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Improving fuselage shape

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slociviccoupe

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Searched and didnt find similar topic so posting one. Just got my pro composites vision plans and sadly not content with the boxy fuselage shape with radiused corners. So with that being said can the fuselage shape be modified. Prety much a poor mans lancair or glassair.
As long as i keep the wing and tail in same positions and same number of bulkheads in same location can i round it a bit more?

Is there any reason bottom of plane is flat? Does the fuselage provide any form of lift?

Ive read that fuselage should be mostly straight at the wings and not to taper in too quickly from the wing trailing edge (beluga belly mod on the lancair).

Im putting all my forms and bulkheads in cad right now stock per plans and another one with little more rounded shape to it. Ill be able to loft skins on it to see fuselage shape. But i would imagine the tip view would look like an airfoil shape and so could the side view.
Also what is good shape or design for fast airplanes?

Lastly when widening the cockpit which is allowable per the plans, does everything in those areas get widened? If cockpit grows additional 4-6" from 40" to 44-46" does wing center section get wider or does the cockpit just extend over the wing center section. If you widen the cockpit does firewall , dash, seatback bulkheads all get widened? Then the baggage bulkhead and rear bulkheads in tail get widened but slowly taper back to the plans built dimensions? Plans say up to 46" wide cockpit is allowable but doesnt say anything more about it.
 

wsimpso1

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Basics are really important in getting a low drag wing and fuselage.

The wing and tail fly, the fuselage goes along for the ride. So we need to let the wings and tail work as if the fuselage is not even there, and do it smoothly. Look at the P-51 for inspiration. Bigger, longer, and heavier than the Spitfire, but faster and can do everything the Spitfire did in the middle of an 8 hour escort mission. Fuselage walls are straight through the wings and vertical. The flow is straightened out by the fuselage before it gets to the wing, and stays straightened out past the wing. Want another more modern inspiration? Look at modern jetliners, our true fuel misers. Round fuselage modified to vertical straight walls from ahead of the wing to behind the wing. Yeah, it does not look sexy, but that sexy tapering fuselage with those huge expanding roots of the Spitfire and P-40 were the result of wanting to make the fuselage look low drag, but not knowing that they were making lots of separation for the wings.

Do your fuselage shaping ahead of the wing and then behind the wing, and make it go cleanly and straight back through the wing. Let the wing do its thing in between. That means that the fuselage does not angle up or down around the wing, and absent the wing, the fuselage would make about zero lift in cruise. If you make it rounded (fore-aft or port-starboard), it is driving flow that messes with the wing. Flat is OK as long as the flow is straightened out going aft only by the time it gets to the wing.

The top of the canopy is rounded, but it is also a long way above the wing. As long as the walls are straight and vertical through the wing with reasonable lead-in and exits, the canopy will not mess with it much. If you can offset the canopy aft a little, some folks believe that is an improvement, but the Lancair Legacy owns Sport Class, and is moving in on Unlimited speeds, so I would have to guess it is still pretty low drag. Then some people do add some volume aft of the wing to smooth that out too. If the fuselage were full just a little further aft, you would not need them either.

As for changing the wing when you widen the fuselage... Why? The span and the wing effects are the same. The assumption in the math is that the buried part of the wing is still working... The basis for that is that the wing is influencing a cylinder of air a wing span in diameter to wash up slightly as it gets to the wing and then be driven downward behind the wing. The fuselage in the middle of it does not change it much, and should not change it at all. If you design the fuselage to interfere with it as little as you can (above paragraphs) the air pressure flowing over the top and bottom of the fuselage at the wing has about the same lift as if the fuselage were not there. Nothing abrupt can happen to that big cylinder just because the fuselage is in there except maybe to add drag to it. The velocity fields and pressures flow over the fuselage, and they have somewhat lower pressure changes, but they also spread out to a little more area too. Net result is usually "the wing behaves like the fuselage is not there" . Now if you had shaped the fuselage to drive air up or down through the wing, then you would be interfering with the wing and make big changes in pressures near and through the fuselage. Back to big drag consequences talked about above.

Want to believe me? Mustangs are fast in the Unlimited class, and Lancair Legacy's are fast in Sport Class. Look at them.

Billski
 

mcrae0104

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The assumption in the math is that the buried part of the wing is still working... The basis for that is that the wing is influencing a cylinder of air a wing span in diameter to wash up slightly as it gets to the wing and then be driven downward behind the wing. The fuselage in the middle of it does not change it much, and should not change it at all.
Low pressure all the way across the fuselage--even on a big dirty low-wing Phantom...

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vapor-cone-9.jpg
images.jpg
 

J Galt

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Several design books don't fully address the issue of the fuse and effect on the section lift of the wing. In Gudmundsson's book (far and away the best one but not perfect) he shows how the section lift drops dramatically at the fuse via analysis by VLM software (I think). He suggests that in order to get a true picture of reality, to run your lifting line calculations using a reduced wingspan, area, and aspect ratio. This is easy to do once you get it set up in excel.

The interference drag is why the Spitfire had a large fillet at the wing/fuse junction. The straight wall fuse doesn't have the interference drag issue as noted above. However the reason the Mustang was faster seems to be increased drag in the Spitfire in the areas of (simplified): windscreen, cooling system, and profile drag of the wing, i.e. laminar vs turbulent flow. Ref Lednicer and J.A.D Ackroyd
Justin
 

wsimpso1

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Billski glad you replied. It was one of your replies i was reading about the fuselage being straight around the wings.
Thanks for the vote of confidence.

There are folks, including the late Mike Arnold, who believe that perfect is actually a fuselage that goes slightly wider as you go aft through the wing. See the AR6. Not sure if I buy it, and the info out there on straight vs slightly expanding has not proven to be definitive yet. I do know that straight with vertical walls works pretty darned well in a bunch of airplanes. Your call.

Billski
 

stanislavz

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I did read on low / high wing to left bottom of fuselage straight for low wing, and top of high wing forlesser interreference drag. Any big roundings may create more vortex than was with rounded edge flat fuselage.

And as one test of Tailwind vs pipistrel Virus shown - flat, boxy shaped fuselage is as good as modern glider like...

But - fuselage center line have to be sames as aoa of airplane at cruise..
 

Speedboat100

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I did read on low / high wing to left bottom of fuselage straight for low wing, and top of high wing forlesser interreference drag. Any big roundings may create more vortex than was with rounded edge flat fuselage.

And as one test of Tailwind vs pipistrel Virus shown - flat, boxy shaped fuselage is as good as modern glider like...

But - fuselage center line have to be sames as aoa of airplane at cruise..

Where is this test ?
 

stanislavz

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I didn’t find a Pipistrel report with data that relates to the Tailwind data. Can you provide a link?

Thanks,


BJC
Found it. It was on some forum, not pdf.


It is interesting to compare the performance of the Pipistrel virus SW to the wittman tailwind. The tailwind was the pinnacle of tube and fabric performance from yesteryears while the Virus SW is touted today as one of the most efficient 2 seater airplanes around. So how do these airplanes compare? Fortunately we have some good objective data for the tailwind from CAFE testing. The Virus SW data is from Pipistrel found at:

Pipistrel Aircraft Virus SW | Pipistrel
the CAFE tailwind data can be found at: http://cafefoundation.org/v2/pdf_cafe_apr/WittTail.pdf
Rotax 912 data can be found at: 912Sperf - Rotax Service - Your Automotive Authority

The tailwind data is at 1425 lbs Gross weight; the virus data is take at 1320 lbs (600 KG ); In an ideal world it would have been better to compare at the exact same gross weight. The difference is not much though and if anything goes against the tailwind

1. The flat plate drag area of both airplanes are very close at about 2.0 ft^2
2. The wetted area drag coefficient of tailwind is a little less than the virus sw given that the virus sw has less wetted area (despite 10^2 ft more in wing area) due to a pinched sailplane type fuselage versus the cruciform fuselage of the tailwind. My estimation is that the tailwind has about 15% less wetted area- tailwind 370 ft^2 versus about ~ 315 ft^2 for virus SW .
3. Given 1 the two airplanes are virtually identical when it comes to max speed at a given power
4. The virus has much lower span loading and higher aspect ratio than the tailwind; hence the virus has lower induced drag than the tailwind
5. Given 4 it is no surprise that the Virus has a higher best L/D than the tailwind (17 to 12.7); the Virus is about 25% more efficient at best glide speed for each.
6. The virus has better structural efficiency ( payload/grossweight fraction) than the tailwind- 0.39 versus 0.51; this is primarily due to the lighter rotax power plant in the virus SW
7. 5 and 6 means the the virus would exhibit better climb performance for a given HP
8. However in recreation flying the airplane is most often flown close to or at the cruise speed; at this speed the the effect of 4 (lower induced drag) is minimal and flat plate drag dominates
9. If the virus sw is flown at the design cruise of 165mph it consumes 18 l/hr per Pipistrel data; working from rotax specs for the 912 this results in about 85HP; working from the drag data in the CAFE testing at the same speed the tailwind will generate 165 lbs of drag at 165 mph requiring about 90 HP (assuming 80% prop efficiency)
10. In other words the Virus is at best just about 6% more efficient than the tailwind flying at 165 mph. At faster speeds than 165mph the difference will be even less.
11. Keep in mind the tailwind in the above comparison would have 562 lbs of payload versus 684 lbs for the Virus SW. However, this is virtually the difference in weight between a lycoming 360 and rotax 912.

In summary, the tailwind, if anything, is slightly cleaner aerodynamically than the virus SW. The SW compensates with lower wetted area resulting in identical flat plate drag area with the tailwind. The Virus SW biggest advantage comes from its higher AR wing (or lower span loading) and structural efficiency . This results in a 30% advantage at low speed and ~ 6% at cruise. And the low end is not a fair comparison as the tailwind was optimized for a higher speed. Put a higher AR wing on the tailwind and a 912 and you get virtually identical performance.

So much for 60 years of progress..... composites, laminar airfoils, FEA, CFD etc versus one man (albeit a very smart one) and his welding torch
 

drgondog

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Several design books don't fully address the issue of the fuse and effect on the section lift of the wing. In Gudmundsson's book (far and away the best one but not perfect) he shows how the section lift drops dramatically at the fuse via analysis by VLM software (I think). He suggests that in order to get a true picture of reality, to run your lifting line calculations using a reduced wingspan, area, and aspect ratio. This is easy to do once you get it set up in excel.

The interference drag is why the Spitfire had a large fillet at the wing/fuse junction. The straight wall fuse doesn't have the interference drag issue as noted above. However the reason the Mustang was faster seems to be increased drag in the Spitfire in the areas of (simplified): windscreen, cooling system, and profile drag of the wing, i.e. laminar vs turbulent flow. Ref Lednicer and J.A.D Ackroyd
Justin
Billski stated the practical approach and desirable results above.

That said, The guiding principles behind Mustang fuselage design were a.) steady velocity gradient from nose to wing via Projective Geometry which embodied conic development in the design layout, and b.) minimizing surface protrusions ahead of the wing/lifting line. Major reason the radiator/aftercooling duct intake was behind the lifting line on the P-51.

Projective Geometry as applied to airframe design, was extracted from the theorems of Pascal and Brianchon (ref NAA Master Lines Manual) and began at NAA in 1940 with the design of NA-73 (Mustang I/XP-51) under Schmued (design) and Weebe - Lines Group. If you Passed Descriptive Geometry and Engineering Drafting, find a copy of NAA Master Lines Manual, by Robert Kurt Weebe. It was passed to Douglas and Northrup and Lockheed during WWII and used at NAA al the way through the F-107 and X-15 and B-70.

Weebe was a little known Giant in airframe design principles.

Justin is correct on the fundamental design differences between the Spit and Mustang and Ledicer's papers on Drag comparisons is the best out there. In addition, all Spifires had a higher CDp for the fuselage.

I did (and still do) disagree slightly with his conclusions that the cooling system of the Mustang did not provided positive 'jet thrust' as his boundary conditions were an Assumed exit temp of 170 degrees in his CFD model. For a radiator temp of 240 degrees F and plenum exit reduced to minimum, NAA tests indicated 195 degrees - and a positive net thrust over the parasite and pressure drag of the 'system'. An extrapolation of Lednicer temp plot beyond 170 shows net Thrust at ~ 180 degrees. Additionally, Ed Horkey stated explicitly that slight net thrust over cooling drag was achieved.
 

Speedboat100

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Found it. It was on some forum, not pdf.


It is interesting to compare the performance of the Pipistrel virus SW to the wittman tailwind. The tailwind was the pinnacle of tube and fabric performance from yesteryears while the Virus SW is touted today as one of the most efficient 2 seater airplanes around. So how do these airplanes compare? Fortunately we have some good objective data for the tailwind from CAFE testing. The Virus SW data is from Pipistrel found at:

Pipistrel Aircraft Virus SW | Pipistrel
the CAFE tailwind data can be found at: http://cafefoundation.org/v2/pdf_cafe_apr/WittTail.pdf
Rotax 912 data can be found at: 912Sperf - Rotax Service - Your Automotive Authority

The tailwind data is at 1425 lbs Gross weight; the virus data is take at 1320 lbs (600 KG ); In an ideal world it would have been better to compare at the exact same gross weight. The difference is not much though and if anything goes against the tailwind

1. The flat plate drag area of both airplanes are very close at about 2.0 ft^2
2. The wetted area drag coefficient of tailwind is a little less than the virus sw given that the virus sw has less wetted area (despite 10^2 ft more in wing area) due to a pinched sailplane type fuselage versus the cruciform fuselage of the tailwind. My estimation is that the tailwind has about 15% less wetted area- tailwind 370 ft^2 versus about ~ 315 ft^2 for virus SW .
3. Given 1 the two airplanes are virtually identical when it comes to max speed at a given power
4. The virus has much lower span loading and higher aspect ratio than the tailwind; hence the virus has lower induced drag than the tailwind
5. Given 4 it is no surprise that the Virus has a higher best L/D than the tailwind (17 to 12.7); the Virus is about 25% more efficient at best glide speed for each.
6. The virus has better structural efficiency ( payload/grossweight fraction) than the tailwind- 0.39 versus 0.51; this is primarily due to the lighter rotax power plant in the virus SW
7. 5 and 6 means the the virus would exhibit better climb performance for a given HP
8. However in recreation flying the airplane is most often flown close to or at the cruise speed; at this speed the the effect of 4 (lower induced drag) is minimal and flat plate drag dominates
9. If the virus sw is flown at the design cruise of 165mph it consumes 18 l/hr per Pipistrel data; working from rotax specs for the 912 this results in about 85HP; working from the drag data in the CAFE testing at the same speed the tailwind will generate 165 lbs of drag at 165 mph requiring about 90 HP (assuming 80% prop efficiency)
10. In other words the Virus is at best just about 6% more efficient than the tailwind flying at 165 mph. At faster speeds than 165mph the difference will be even less.
11. Keep in mind the tailwind in the above comparison would have 562 lbs of payload versus 684 lbs for the Virus SW. However, this is virtually the difference in weight between a lycoming 360 and rotax 912.

In summary, the tailwind, if anything, is slightly cleaner aerodynamically than the virus SW. The SW compensates with lower wetted area resulting in identical flat plate drag area with the tailwind. The Virus SW biggest advantage comes from its higher AR wing (or lower span loading) and structural efficiency . This results in a 30% advantage at low speed and ~ 6% at cruise. And the low end is not a fair comparison as the tailwind was optimized for a higher speed. Put a higher AR wing on the tailwind and a 912 and you get virtually identical performance.

So much for 60 years of progress..... composites, laminar airfoils, FEA, CFD etc versus one man (albeit a very smart one) and his welding torch

So if you electrify Tailwind...you have a winner ?
 

stanislavz

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So if you electrify Tailwind...you have a winner ?
Nope. Better electric airplane is done fron Rutan boomerang. Because you may safely place batteries separated from crew. I will never seat in same fuselages, as batteries.

But - Tailwing w8 with rotax 912/914 done on lighter side due to lighter engine - you may have aircraft comparable to top build composite airplanes in rag and tube technology.
 

Marc Bourget

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Supplementing the comments above, John Thorp explained fuselage, as an airfoil, could develop lift coefficients far above a wing, but at even greater drag coefficients -arguing against the approach.

Without attribution John Thorp, (prior to Arnold's articles), told me it was easier to control wing-fuselage separation by coordinating the widest portion of the fuselage at the wing trailing edge. But he went further, emphasizing his "poor man's area rule" - an integration of the various shapes to minimize the transverse movement of air about the fuselage as it flows from nose to tail. Arnold recognizes this as well but explains it differently than Thorp.

Finally, referencing the Thorp and Wittman designs, both employing "boxy" fuselages. If you understand that both designs were initiated in the early years of "legal" homebuilding, flat fuselage surfaces were easier for home "building" and, as John explained, the higher drag of a square flat plate vs. a circular flat plate can reduced if the "corners" coordinate adjacent pressure distribution curves so as to "shed" air equally, avoiding transverse flow.

FWIW !
 

stanislavz

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Supplementing the comments above, John Thorp explained fuselage, as an airfoil, could develop lift coefficients far above a wing, but at even greater drag coefficients -arguing against the approach.

Without attribution John Thorp, (prior to Arnold's articles), told me it was easier to control wing-fuselage separation by coordinating the widest portion of the fuselage at the wing trailing edge. But he went further, emphasizing his "poor man's area rule" - an integration of the various shapes to minimize the transverse movement of air about the fuselage as it flows from nose to tail. Arnold recognizes this as well but explains it differently than Thorp.

Finally, referencing the Thorp and Wittman designs, both employing "boxy" fuselages. If you understand that both designs were initiated in the early years of "legal" homebuilding, flat fuselage surfaces were easier for home "building" and, as John explained, the higher drag of a square flat plate vs. a circular flat plate can reduced if the "corners" coordinate adjacent pressure distribution curves so as to "shed" air equally, avoiding transverse flow.

FWIW !
Plus on top of that - boxy shape have cd of 0.27, circular or eliptical 0.2 But in a two seater you circular shape will have to be wider to accomodate two peoples.
 
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