Which is more efficient - bullet shape or pure teardrop?

Discussion in 'Aircraft Design / Aerodynamics / New Technology' started by Thunderchook, May 29, 2017.

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  1. May 30, 2017 #21

    Sockmonkey

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    Only if you plan on sticking wings on the thing.
    The simple explanation (that will make actual aerodynamicists cry) is that the wings suddenly increase the frontal area a lot at the particular cross-section of the fuselage where they're mounted, so you have to subtract some of it from somewhere else.

    Aaaaand I just noticed that several people already pointed this out. Darn my attention span.

    On a related note, notice how the back ends of some of those teardrop designs taper more sharply at first before flowing into a longer, thinner tail? I *think* this is because the air getting squeezed past the body is going to "snap back" more quickly just past the widest point, and then slower and slower as the pressure lessens.
     
    Last edited: May 30, 2017
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  2. May 30, 2017 #22

    Dan Thomas

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    Or aerodynamic drag opposing gravity is pulling it into that shape. Same as a slow drip of a viscous fluid. Jewellry made of molten glass drips:

    ae921c7f4407bcebb832594859952d8f.jpg
     
  3. May 30, 2017 #23

    Sockmonkey

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    I mean when something is built in that shape on purpose.
     
  4. May 30, 2017 #24

    Thunderchook

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    Thanks.
    All interesting responses.

    So.... I wonder why more aircraft (particularly glass/carbon fibre) aren't specifically designed with an intention to adhere to this design more?
    I understand that there are overheads to be considered, e.g. pilot and passenger have got to sit somewhere... and you've gotta attach wings somewhere on it too... and you've gotta make sure it all balances out correctly etc..

    So, is the increase in efficient not marked enough to warrant going out of your way to make the "teardroppiest" aircraft imaginable?
    The Gee Bee Air Racer was a good try for its day.
    Has there been any more recent attempts?
    If done purely for research purposes, you could include an electric engine (to get around those draggy air intakes etc) or make a few small jet engines (a la Jet-powered Cri-Cri.)

    Anyone thought of this?

    Thunderchook.
     
  5. May 30, 2017 #25

    Swampyankee

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    Two issues are packaging, that is putting stuff into the fuselage, like people, and manufacturing. A third is that the wings and empennage will change that ideal. Streamlined bodies are unstable, so stabilizing surfaces need to be added. Because these add drag, the fineness ratio of a fin-stabilized streamline body, like a dirigible, is much greater than the ideal when stability is not a concern.
     
  6. May 30, 2017 #26

    cheapracer

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    Thanks for the 4 or 5 'coke bottle' answers :)
     
  7. May 30, 2017 #27

    BJC

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    See http://www.davemorss.com/testingsafety.html for one example.


    BJC
     
  8. May 30, 2017 #28

    wsimpso1

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    already covered...
     
    Last edited: May 30, 2017
  9. May 30, 2017 #29

    wsimpso1

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    To really get into it, in subsonic airplane design, making the fuselage low drag is not the same thing as making the airplane low drag. To really make an airplane low drag, the lifting and stabilizing foils have to be low drag, the people and stuff has to be low drag, and the thrust machine needs to be low drag.

    To start, a really low drag looking fuselage shape that interferes with the wings and tail will be a higher drag airplane. Put simply, the wing has to work VERY cleanly with the fuselage to get drag down. Simple way to do this is to make the fuselage walls vertical and the fuselage section straight through the wing. The wing then hardly knows the fuselage is there. Spitfire and P-40 fuselages cause all sorts of evil, and even the slight fuselage bulge at mid wing of the RV6 and its brethern causes significant issues. If you have CAD, lots of mold making capability, and CFD for the whole airplane, you can shape the fuselage and wing together for good behaviour. This is done in sailplanes and the airfoils are tailored so that despite what would otherwise be unfavorable shaping, they maintain very favorable velocity profiles.

    Engine cooling is another huge issue. Huge amounts of air are involved, you want to minimize it, and make the air you are disturbing mess with the foils as little as possible.

    Once you have achieved letting the wing and tail work well and letting cooling occur with as little drag as possible and without messing with the wing and tail, you can clean up the fuselage... The P-51 Mustang was as faster and more fuel efficient than the Spitfire while being MUCH bigger than the Spitfire on the same engine for these reasons. Does its fuselage look like it should be low drag? not by itself, but other stuff is more important.

    Billski
     
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  10. May 30, 2017 #30

    BJC

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    The "area rule" traditionally applies to transsonic flight, but several designers, John Thorp among them, have said that it also is relevant to low speed (< 0.4 M) drag reduction for conventionally shaped (i.e., the wing is mounted to the fuselage rather than well away from the fuselage on a pylon) airplanes.

    Barnaby W. or anyone else here: Do you have any experience in this area?


    BJC
     
  11. May 30, 2017 #31

    Highplains

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    I was the first to use this effect on Quickie 500 pylon racing model designs about 25 years ago, about 20 years before hearing of Thorp's concept which is very similar. Quickie's, per the rules, were not allowed the use of wing fillets. Most designs of the day had the maximum width of the fuselage more or less aligned with the thickest portion of the wing which was kind of the natural look. But this promoted flow separation and this big suck limited other efforts at drag reduction. At one contest where conditions of testing time and zero wind allowed, my airplane peak speed on the course in level flight but turning about every thee seconds was 172 mph on radar. The next closest was about 164-5 and the vast majority were below 160. All with Nelson racing engines that were very consistent. The primary difference was moving the maximum width to the trailing edge on my design.

    This however, is not the only way to skin the cat. So called "shock pods" can achieve similar results, and may be implemented in a couple ways. One way is at the fuselage, and it basicly adds cross sectional area at it's junction with the wing. The other method is actual pods attached to the wing to give the air a push in the right direction and it also makes it easier to add flap tracks. I think it was the Convair 990 that used this method to gain speed. from Mach .8 to about .9. Like other great ideas, these were also worked on by Whitcomb, but the first to do them may have been a German, Dr. K├╝chemann which were known as K├╝chemann carrots. Finally a great example is the Russian Tu-95 bomber that used them biggly.
     
    Last edited: May 30, 2017
  12. May 30, 2017 #32

    Norman

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    What Thorp did was actually inverse pressure gradient matching. He did call it "poor man's area rule" but they are not the same. Area rule is about minimizing the energy that goes into shock waves, inverse pressure gradient matching is about minimizing interactions at the intersections of airplane parts so the boundary layer doesn't separate. inverse pressure gradient matching will not produce a wasp waist AKA "Coke bottle" because it is the exact opposite of what you need for minimum interference at low speed. You don't see a Coke bottle on many modern supersonic planes because of the higher drag at low speed and that it reduces the volume of the fuselage. It turns out that you can put your cross sectional area correction bits in other places. The upper deck on the 747 wasn't designed as area rule on purpose but it does part of the job. Someone already mentioned that fat tailpipes also do part of the job. If you pay attention to airliners you may have noticed that the flap rail housings are huge. This is not because the flaps produce so much force that they need huge fairings, in fact there's a lot of empty space inside those things. They are called "anti-shock bodies" and replace the volume of the narrowing of the Coke bottle fuselage at the wing intersection thus allowing big jets to have area rule AND a clean intersection at the wing root.
     
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  13. May 30, 2017 #33

    Xanadrone

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    What about a fourth (sub)version of the initial OP question, even if my third one (the Goldschmied shape) remained unanswered: would it be more efficient in drag reduction the modelling of a 3D fuselage shape starting with an airfoil profile known for good L/D ?!

    Like our revered (ex?)colleague @Karoliina tried to explore in her blog once, taking the NLF414F airfoil with a very low drag value at Reynolds nr.=10 mil., decambering it, changing the thickness at 26% etc. and then simply rotating it:
    https://karoliinasalminen.wordpress.com/2009/05/06/fuselage-shape-optimization-2/

    P.S. @cheapracer: I must congratulate you for the really innovative My Extraordinary Plane Build thread, but here is also the 6th Coke-bottled answer for you:
    - Yes, that shape seem to be ideal, but only if the coke bottle has the cap on -- or better a spinner instead. Or it must fly with the bottom upfront? :gig:

    ...A bit more seriously, almost all the previous posters were not only very informative, but also logic in their various analysis - including the blody coke bottle shape.
     
  14. May 30, 2017 #34

    BJC

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    Thanks, Norman.


    BJC
     
  15. May 30, 2017 #35

    Marc Bourget

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    Norman,

    Appreciate your description of John Thorp's approach. I benefited from 15 years of his mentoring and would add two additional points. First, while separation is an obvious concern, another benefit is what I would call (don't believe John ever applied a term to this point) reducing transverse mass displacement of the air. Of course, transverse flow evidences separation, but it has effects on the mass of air outside of the boundary layer. Others have spoke of this approach, (Wasabi and The Hirth powered bird that went over 200?) Second point, on the T-18, he put much effort into coordinating pressure distribution curves. For example, in designing the spinner/cowl for the firewall forward, he used 5 separate PDC's. The unschooled felt the angular empennage of the T-18 should have been rounded to reduce drag, but John explained that the proper matching of PDCs reduced the drag differential to such a low amount that is wasn't worth the (relatively high skilled) effort to form the metal to achieve a small net benefit.

    Onward and upward
     
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  16. May 30, 2017 #36

    Aesquire

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    The Martin B-26 bomber ( not to be confused with the Douglas A-26, later re-named the B-26 after all the Martins had been long retired ) was shaped by taking the symmetrical low drag airfoil chosen for the wing, and using rotations of it for the fuselage and engine nacelles. ( modified, or course, with cockpit, etc. )

    This bomber had a very good record in combat, and a horrible one in training. It's a good example of one of MY pet peeves, designing the minimum sized wing for speed, and then when more gear is installed, having a high wing loading and stall speed, in the case of the B-26, a very hot plane for it's day. The pilots training on it had never flown anything with such a high wing loading and landing speed. Very few ever had. The wing was made bigger to compensate, but the weight increases kept on coming and it never did catch up.

    The combat record, however showed that raw speed and sturdy construction was a winning combination for low to medium altitude tactical bombing. Being in a hard to catch plane was a good thing. At the end of WW2, they were all scrapped very quickly, despite the performance.

    to be fair, most WW2 airplanes were designed before the lessons of the Battle Of Britain. That taught allied designers that self sealing fuel tanks, & armor for the oil tank and pilot, was a must to survive air combat. Most of the pre-1940 designs got loaded up with "extra" needed mass and suffered for it. The few planes to be designed after the Battle were bigger and tougher and the increased power available from improved fuels and higher compression engines compensated, mostly. ( I'm trying to think of a single American designed after 1941, aircraft engine that made it into service. I'm blank. ??? )
     
  17. May 30, 2017 #37

    Aesquire

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    Here's an article on Mike Arnold's AR-5. He has some serious fans here. ( I'm one )

    Mike used Thorpe's "poor man's area rule" principle, to great effect. 213 mph on 65 ( or a bit less ) HP? dang.

    of course attention to detail, and simple, ( but hard ) things like keeping the landing gear out of the prop blast contributed to the low drag.

    https://airscapemag.com/2016/12/05/arnold-ar5/
     
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  18. May 30, 2017 #38

    Highplains

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    I seem to recall an article or NACA study on rotated airfoils that made the point that the Y ordinates were taken to the 1.5 power to work properly for a three dimensional body for lowest drag. Simply rotating an airfoil won't do it.
     
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  19. May 30, 2017 #39

    autoreply

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    Or go old-fashioned.

    I did some design work on a pretty hard problem. (Fuselage/wing fairing) and didn't want to go through all the CFD while still not having a validated model. Then I recalled Aircar's fondness about watertanks. A weighed down model, stabilized with a simple X-tail took a few trials (lots of weight added) to get pretty close to the desired Re-range, while dye on the nose gave a reasonable idea of the flow pattern.

    After the first try we put a water-filled garbage bag under the trajectory; made it possible to re-do the experiment with the same model. Discuss the dye with the owner of the swimming pool first and practice breathing through a PVC tube beforehand.
     
  20. May 30, 2017 #40

    SpainCub

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