stunthanger.com
Design => Engineering board => Topic started by: Howard Rush on March 25, 2013, 04:33:10 PM
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Summary
I used XFoil to calculate natural boundary layer transition for the Impact mean aerodynamic chord airfoil at various flap settings. XFoil says the upper-surface transition is about 23% to 43% chord for stunt operation. XFoil is unimpressed with turbulators at 15% chord.
Method
I used XFoil via Profili Pro. I used a Reynolds number in the ballpark for stunt. I assumed incompressible flow (Mach = 0) and picked 6 for the XFoil Ncrit value on the advice of real aerodynamicists. I calculated my own airfoil shapes with deflected flap, rather than using Profili's flap function. XFoil barfed when I tried to replicate the actual flap hinge geometry, so I faired the wing to the flap with straight lines between tangents to the wing TE and flap LE or to the point where the LE or TE curve transitioned to a straight line. It appears that XFoil or Profili went on to do additional smoothing. This smoothing may have perverted the airfoil shape some.
I had XFoil calculate lift, drag and chordwise transition locations for the clean airfoil, then I fixed transition at 15% chord and had XFoil calculate lift and drag for that configuration.
Results
Folks use things that are or are thought to be "turbulators" on stunt airfoils. My understanding of a turbulator is that it's a doodad that causes the flow to transition from laminar to turbulent at its chordwise location. XFoil seems to be using that definition, too. The first attachment shows lift and drag vs. angle of attack for the clean airfoil. Next is the transition point for the clean airfoil. Xtr is the fraction of distance from the LE of the wing to the TE of the flap. The third attachment is a plot I made of transition vs. Cl, which I think is a little more interesting than transition vs. angle of attack when comparing different flap settings. The fourth attachment is lift and drag vs. angle of attack of the clean airfoil and of the airfoil with transition forced at 15% chord for the same flap settings. The effect of the turbulator is ho-hum at best.
I tried putting turbulators at 20% chord, too. Results looked about the same as for 15%.
Vortex generators do something different.
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"simulation is like masturbation ..... if you do it long enough you think its the real thing"
The other evening I was looking at the difference of a 0008, 0010, 0012, 0015, and 0020 using Xfoil.
Now, a 0008 is a Ringmaster.
I know for certain that a Ringmaster won't go to a full angle of attack of 10-12 degrees without a stall, and it shouldn't perform anything like a 0020.
Xfoil doesn't show that much difference between all the thickness's. I don't want to dump on Xfoil or CFD, it has its place for sure, but .....
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I did that, too (the XFoil, that is), to compare with an old NACA report: http://stunthanger.com/smf/index.php?topic=24769.0. Looks suspect, doesn't it.
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"Another area in which XFOIL differs from experiment is in the prediction of turbulent separation. The location of turbulent separation predicted by XFOIL is significantly further aft than that observed in the wind tunnel. For most cases tested, the XFOIL predictions of the chordwise location of turbulent separation diff er from experiment slightly as the trailing edge flow fi rst separates, with the discrepancies becoming greater as the angle of attack is increased. This tendency of XFOIL to under-predict the extent of turbulent separation results in a stall angle of attack that is greater than that seen in the wind tunnel data."
This is from AIAA 2003-4067, Surface Oil Flow Measurements on Several Airfoils at Low Reynolds Numbers, McGranahan and Selig.
Although not symmetrical airfoils, the phenomenon is the same.
AIAA 97-0511, Boundary Layer Trips on airfoils at Low Reynolds Numbers, Lyon, Selig, and Broeren, has some good trip data, especially 2D vs 3D trips.
Frank
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Howard, one thing that sims usually won't do is account for airframe vibrations or deformations. It doesn't take very much vibration to trip the flow as you reach the critical Re.
The empirical data almost always show the flow transitioning before the theoretical prediction.
We used to demonstrate this in an oil tube. We's push the Re up to close to a million and then a car would go by on the road out front and the flow would trip!
Add in turbulence and the low energy of a model...why not just use the motor's energy to control the flow? Add a turbulator and basically you use HP from the motor to add energy to the flow to keep it attached. With a CLPA ship I think ( and YMMV, of course) that boundary layer thickness and hence drag isn't a real issue - heck the chord is only a foot - but separation is the enemy. We fly CL at such low flow energies that the air doesn't have the momentum it needs to stay attached. Sure, it takes some HP to add vorticity ( and hence, energy) to the flow but it's not like modern stunt ships are running out of thrust these days!
Chuck
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Yep on both counts. Many folks have observed that it's harder to fly an electric-powered airplane level than it is the same plane with a lot of engine vibration. We suspect the vibration is affecting transition. Stunt is indeed a peculiar case with respect to drag. Drag is not a big deal and is sometimes a virtue.
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Yep on both counts. Many folks have observed that it's harder to fly an electric-powered airplane level than it is the same plane with a lot of engine vibration. We suspect the vibration is affecting transition. Stunt is indeed a peculiar case with respect to drag. Drag is not a big deal and is sometimes a virtue.
I haven't flown enough of them to notice it myself, or diagnose it, but I have seen a number of electrics that didn't seem to want to fly level or in straight lines very well. I had presumed it was because of the (usually) giant props (usually) spinning in the wrong direction in conflict with God's plan* but the "too smooth to stay attached" theory sounds pretty good.
It also explains why turbulators shouldn't work, but frequently do. I like it.
Brett
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I remember Christph H. from germany wrote (4 years ago?) that his model hunts after rework for electric and also Alex S. had model which hunted after such rework. We tried many things without too much success so the "vibration" theory was the only answer which seems to be real. However prop on IC motor also makes another air quality for elevator because it is usually larger (powerwise) and also its angular speed (and this AoA) in unequal. Anyone heard about hunting large 4C model? Also models with low AR tails (means inside prop stream) seem to be safer way.
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I have heard this also, however I have not experienced this. I have converted 2 Impacts and both tracked as good as before. I have also built numerous ones for electric with low and high aspect ratio tails that have all tracked fine.
In the conversions that didn't work well, are you sure you got the motor alligned corectly? Maybe a shim or two on the motor mount might shed some light on this issue. If it really is the vibration missing, why have all of mine worked?
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It also explains why turbulators shouldn't work, but frequently do. I like it.
Brett
I have two of those electrics which refuse to fly really solid straight & level (even less so when inverted.) One has +/- zero motor thrust and the other one +1° up (I fly pushers). Both have +/0° zero stab A.o.A.
Now I've found a self adhesive tape turbulator (0.5 mm or 20 thou. thick and 1/4 in wide) on http://www.md-aeroshop.de/de/Turbolator-Zackenband-60 (see picture). Where, related to the high point of the wing, should I put it on?
rgds Peter Germann
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I'd put in on the leading edge of the stabilizer, but I don't know if that would work. Please tell us what happens, wherever you put it.
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Peter,
0,5mm sounds a little thick. We have found a 6mm wide and 0,18mm thick zigzag to have same effect (but more stability) than a 0,3mm straight turbulator. But that's in F1A and maybe not comparable. But anyway, the thin zigzag works better at both ends of speed range, from about 10km/h glide to nearly 200km/h in start.
Remember that I have the cutter..:)
L
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I have heard this also, however I have not experienced this. I have converted 2 Impacts and both tracked as good as before. I have also built numerous ones for electric with low and high aspect ratio tails that have all tracked fine.
In the conversions that didn't work well, are you sure you got the motor alligned corectly? Maybe a shim or two on the motor mount might shed some light on this issue. If it really is the vibration missing, why have all of mine worked?
Paul,
Have you balanced the rotor in the motor? I've heard via the grapevine the current world champ wouldn't think of flying a voltomotor without doing so.
Just trying to help.
Ted
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Paul,
Have you balanced the rotor in the motor? I've heard via the grapevine the current world champ wouldn't think of flying a voltomotor without doing so.
Just trying to help.
Ted
maybe balance the outer case and the magnets, plus shaft hardware ;D ;D ;D ;D ;D
Randy
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Incoming from left field.....and without trying electric....I'd have to wonder if the too- tight ball link controls in electric ships don't have the IC vibration to break the friction....just a burnt offering.
Dave
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Battery Operated Boyfriends (aka "BOBs" )are cheap and easy to buy online... LL~ Steve
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I'd put in on the leading edge of the stabilizer, but I don't know if that would work. Please tell us what happens, wherever you put it.
The zig-zag turbulator at 35% wing chord did not help really much. Tracking, or perhaps better "altitude hold" in level and (more so) inverted flight, is still not good enough. Next I will try putting the turbulators near the leading edges of the stab.
Peter Germann
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... Next I will try putting the turbulators near the leading edges of the stab.
Removed 35% wing chord zig-zag turbulators
Installed zig-zag turbulators at 1/2 in from stab leading edge
Tracking not as good as with 35% wing chord zig-zag turbulators
Turbulators on stab did not show any effect
Next:
Remove stab turbulators
Add turbulators at 10% of half span wing chord, parallel to leading edge
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Next:
Remove stab turbulators
Add turbulators at 10% of half span wing chord, parallel to leading edge
This now has helped a lot and my tracking issue is gone. No unwanted side effects, no impact on power consumption.
Perhaps I should have installed the turbulators at 10% midwing chord first, before going the extra mile by replacing the empennage? For details, see the complete trimming process and current configuration in my Max Bee Two post in "List your setup"
My sincere thank you to all having commented here, your help was truly instrumental.
While I am not qualified to draw a valid conclusion from this experience, it nevertheless seems that adding turbulators near the wing's leading edge can help improving tracking in both level and inverted flight. I am unable to define the original cause of this instability, too.
When I recently helped launching an I-Beam ST.60 airplane, lightly touching the open bays wing covering made me think of the earlier mentioned effect of motor vibration on boundary layer behaviour. Could it be that this effect has occasionally prevented level flight instabilties? Which when converting the airplane to electric re-surface?
Peter Germann
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This now has helped a lot and my tracking issue is gone. No unwanted side effects, no impact on power consumption.
Perhaps I should have installed the turbulators at 10% midwing chord first, before going the extra mile by replacing the empennage? For details, see the complete trimming process and current configuration in my Max Bee Two post in "List your setup"
My sincere thank you to all having commented here, your help was truly instrumental.
While I am not qualified to draw a valid conclusion from this experience, it nevertheless seems that adding turbulators near the wing's leading edge can help improving tracking in both level and inverted flight. I am unable to define the original cause of this instability, too.
When I recently helped launching an I-Beam ST.60 airplane, lightly touching the open bays wing covering made me think of the earlier mentioned effect of motor vibration on boundary layer behaviour. Could it be that this effect has occasionally prevented level flight instabilties? Which when converting the airplane to electric re-surface?
Peter Germann
Peter
I am curious if you have added VGs or turbulators on the stab? did it help or make no difference.
I found on 2 models that adding parasitic drag at the back of the plane helped tracking greatly. You can add it to the stab, Hor. or Ver. even adding caplets (stab end caps) helped.
Randy
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Peter
I am curious if you have added VGs or turbulators on the stab? did it help or make no difference.
I found on 2 models that adding parasitic drag at the back of the plane helped tracking greatly. You can add it to the stab, Hor. or Ver. even adding caplets (stab end caps) helped.
Randy
Randy,
Very interesting. It's the first I've heard of this solution. I have wrestled with poor level tracking since I started flying. I figured that small handle inputs around a sensitive neutral were the main cause. For caplets, is it at the 4 intersections of stab and caplet that the parasitic drag is generated? So, a design with twin tip rudders, such as the Olympic, would also produce the same effect? Do you suspect that this is a "catalyst" solution, that is, a little drag does the job, and added drag doesn't help? Of course, it depends on how much is "a little". Thanks.
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Randy,
Very interesting. It's the first I've heard of this solution. I have wrestled with poor level tracking since I started flying. I figured that small handle inputs around a sensitive neutral were the main cause. For caplets, is it at the 4 intersections of stab and caplet that the parasitic drag is generated? So, a design with twin tip rudders, such as the Olympic, would also produce the same effect? Do you suspect that this is a "catalyst" solution, that is, a little drag does the job, and added drag doesn't help? Of course, it depends on how much is "a little". Thanks.
Hi Kim
Yes I have several triple rudder designs, and I have even took the back of a straight down rudder..one that is 90 degrees to the fuse and tack glued a strip of 3/8 inch to the trail edge, this made the end of the rudder pretty dirty, and I would expect that just enough is good, and too much would not be better.
I would also like to try some of the small wings that you see many of the RC ships using, but use them back near the stab rudder area.
I also had a weird occurence with my APEX, a triple rudder design I flew at the NATs around 1989-90, it hunted in level flight only at neutral., I tried everything..no luck, what I found later was the cause was the end of the elevators and flaps, and the fillets were capped with 1/64 ply and the tolerance in the gap was very tight, it did not touch but was very small, the side eX tube blew oil on the fillets , the oil would make the flaps and elevators stick as they cross at neutral, and it would be very sticky trying to hold level, I finally found this, opened up the gap, and the hunting went away completely.
Randy
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I am curious if you have added VGs or turbulators on the stab?
I have added 4 ea. 12in long self-adhesive zig-zag turbulator tapes at 1/2" from the stab leading edge, Randy. It had no noticable effect whatsoever.
Adding parasite drag at or even behind the empennage might be a good idea, too. Perhaps we should take a look at what the indoor 3-D RC flyers do with their airbrakes?
Peter
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Hi Kim
Yes I have several triple rudder designs, and I have even took the back of a straight down rudder..one that is 90 degrees to the fuse and tack glued a strip of 3/8 inch to the trail edge, this made the end of the rudder pretty dirty, and I would expect that just enough is good, and too much would not be better.
I would also like to try some of the small wings that you see many of the RC ships using, but use them back near the stab rudder area.
I also had a weird occurence with my APEX, a triple rudder design I flew at the NATs around 1989-90, it hunted in level flight only at neutral., I tried everything..no luck, what I found later was the cause was the end of the elevators and flaps, and the fillets were capped with 1/64 ply and the tolerance in the gap was very tight, it did not touch but was very small, the side eX tube blew oil on the fillets , the oil would make the flaps and elevators stick as they cross at neutral, and it would be very sticky trying to hold level, I finally found this, opened up the gap, and the hunting went away completely.
Randy
The strip attached to the rudder TE was actually the first thing that came to mind when I read your drag at the rear idea. I assume if the trailing edge of the rudder is raked either forward or back that the angled drag strip (a new meaning for the term "drag strip") would produce some unwanted pitch input, a forward rake pitching the nose down, and rearward rake pitching it up.
I don't follow RC, so haven't seen the little wings you mentioned or the airbrakes Peter mentioned. Does anyone have the name of one of these RC models that I can search, possibly on the Model Aviation website? Thanks.