little bit older ones. Do they keep them in the safe deposit box? Same with FX-81, this airfoil isn't extremely new, not to mention some HQ.

    I was able to find only 2 DU airfoils seems that one of them was used for winglets of ASH 25 E and ASW 22, found also one HQ 35 both here http://www.rcsoaringdigest.com/OTW/on-the-wing1/24DU86-084-18.pdf.

    But what amazed me the most is a potential usage of wind turbine airfoils, I know that this sound strange, but it might not be so silly idea. DESIGN|A|PLANE: Interesting aircraft design - LH10

    I did short comparison FX79 vs NACA 66 series, for same lift NACA wing was about 30-40% bigger but even despite it the total drag was lower 15-20% for particular Re numbers ( for same AOA, it might change for different AOA, and Re). Of course I wouldn't put much trust in those results, it would be necessary to make proper simulations and calculations.

    Don't take what I wrote above so seriously, I was reading aviation related books (Raymer, Abbott, etc) only for about month, put them on the shelf about 18 months ago (I had other things and problems), but I just picked them back again ;).

    Seb
     
  16. Dec 18, 2010 #56

    orion

    orion

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    The race plane did not use anything more exotic mainly because we really didn't need to. The design point for the race is actually the turn, not the straight, which sort of forces you to use sections that have a better l/d at the higher lift coefficients.

    I like that PZL M-17. Any idea what happened to it?
     
  17. Dec 18, 2010 #57

    autoreply

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    Well, those airfoils are a major part of why they sell quite a lot of gliders, so I too would keep them as secret as possible. Measuring them from a wing is fairly easy though...
    Well, "real" windturbines aren't that different from aircraft, except that stall isn't too interesting. I'd be very careful with them, because stall might be dangerous with those foils.
    Also the LH10 is quite optimistic. To get to 200 kts they'd need about 15% less drag than the AR-5... that's optimistic. Realistic cruise @ 75% would likely be 155-160 kts, which is still very high, in fact it'd still be the fastest Rotax 912S powered aircraft.
     
  18. Dec 18, 2010 #58

    Mac790

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    Orion there is very little info about it even in Polish, from what I found they used smaller engine than was originally desired for it (less power), and improper propeller, because of it performance was worse than expected. They built only one prototype, which was given to Rzeszow UNI.

    Ok thanks it make sens, but I'm still wondering, is it something wrong with 66 series, I have a copy of Ribblet's book, I did a comparison in CFD some time ago, between NACA 64A212 and Ribblet's improved version of it GA37A212, generally Ribblet's airfoils had slightly lower cd, and slightly higher cl, at most AOA, but for what ever reason he didn't improve 66 series.

    Yes, I've heard about it, more info here "how to steal an airfoil":gig: http://www.hpaircraft.com/hp-24/update_8_december_06.htm

    Seb
     
  19. Dec 21, 2010 #59

    Mac790

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    Found coordinates for HQ 17 (looks even better with camber reduced to 2%, light green), L/D is impressive special vs FX 79, you might also want to check out NLS 1015 but 20% thick (has lower cm than other NLS's) Airfoil Investigation Database - Showing NLF(1)-0115, it's Karolina idea DESIGN|A|PLANE: NASA NLF-115-20% funny story I found there (on that blog) also Raymer comment under one thread, even Raymer read it :). I did short test in XFLR5 for the wing and 20% 0115 had better L/D than FX 79, at all AOA (if I remember correctly), in a week or two I should have access to proper softwares so I might send you PM with better results.

    Seb
     

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  20. Dec 21, 2010 #60

    topspeed100

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    Gentlemen,

    I have always insisted that some foils look alike...and these two have been used in sailplanes as well.

    What could be difference of these two shown ...and yes NFL0414 does look like a Mustang foil..H model root foil...thus I think a greater speed could be obtained with NFL0414. The speed region is clearly higher.

    The FX62K153/20 has insane L/D; http://www.worldofkrauss.com/foils/247

    Eppler 435 looks also silimar but has totally different stall angle; http://www.worldofkrauss.com/foils/871

    NFL0215f seem to be the best compromise; http://www.worldofkrauss.com/foils/1266
     

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