Good Day all,

I have been spending a lot of time analyzing many different aerofoil profiles to optimize said profiles for a very specific flight regime. If there are any experts with good knowledge on foil analysis, Xfoil, laminar flow analysis, etc, I would love to have some discussions with you.

In essence I wish to optimize flight range, ie, kilometers flown, not endurance, under the following constraints:

AC AUW - 4.5kg, conventional form, ie, not flying wing, but with V tail, and ailerons and flaps.

Wing span max allowed 2.2meters, prefer 2meters

Max chord 310mm

Preferred cruise speed around 18m/s to 22m/s max

Practical Reynolds numbers from 380,000 to 450,000

Electric flight, prop in the nose.

Fuselage is low drag, whole A/C is fully composite molded.

I can use flaps for takeoff and land, but want MAX L/D ratio at around Re=400,000

I have analyzed a dozen wing profiles but would like some lateral insight....

I currently use a Clark-z foil, with max thickness of 13%, but want to improve on that.

Thanks...

Joe

The Nampilot

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Replies

  • Hi Joe,

    I'm very late to this discussion. I curious to know what foil you settled(if there is ever such a thing) on.

    Cheers,

    Antonie

    • More wing loading (80-100gr/dm2) and flaps. Profile not in the first place. For example ClarkY 10% camber 2-2,5% will go.

  • Hi,

    I check with PredimRC which is an efficient tool to design airplane : http://www.jivaro-models.org/predim_rc/page_predim_rc.htm

    It seems that with 4.5kg, an aspect ratio of 10 gives the best L/D at 21m/s (around 75km/h), but with this ratio, and wingspan only of 2.2m, the wing load is quiet high (93g/dm2) which means a landing speed of around 12m/s (45km/h).

    Reynold number are slightly lower at 21m/s (320k instead of 380k) and at landing speed, it's only 190k.

    I think that F3J like airfoil should be considered, for example, there is the NM41 serie with thikness up to 12% and camber up to 2.5% that could be a good candidate : http://kipecoul.e-monsite.com/pages/profil-f3j/serie-nm41.html

  • Have just had a look at the S7075 - it does look good however it is only 9% thick - did you not want a thicker airfoil?

  • Getting to the discussion a bit late.  Have also looked at a few airfoils including the Eppler series and SD series from Michael Selig.  My favourite all round real world airfoil is the SD7037.  The best flying glider I have ever owned had an 7037 and it was easy to fly and had great performance.

    Tuning your plane to one CL value is OK however all the analysis programs rarely work out in the real world.  Prof Eppler is the most strident in this.  Most of the later programs are based on his work.  Small imperfections in the surface can have MAJOR effects in the performance of the airfoil.  If you are vac-bagging foam wings then it is quite unlikely that you will get the airfoil completely accurate.  A foil with a wide drag bucket will give you a range of CLs that you can work with even if the surface is not perfect.

    Another consideration is aspect ratio.  Low drag gliders have very high AR wings for a reason.  However if you want to run at 380 000 Re you can't make a 2M wing with a 10:1 or higher AR and have a chord sufficient to get the Re you want at the speed you want.  Also if you taper the wings the tips will operate at a lower Re and be in the lumpy drag bucket of low Re where there are dragons in the handling.  At the low Re regime that we operate in, bubbles form on the upper surface that can break without warning stalling the wing.  Martin Simons also talks about using higher camber airfoils at the tip to give geometric washout - again you may have to use two airfoils to avoid tip stalling.

    I would suggest you look at the HQ series of airfoils.  You can select the exact camber and thickness you want as it clearly stated in the airfoil name without ambiguity.

  • I'm working on a similar problem. I decided to use a wing that has full span camber flaps. That would give me enough lift at low speed and reduce drag at high speed.

    Also, I figured it would be more efficient to set both minimum air speed and minimum land speed. Minimum air speed would be @ best L/D. The plane would fly at that speed when there's no wind or when flying downwind.

    Minimum land speed would be something like 5 m/s. The plane would fly upwind at that speed.

    Depending on the wind conditions the plane should fly a considerable amount of time near the best L/D.

  • Hi Andrew,

    I believe we are both in agreement...
    I am trying to optimize for the cruise, and not for minimum sink rate flight speed ( Vms ).

    Min sink rate occurs where Cl*1.5/Cd is max, ie, where the power factor is greatest - and is normally close to the stall flight speed and angle.

    I wish to optimize the aerofoil for a speed where the drag versus lift gives me the best range ( km flown), at a specific speed and Re.

    Obviously I will not take off at 21m/s, nor land so, I require a Cl of around 1.1 for launch, with liftoff at 11m/s at Re = 230,000, at 1000m ASL 30deg C
    My cruise Cl is around 0.31, at Re=404,000, as above.

    The current aircraft with the Clark-Z performs admirably under these condition.

    The current specs are -
    1800mm wing span, 315mm chord, rectangular wing, Clark-z 12% thickness, full span ailerons.
    Weight = 4.5kg, all up.
    Takeoff at 1000m ASl, 35deg C is in 22meters, @ 11m/s airspeed on wheels up.
    Climb rate is set to 3.5m/s
    at 15meters above ground airspeed is 19m/s
    at 20meters above ground airspeed is 21m/s, the required setpoint.

    I cannot go thinner in wing thickness - I am constrained by antenna, etc in the wings. Min wing thickness ( at max thickness point) is 35mm.

    So what I am trying to achieve is to cover more distance with the same battery capacity.

    What I am trying to resolve is how to determine a more appropriate aerofoil to achieve this, all the other elements being equal, thats all. I already have all the other parameters known, wing size, Cl for takeoff and land, Cl for cruise, etc. These are known and proven from the current aircraft.

    So, if I take the Aerofoil, in an ideal simulation, and optimize it, and the optimized results are better than the same simulation process executed on the existing Clark-Z wing, then the 'new' wing made with the new aerofoil will be better, regardless of the aircraft drag. Wing shape, ellipticity,etc, are not the driver here. These elements remain constant, regardless of the type of wing I fit, so the wing must give me compatible Cl at launch, land ( with Flaps or otherwise), and an appropriate AoA, for the appropriate Cl, with an appropriate drag, at my flight speed - These 'appropriates' are what I wish to determine.

    In Martin Simons book - 'Model Aircraft Aerodynamics' he covers very well the difference between flights speed for minimum sink rate and for maximum range. However, for max range all Texts indicate that this should be at a speed where the Cl/Cd is maximum. The problem with this concept is that this occurs typically at HIGH Cl's and Cd's that are higher than the minimum Cd's from the Cl - Cd polar diagrams. This tends to result in AoA for most practical aerofoil s in the region of 3 to 5 degrees. But in the speed part of glider flights, they optimize for the lowest drag, and Cl is very low at these points, but the Cl/Cd curve is well below max for these applications. I cannot associate these two concepts with each other mathematically and that bothers me!

    Intuitively , the polars tell me that , if I require a Cl of 0.3 for cruise speed, then I should try to use an aerofoil that will give me the lowest drag at that Cl.

    This is not what the Max range concept indicates - ie, fly at the speed where Cl/Cd is greatest.

    Joe
  • Hello Joe,

    Forgive me if I'm teaching granny to suck eggs, but have you seen the work of the late Maynard Hill?

    BTW, if you ever get around to building an airframe, I'd like to propose a few engine options that seem to be missing from general discussions around the place.

  • Not enough information really. You need to also decide the minimum speed you want to try to achieve as this will dictate the size of your wing. Flaps will increase your CLmax a little, but still not massively so.

    To illustrate, if you chose a foil with a "square" polar drag "bucket" such as an E374, you could operate at a much higher CL with relatively limited drag penalty, whereas the Clark-Y, having not so different a real-world best L/D, has its drag increasing much faster with increasing CL because the drag bucket is much more rounded. With a Clark-Y, you would then have to operate with a much larger wing area to achieve nearer the best L/D. The E374 would allow you to operate further away from the best L/D point using a smaller wing for a lower overall drag.

    The reality is that using an E374 in this way would give you a really high stall speed.

    So, I say consider your min speed requirement carefully and use this to drive your wing area. Then work out what your CL will need to be for your cruise speed. Then go looking for the appropriate aerofoil with the best Cd at this CL

    Of course, aspect ratio and induced drag also comes into the picture to confuse things a little more...
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