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Thread: Aerodynamic modifications

  1. #11

    Default Re: Aerodynamic modifications

    The other big name in aero in the racing world is Bruce Boland. There's a nice biography on him here:

    http://www.warbirdaeropress.com/arti...and/Boland.htm

    Sadly, he's no longer with us.

    Bruce Lockwood may also have some insight the share into aero mods; he's posted in the past about tests with Dago Red and Miss America done by comparing speeds at known power settings, though off the top of my head I don't remember the specific mods. He does give a summary of some of the things done to Dago Red here:

    http://www.aafo.com/hangartalk/showt...4752#post74752

    This thread is also interesting:

    http://www.aafo.com/hangartalk/showt...e-Road-to-Reno

    Much of what I've observed is as much about feeding and cooling the engine (cooling drag) as it is about reducing airframe drag. Of course, with the mustangs the belly scoop is the big question. Both Strega and Voodoo did away with the boundary layer splitter. Presumably the radiator doesn't mind BL air and removing the intersections cuts down on the drag.

    One thing that I've noticed is that the mustangs by and large have gone back to the stock spinner - nobody uses a pointy one anymore, and it seems to me that I've heard that was because of a loss in manifold pressure (or was that the induction 'smile?'). Wonder if that had any effect on the Bear, given the persistent rumour that they had lost MP with the cowling scoop. Rumour and innuendo...

    Way back when NACA did tests on the P-51 to determine critical Mach - (maybe even with the NACA P-51 that attended this year) and found it to be about 0.7. Even with the 520TAS mentioned for some planes this year, the racers are just getting there, and that's probably the limit with the basic airframe.

    However. digging around in Hoerner does give an idea of how much just cleanup is worth: A presumably 'clean' P-51 has a sharper 'knee' at the drag rise than a 'dirty' Bf-109; likewise the higher the basic drag coefficient, the lower the critical Mach number. Hoerner also advocates for large fillets to cut down on wing/fuselage drag.

    Wish I had more answers than questions. Hopefully this'll get to be a good discussion.

  2. #12
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    Default Re: Aerodynamic modifications

    Quote Originally Posted by L.E.D. View Post
    Much of what I've observed is as much about feeding and cooling the engine (cooling drag) as it is about reducing airframe drag. Of course, with the mustangs the belly scoop is the big question. Both Strega and Voodoo did away with the boundary layer splitter. Presumably the radiator doesn't mind BL air and removing the intersections cuts down on the drag.
    Actually... the scoop on Voodoo still has somewhat of a boundary layer splitter, as the front of the scoop still hangs down and away from the bottom of the wing. It's just much smaller than stock. The scoop on Strega blends right up against the bottom and doesn't do this...
    Last edited by RichH; 10-02-2013 at 01:06 PM.

  3. #13

    Default Re: Aerodynamic modifications

    Quote Originally Posted by RichH View Post
    Actually... the scoop on Voodoo still has somewhat of a boundary layer splitter, as the front of the scoop still hangs down and away from the bottom of the wing. It's just much smaller than stock. The scoop on Strega blends right up against the bottom and doesn't do this...

    Well.. oops, you're absolutely right. Maybe I was thinking of the Dago Red/RRIII scoop.

  4. #14
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    Default Re: Aerodynamic modifications

    Very informative. I knew Bruce back in the Tsunami days, great guy, passionate about the sport.

    More accurate information in testing mods can be achieved by flying what I call a speed triangle at different altitudes and recording gps speeds. The ground speeds, or gps speeds, at three different headings can be resolved to provide true airspeed and removes instrumentation error.

    For RR III to reach 490mph on the course, sounds like removing the BL splitter from the scoop configuration, paying attention to oil cooler duct routing, extending the afterbody fairing, and the trailing edge fairing help more than installing a tiny canopy. Of course ingesting the BL in the scoop will generate duct rumble, but there are ways to mitigate that internally. The scaled Mustang guys running in sport would see a speed increase simply by installing the wing trailing edge fairing, and keeping the BL splitter wouldn't detract from the visual presentation too much I would think.

    Roundy spinners are best because the air flowing around them accelerates early and the pressure integration produces a thrust. The pointy spinner delays the acceleration toward the back end of the spinner and can see how that would incrementally lower the pressures at the 'smile'.

    Drag divergence Mach number reduces with increasing lift coefficient, or g, in the turns, and clipping the wings reduces wing area and the lift curve slope. There's no appreciable compressibility drag for the Mustangs in the straights, yet, but they're bouncing up against it in the turns.

    The link is a simple compressibility primer,

    http://adg.stanford.edu/aa241/drag/cdcintro.html

    The large fillets on the Sea Furys are a wonderful thing. Looking at the exhaust, oil trace flow after a race depicts nice, tight flow.

    As a general, and encouraging observation, there appears to be a good knowledge base here.

  5. #15
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    Default Re: Aerodynamic modifications

    Quote Originally Posted by Curt_B View Post
    Roundy spinners are best because the air flowing around them accelerates early and the pressure integration produces a thrust. The pointy spinner delays the acceleration toward the back end of the spinner and can see how that would incrementally lower the pressures at the 'smile'.

    Drag divergence Mach number reduces with increasing lift coefficient, or g, in the turns, and clipping the wings reduces wing area and the lift curve slope. There's no appreciable compressibility drag for the Mustangs in the straights, yet, but they're bouncing up against it in the turns.
    It's remarkable how good those engineers were back in the day!
    ...perhaps they should put the "Tiger Claw" prop back on Strega sans pointy spinner... lol

    Makes one wonder about the prop tips on the unlimiteds(cambered airfoils). PM looks awesome in that respect... Strega has the slightly clipped and raked tips...

    A leading edge extension(wing airfoil mod) was installed on the Rare Bear at one time in her life. Also, some years ago Matt Jackson provided us with interesting info on Bears wing(stock airfoil) reaching critical mach number in flight.

    Lots of interesting things have occurred over time. One wonders what will be next... The freedom of experimentation is a beautiful thing.

  6. #16
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    Default Re: Aerodynamic modifications

    Quote Originally Posted by John View Post
    Also, some years ago Matt Jackson provided us with interesting info on Bears wing(stock airfoil) reaching critical mach number in flight.
    Would like to see that if anybody can dig it up. Can't remember if I analyzed the RareBear wing or just looked at the geometry and didn't bother. That poor engine having to contend with that, though there are some things aerodynamically that can be done to help some.

    Quote Originally Posted by John View Post
    Lots of interesting things have occurred over time. One wonders what will be next... The freedom of experimentation is a beautiful thing.
    But the costs with current fuel prices have to be horrendous. Helps a little to schedule aero flight testing with regular maintenance flights when possible since the sponsorship isn't there. Drags out development time and it's good to be right the first time because second chances are hard to come by.
    Last edited by Curt_B; 10-02-2013 at 07:37 PM.

  7. #17
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    Default Re: Aerodynamic modifications

    Perhaps Wild Bill Kelso will chime in on how well the aero mods on PM worked this year. I've heard in testing they went very fast (at Reno). If you look at PMs wing tips, then look at a few of the sport planes and some of the f1 planes, you might see some similarities. They also changed the canopy, the scoop, and added strakes this year. It seems to me that these guys are looking at new ways to address issues that have been deemed a good enough fix by the other major players, the difference being the number of prop blades up front and the additional heat generated by the much larger engine.

    Speaking of interesting mutations of the P51, MA2 had some creative ideas for lowering drag such as the NACA ducts for both the carb intake and for the radiator scoop. According to the book "Griffon Powered Mustangs" once they had a few things sorted out, both worked very well. I can't say if the NACA duct for the radiators would work well with a stock Mustang wing, but it might. I think if you are interested in seeing an unlimited P51 evolve into a more modern era, keep an eye on what the PM guys do. They seem to be forward thinking and not that afraid to try things that have been said "will never work on an Unlimited even though they might pay large dividends on F1s and sport racers.

    Great thread.

    Will

  8. #18
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    Default Re: Aerodynamic modifications

    Quote Originally Posted by RAD2LTR View Post
    Perhaps Wild Bill Kelso will chime in on how well the aero mods on PM worked this year. I've heard in testing they went very fast (at Reno). If you look at PMs wing tips, then look at a few of the sport planes and some of the f1 planes, you might see some similarities. They also changed the canopy, the scoop, and added strakes this year. It seems to me that these guys are looking at new ways to address issues that have been deemed a good enough fix by the other major players, the difference being the number of prop blades up front and the additional heat generated by the much larger engine.

    Speaking of interesting mutations of the P51, MA2 had some creative ideas for lowering drag such as the NACA ducts for both the carb intake and for the radiator scoop. According to the book "Griffon Powered Mustangs" once they had a few things sorted out, both worked very well. I can't say if the NACA duct for the radiators would work well with a stock Mustang wing, but it might. I think if you are interested in seeing an unlimited P51 evolve into a more modern era, keep an eye on what the PM guys do. They seem to be forward thinking and not that afraid to try things that have been said "will never work on an Unlimited even though they might pay large dividends on F1s and sport racers.

    Great thread.

    Will
    I enjoy the F1, Biplane, and Sport hangars because of the ideas, which are transferable to other classes since air is air. The people are refreshing as a lot of them don't have aero degrees I imagine, are always learning and come up with great concepts. Goes to show that drive and desire can be more rewarding than degrees and computers I imagine. The unlimiteds are neat too, but the development seems to have stopped years ago - development costs?, recession?, everybody's happy at ~500mph? Did I hear the jets have a speed limit now?

    Wouldn't think that NACA scoops would work well in those applications, but I've been wrong before - sometimes.

  9. #19
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    Default Re: Aerodynamic modifications

    Apparently the NACA intake scoop doesn't work well on a Merlin as the Tsunami team found out, but can work well on a Griffon due to a different mounting location (lower allowing the airflow to straighten out before entering the carb.) The radiator scoop allowed the radiator and oil cooler to sit deeper in the fuselage of MA2. Another interesting thing I just noticed while looking at some pics of MA2, the wing tips are somewhat similar to the ones on PM now. Obviously not identical but similar. It would be interesting to see what MA2 would look like now had the accident never happened. Looking at race speeds of MA2 and PM from they were in the same ball park. Had Gary Leviz run props like what PM has now, the results might have been pretty impressive. They were good for what, 60 mph on PM last year? MA2 was cleaner with a potentially faster wing, it certainly makes me wonder, what if...

    I agree, the Unlimiteds have sort of reached a point of stagnation to some degree. The new Rare Bear program might show some good results in time. Its a new crew trying far different things, and a few that were explored in the past but were possibly deemed as dead ends. With the new crew and a different way of looking at ideas, they brought back the leading edge mod that was tried awhile back, but did it better this time around. The intake block off idea seems to have worked this year. I think given more time to really massage the beast, we could see it back on top.

    The PM team still seems to be the ones doing all the serious innovation and boldly going in new directions that very well could pay off big time. This year they went slower than they did last year, but I'll bet they learned a huge amount and brought home tons of data that will likely prove ideas out, disprove a few (like the carbide valve fingers that ate cams) and come next year assuming they get some good luck (they were a little short on that this year) they might be right there pushing Strega and Voodoo right to the breaking point. (Yeah I'm sure a few people are laughing at that thought. All I can say is time will tell. ) They certainly have people putting some serious effort into making it happen. There are a lot of bright guys on that team who all want to prove the naysayers wrong. I really look forward to seeing what these guys do next.

    Will

  10. #20

    Default Re: Aerodynamic modifications

    I found Matt Jackson's comments on the Bear at critical Mach:

    http://www.aafo.com/hangartalk/showt...4182#post54182

    And some other stuff:

    http://www.aafo.com/hangartalk/showt...5231#post15231

    And this thread, while just plain long, is worth the reading. The link starts in the middle with discussion of
    Sea Fury 3350 cowlings, but the whole thing is gearhead heaven:

    http://www.aafo.com/hangartalk/showt...foil#post49780

    I haven't read this paper yet but maybe Curt_B will like it:

    http://aero-comlab.stanford.edu/kasi...aa.06-0048.pdf

    However, what's got me hitting the books tonight is the subject of change in Mcrit with lift coeff.

    If we start with an 8000lbf P-51 with 220ft^2 of wing, at 5000ft at 500mph (733fps; M = 0.66), it needs a Cl of 0.07.
    3.5G would require Cl = 0.25. Mcrit for the NACA 66-210 goes from about 0.77 to 0.75, which doesn't seem like it would hurt too much, at least for the wing, but it's clearly not the whole story. Lots of assumptions there: wrong airfoil, guessed at the weight, guessed at the area, wing alone with no interference effects... I'm out of my depth, here; how much does Mcrit for a whole airplane depend on Cl and/or alpha?

    In any event it seems clear that keeping Cl low pays - which makes Precious Metal's wing extensions all the more interesting.

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