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Suitable airfoil large thickness, high RE

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Norman

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What your opinion on using a series 65 airfoil on a delta?
For the root a 65xxxx would be fine but you'd have to either find a reflexed version or make your own because even a small pitching moment coefficient produces a strong torque on such a long chord.
 

BJC

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Put the same thrust to weight in a full scale, and it, too, will fly great.


BJC
 
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TFF

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That configuration is always stable in thrust vector mode. Expanding the flight envelope, fly it fast and note characteristics, and fly it at various speeds and cut the power for emergency landings. Slow, medium, fast to no thrust. A real plane has to handle those situations, doesn’t matter how rare the possibility really is. You want to survive those possibilities.
 

berridos

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Wonder in how far those tip ailerons are beneficial in vortex mode. I guess for emergency landing your gliding is awful, but touchdown speed is nice with power off.
 

TFF

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When flying power off envelope, make sure it’s at altitude. Gliding distance is expected to be bad; controllability under full glide is the real test.
 

berridos

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Is there any paper that explains how to reflex an airfoil in a reasonable manner, to eliminate pitching moment? The 654221 has a tiny downward kink at the te.
 

Norman

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Is there any paper that explains how to reflex an airfoil in a reasonable manner, to eliminate pitching moment? The 654221 has a tiny downward kink at the te.
Two ways I use.

The faster way is to bring it into your CAD program as a spline that you can trim to and draw 60 or so vertical station lines with a few extra lines near the LE for higher panel density there and in the last 30% of the chord for acuracy in your modified trailing edge. Then trim all of those lines so you have end points to snap to later. Now snap a spline to the end point of the original airfoil and the mid-points of the station lines to draw the mean line. Now draw in the modified section that you want for your mean line. Then using snaps <mid-point> and <intersection> grab each station line by it's midpoint and move it to the intersection with the new mean line. Then snap a spline to the end points of all the station lines to make the new airfoil. Now you have a reflexed version in your CAD. Getting it into a .DAT file is another project altogether.

The slow way is to go into <Direct Design> in XFLR5 and use the <Set Flap> to add a negative flap. Then use mixed inverse (don't use full inverse for this) design to clip off the spikes in the pressure distribution from the sharp bend. When you save this it'll try to append "modified" onto the airfoil name. I suggest changing that to "mod #" because this is going to be an iterative process and the names will get really long and useless if you just let the program choose names. Now that you've got the new airfoil from the inverse design go back to direct design and de-rotate it (you may have to save the project then close and reopen before <derotate> will work) and <refine globally> then go to the analysis window to check your results. Here's an example of an NACA 65(4)-221 that I was playing with. One of the problems with the NACA 6 series sections is that CLmax is low. That's a consequence of the camber not the thickness. I've also attached a center-section profile that I've been working on to illustrate that you can do better than NACA 6xxxxx.
 

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Norman

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One of the problems with the NACA 6 series sections is that CLmax is low. That's a consequence of the camber not the thickness. I've also attached a center-section profile that I've been working on to illustrate that you can do better than NACA 6xxxxx.
This statement needs some clarification. Absolute CLmax is affected by thickness but not very much. For any given family of airfoils usable CLmax increases with thickness up to some thickness then drops off with increasing thickness. A graph of airfoil power factor shows the cl for best angle of climb and minimum sinking speed, any AoA steeper than that is on the edge of stall, and the part of the graphs beyond max power factor produced by panel codes is always optimistic and not to be trusted. I didn't have a family of NACA 6xxxxx or reflexed sections so I generated a family of 43xx sections with the NACA section generator in XFLR5 (hope you're not color blind). This shows that power factor is highest for the 13, 15 and 17% airfoil sections, which is fairly typical for other sections. Now when you get into highly laminar airfoil sections absolute thickness has similar effects but thickness distribution becomes a problem, especialy for reflexed airfoils. Reflexing a 40% laminar airfoil section works OK but a 50 or 60% laminar airfoil usually doesn't work too well.
 

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berridos

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Thank you very much. Reflexing foils is a research field in itself. Yesterday i imported foils into solidworks and i am wondering really if the exercise wouldnt be much more straightforward if i modify the camberline in excel applying functions. I need to learn the effects of starting the reflex in different chord stations (starting from the thickest point of the foil and moving to the TE) and afterwards understanding the progression functions to apply in order to bend the camber upwards.
The idea of using a nose stall prone foil versus sharpening is brilliant. After reading a number of papers i got the feeling that sharpening offers really 0,1 incremental CLmax but ruins the allround performance of the foil. The sweep seems to be more critical.
But again the rule that vortex starts at 50-55º sweep isnt written in stone either. Some papers show that at 40º eddies show up in the oil tests.
As i plan to sweep the center section 50º and the outward panels 60º a compromise would be to install stallstrip at the beginning of the outer panels to trigger there stronger eddies that stabilise the wingtip stall tendencies.
Bart Verhees states that his delta doesnt land in vortex mode, but how can it be than, that his landing/touch down attitude is 30º. He uses a 63020 without sharpening.
Yesterday i reviewed the rate of climb of his two seater and it is stellar compared to its peers. 1300fpm on 100hp.
Returning to the reflex exercise, the single seater flies strongly nose high. I guess i will need to modify a pitch neutral airfoil in order to balance that attitude also.
 
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Steve C

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Vortex mode seems dangerous for a light plane. It creates a lot of drag and large decent rates can develop. Perhaps with a very light wing loading it might be ok.

A friend who flew the F-106 said 25,000lbs thrust was not sufficient to arrest a decent in that mode. He also added that rudder was more useful than ailerons at that point because of strong adverse yaw.
 

rotax618

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I designed and built this model over 12 years ago, I was not flying it, most of the landings were dead stick because we couldn’t keep the engine running. You can see by the landing speed that there is vortex lift at play. I believe that it would be possible to slow an airplane like this to deliver not too fragile packages.
 

poormansairforce

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I designed and built this model over 12 years ago, I was not flying it, most of the landings were dead stick because we couldn’t keep the engine running. You can see by the landing speed that there is vortex lift at play. I believe that it would be possible to slow an airplane like this to deliver not too fragile packages.
What was the wingspan?
Weight?
Engine size?
Thanks
 

rotax618

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It is so long ago, the model still exists it is a bit too big for my garage so I left it in a friend’s shed, since then I built a hangar but haven't bothered to pick up the model. I think I still have the drawings that it was built from - the drawing was done using an old version of Visual Cadd. It wasn’t the only model built to investigate faceted lifting bodies and vortex effect.FD1031EF-EF9E-4BBF-ADD5-B7A3C5B54D5B.jpegA81F25EE-3B8D-43AB-B92B-A330CE98F2AF.jpeg
 
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