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Old Jul 10, 2010, 01:33 AM
Master of the Wind
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Further explorations

Did some 2d analysis on a whole stack of airfoils including DS19, most of the HN F3J, big plane, and F3B series, etc. In the end I came up with the idea that one way to go faster is to go thinner. This thins out the herd pretty fast and left me pretty much with the HN327 for a design study. I took the Supra planform and distorted it to 13.25" root and 160 span. I then played with the chords and the stock airfoil to get close to elliptical lift distribution at max L/D. I then made a new tip airfoil by thinning, decambering, and moving the high points pretty much following the AG40 series. The in between foils were then generated by blending the two foils on chord. Finally the chords were adjusted just a little more to optimize the elliptical distribution with the new airfoils. This wing looked pretty good in 3D. Min sink not as good as the Supra but max L/D at a higher speed and the polar was pushed out to a higher velocity, pretty much parallel. Still pulls a low save better than any of the poly planes. Stuck it in the spreadsheet to see what pops out.

On the big day we still have pretty much a 3 way tie between the Supra, 12 inch V2 foils, and the HN327 design. On the light inverted day the Supra wins out by superior min sink. On the inverted good day, the HN327 design wins by a little by being a little faster.

You can burn a whole day on one of these, at least in the early part of the learning curve. Next I will attempt to guesstimate the MXC with some crude dimensions knowing it was at least based on the MH32. After that maybe play around with the DS19 to see what can be squeezed out of it. Also some of the more far out options like the HN big plane series, older HQ series, and anything else that looks interesting.
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Old Jul 12, 2010, 01:17 AM
Master of the Wind
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United States, CA, San Jose
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Spent most of the weekend with XFLR5 playing around with various root airfoils to try to beat the Supra and can't do it yet, though I think it can be done. Learned a few things.

For a poly design the root airfoil is everything. For camber change the root airfoil is not so important as how it responds to camber. I can envision an airfoil set that is never flown in its "cruise" setting.

I'm missing a step in the analysis, to check the stations along the span and make sure they all peak at the same velocity. There may be something to be gained here.

Some airfoil sets (AG40 series) decrease in camber toward the tip.

Others, (AG23 series and the thinned versions XFLR5 likes so much) increase in camber toward the tip. I'm completely out of my element here and need to do a little reading on this. Once I know what is going on I will look for something in the "library" that looks like that thinned AG23 but is designed for camber change. Work with that and experiment a little with the size and AR of the wing and I think there is some room to squeeze out a little performance. Target window is still to at least tie the big Supra on the inverted days and take any other gains on the big days.
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Old Jul 12, 2010, 02:10 PM
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Originally Posted by G Norsworthy View Post
Spent most of the weekend with XFLR5 playing around with various root airfoils to try to beat the Supra and can't do it yet, though I think it can be done. Learned a few things.

For a poly design the root airfoil is everything. For camber change the root airfoil is not so important as how it responds to camber. I can envision an airfoil set that is never flown in its "cruise" setting.

I'm missing a step in the analysis, to check the stations along the span and make sure they all peak at the same velocity. There may be something to be gained here.

Some airfoil sets (AG40 series) decrease in camber toward the tip.

Others, (AG23 series and the thinned versions XFLR5 likes so much) increase in camber toward the tip. I'm completely out of my element here and need to do a little reading on this. Once I know what is going on I will look for something in the "library" that looks like that thinned AG23 but is designed for camber change. Work with that and experiment a little with the size and AR of the wing and I think there is some room to squeeze out a little performance. Target window is still to at least tie the big Supra on the inverted days and take any other gains on the big days.
I'm curious about the effect of moving the max thickness point closer to the LE like you see in the AG series and especially in the HT12-14 fin and stab airfoils. Would be nice to hear from an aeronautical engineer on this subject.

We flew Super Supra after the contest last saturday. Launches very well and Bill actually got a pretty impressive zoom out of her off launch which you dont usually see with an XC ship. It really does fly like a Supra, just bigger and faster. She responds very well to reflex and we now have her loaded to near full 11 lb limit. Moves around the sky very well. At 11 lbs still thermals like a dream, no tips stalls. On one flight I was setting up for landing and hit a bubble and managed a low save from 50ft and was able to turn extremely tight in the small thermal. Still trying to sort out how to best set up the transmitter. I like to go as simple as possible but transmitters are always a personal thing. We now have camber on the left slider and elevator speed trim on the right slider. I dont think we need any down elevator with reflex but its there on the right slider for those situations where you need it (sink, final dash, etc)

also, the wing bottom surfaces are now painted flat black which really makes a difference. the native carbon bottom surfaces still had a shine out of the vacuum bag which was quite reflective and reduced visibility. flat black is the best I am now convinced.

she's pretty much race ready at this point.
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Old Jul 14, 2010, 12:08 AM
Master of the Wind
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United States, CA, San Jose
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The more I learn, the less I know. Here are a couple of really good links with a general approach.

http://www.rcgroups.com/forums/showp...3&postcount=75

I typically seek a taper/washout/airfoil distribution which gives a good local Cl distribution in the tightest practical sustained turn.

If anybody has a shortcut how to figure out the wing velocities in the tightest practical sustained turn, let me know. I will ponder on it and read up a bit more.

http://www.rcgroups.com/forums/showp...3&postcount=17

Lots of gems buried here on RCG if you are willing to spend the time.
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Old Jul 14, 2010, 01:16 AM
Master of the Wind
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OK the thermal turn conditions I can get from physics 101 and Simons. More importantly would anyone like to comment on the difference between designing a wing with XFLR5 and then flying it with the parasitic drag of the fuse so it actually behaves something like the polars in John Ellias' tests? What is the penalty for doing wing only and would it be worth the effort to try to correct for it?
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Old Jul 14, 2010, 01:55 AM
yyz
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Greg,

Have you analyzed your flights to determine what percentage of time you are spending thermaling? I would spend the same percentage of time in your design worrying about the design for thermaling flight.

IMHO, the long and short is: design for cruise and make sure it thermals well enough,

Mike



Quote:
Originally Posted by G Norsworthy View Post
OK the thermal turn conditions I can get from physics 101 and Simons. More importantly would anyone like to comment on the difference between designing a wing with XFLR5 and then flying it with the parasitic drag of the fuse so it actually behaves something like the polars in John Ellias' tests? What is the penalty for doing wing only and would it be worth the effort to try to correct for it?
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Old Jul 21, 2010, 01:14 AM
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United States, CA, San Jose
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I've not been idle. Took a look at a proprietary F3F design sitting unbuilt on my kitchen floor and blew it up. Pretty nice work as it about ties the Supra but not better for the tasks so it remains an F3F design that I will get around to building one of these days. Looked at the MG-06 and got nowhere. That Supra is pretty good. I know enough to see now how it was optimized. You can make it smaller and make it faster at the expense of a little low save, but not necessarily "better". My only hope is to find an airfoil that is better optimized at the Re we are flying at vs what you see on a 55 oz TD plane. And be able to modify the tip so that the whole package works.

Played around with the DS19 and could do almost as well, may come back to that one as I have learned more since. Found the HN483 which is a pretty successful F3F airfoil and somehow got missed in the first list of HN I looked at. At first cut if I am reading this right the tip is a bit peaky and will need some modification to work right with the root in the thermal turn and straight line speed.

Looked at some of the HN series designed for >3.5m planes. Very thick and I'm guessing targeted at the scale market to maximize stiffness in high aspect ratio wings. If I have time I will go back and look at those just to be sure.

Just picked up a Pike Perfect and it would be nice to take a look at those but what you can blow up off their web site is awful fuzzy by the time you start sticking points on it.

Thinking about it the only difference between our task and what the Supra was designed for is that we do spend a fair amount of time in straight line min sink where the TD plane is either banked up or in cruise. Not sure if it would affect the final design as in every contest in recent memory except the last Sacramento there has been at least one ultra low save.
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Old Jul 23, 2010, 01:11 AM
Master of the Wind
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Looked at the HN483. Responds well to tip blending at least at the first cut. The full wing is respectable, actually winning the inverted day by a little. Looks like the performance falls off a bit if you get too far off the max L/D compared to the Supra. I'll keep playing with this one if I have some time to burn. The middle blends were just guesses and I may be able to do better with some combination of planform and airfoil. I have nothing against the AG40 series and will probably end up there when this exercise is finished. It's worth playing around a bit more just for the sake of finding out what is really going on here.
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Old Jul 23, 2010, 02:10 PM
agony sweetns the victory
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Greg,

A friend of mine Anthony Beck (also here on RCGroups as AMBeck), recently sent me a series of AG23/24 based airfoils that he modified by rounding the nose change the max camber point and thining and they have very nice results when compared against their respective originals. Contact him, and I know he will send you his set of airfoils for your comparative analysis.

btw, and answer to your previous question back on post #47

"Some airfoil sets (AG40 series) decrease in camber toward the tip. Others, (AG23 series and the thinned versions XFLR5 likes so much) increase in camber toward the tip. I'm completely out of my element here and need to do a little reading on this. Once I know what is going on I will look for something in the "library" that looks like that thinned AG23 but is designed for camber change."

There are two types of "twist", geometric and aerodynamic (but LIFTROLL doesn't differentiate between them). Geometric is washout/washin induced by physically twisting the wing as it is built or covered or by sweep in the wing's planform. Aerodynamic twist is imparted when you change to airfoils (series) of different cambers. The difference in airfoil cambers between the root and the tip of (of a series) for each panel can be added into LIFTROLL as twist (so can Geometric twist, because LIFTROLL doesn't differentiate between the two types). This is sort of an averaging cheat that gets you in the ballpark, but not at home plate. Next, if you can make use of Adam Till’s Sailplane Design spreadsheet then you are significantly closer to homeplate. It is essentially the next step up from LIFTROLL because Adam Till’s Sailplane Design also incorporates all the XFoil data. If you're already using XFLR5 then that may give you better results??? It may be a slight side excursion to see if the two give the same answer or not, I've never checked, but if nothing else good for a double check of the results.

Keeping tuned into your analysis, Todd
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Old Aug 06, 2010, 07:54 PM
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Got the modified AG23/24. I'll look at it as a poly and also as a flapped airfoil just to see what it does.

Took some time off for other activities but next on the list is to look a 4 root airfoils, DS19, HN217, HN483, and HN990 to see what has the most potential. Ignore the section and see which one responds best to camber and reflex. Then jump to the first break and try to optimize, finally find a tip that works with the whole package. I credit this approach to GT in the hand launch thread. Previously I had been looking at root then tip, and blending for the intermediates. Not sure how much it matters but this is more than anything an exercise to learn the method. In the end I would have to come up with something significantly better to justify scrapping the Supra tooling and starting over.
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Old Aug 12, 2010, 06:49 AM
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Thinking about it the only difference between our task and what the Supra was designed for is that we do spend a fair amount of time in straight line min sink where the TD plane is either banked up or in cruise. Not sure if it would affect the final design as in every contest in recent memory except the last Sacramento there has been at least one ultra low save.
Why do you pend time cruising at minimum sink? I fly full scale and we very rarely cruise at min sink as the L/D at min sink and a speed just faster than Max L/D are the same. For example the min sink L/D might be 34:1at 42 kts, Max L/D 38:1 at 52 kts; and at 60 kts the L/D is 34:1. At 60 kts the sink rate is higher but I get to see more air and up the probability of finding lift. Overall, the penalty for the higher sink rate is well worth it since finding a thermal with better lift than the price paid for it. Not quite full on MacCready theory but a good rule of thumb.
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Old Aug 12, 2010, 01:25 PM
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Straight line min sink is when you are moving down the road in a thermal that covers a large area. For example you think you topped out at 1200m and decide to go but you run into something bigger as you leave, and you run for a mile or two at min sink until you find the real end of the thermal. As long as the visibility is good you take the free energy. When the thermal runs out you switch over to STF. The comparison was with the thermal Supra, which would almost never go in a straight line at min sink. It either is cruising to look for lift, or circling when it finds lift. If I understand right Dr. Drela designed the plane for handling in the tight circle. For the XC model you might be willing to give up a bit of this and optimize the plane to fly straight at min sink. It all depends on whether you optimize for low saves or flying high and just changing the velocity depending on the tone.
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Old Aug 16, 2010, 02:57 PM
launch low, fly high
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Greg,

You have come a long way on the path of engineerd! One more little piece to think about... A big part of low level saves is maneuver margin. That is, the capability to get a little extra Cl when one needs it to make an adjustment in a thermal turn when already banked up and thermalling at something close to optimal minimum sink. This is where some airfoils get in trouble, and some shine. BTW, this may not be an easy one to quantify in WRT the typical design objective function. In particular, how does one define the relative weighting on this margin, and how much Cl maneuver margin is acceptable or desired?

One thing to be wary of, is a design that requires a tight range of TE deflection and/or flight Cl to provide the optimal performance. The next level of optimization drives one to analyze off-optimal design performance, which in actuality better matches real life flying...
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Old Aug 16, 2010, 03:14 PM
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Straight line min sink is when you are moving down the road in a thermal that covers a large area. For example you think you topped out at 1200m and decide to go but you run into something bigger as you leave, and you run for a mile or two at min sink until you find the real end of the thermal. As long as the visibility is good you take the free energy. When the thermal runs out you switch over to STF. The comparison was with the thermal Supra, which would almost never go in a straight line at min sink. It either is cruising to look for lift, or circling when it finds lift. If I understand right Dr. Drela designed the plane for handling in the tight circle. For the XC model you might be willing to give up a bit of this and optimize the plane to fly straight at min sink. It all depends on whether you optimize for low saves or flying high and just changing the velocity depending on the tone.
I can confirm the tight thermal - low save capabilities of the scaled up XC Supra (#9-Super Supra). It is easily as good as my regular TD Supra in this regard. Right now I think my flight strategy will be TE thermal camber down low and tight in smaller thermals, best L/D cruise betw thermals in nuetral air and reflex in sink or headwind. I would like to try using thermal TE camber at min sink in cruise situations as you describe when I dont need to circle but want to capture additional energy while flying thru lift (dolphin soaring??) but I'm worried about stability and getting into those dreaded "whoop-dee-doos" while at high altitude. I'll just have to try it and see what happens. I think Bill's 10G has the capability to introduce the TE camber with a delay over a second or two after flipping the camber switch which may help with this, not sure.
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Old Aug 16, 2010, 11:39 PM
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I think I will play around a bit with a few more foils on my short list to get more experience learning what is really going on here, and then tackle the last thing I have been dreading, actually come up with some sort of function for weighting all the factors in the design. Not sure I can do any better than gut feel based on experience, but it is at least worth thinking about.

On course at Montague this year we actually only had one low save on the runs we kept. All the rest was done by flying slow and fast depending on the tone. That said when you need a low save it better work.

The Supra foils are looking pretty good based on both XFLR5 and the flight experience. Plus look how many Supras and Icon2's there were at the F3J worlds. There must be something good there.
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