Quantcast

Some design musings

HomeBuiltAirplanes.com

Help Support HomeBuiltAirplanes.com:

Tiger Tim

Well-Known Member
Joined
Apr 26, 2013
Messages
3,300
Location
Thunder Bay
What about a touring motor glider? Burt Rutan seems to like the various loopholes in that class so there might be a whole market just waiting for you...
 

pictsidhe

Well-Known Member
Joined
Jul 15, 2014
Messages
8,024
Location
North Carolina
You could ditch the horizontal stabiliser and make the diamond bipe a tandem wing.
I've even been pondering a triple winged 'tandem', if that makes any sense, a biplane front, single rear
 

autoreply

Well-Known Member
Joined
Jul 7, 2009
Messages
10,753
Location
Rotterdam, Netherlands
Duncan,

I've been following your various design musings over the years. Why not focus on what you every time seem to come back to? A sexy composite plane, 35-45 kts stall range, 40-60 hp, 150-ish kts WOT cruise. There is a small but substantial market for such a plane and many pilots are looking for something like that.
 

Head in the clouds

Well-Known Member
Joined
Mar 11, 2012
Messages
1,983
Location
Gold Coast, East Coast of Australia
Duncan,

I've been following your various design musings over the years. Why not focus on what you every time seem to come back to? A sexy composite plane, 35-45 kts stall range, 40-60 hp, 150-ish kts WOT cruise. There is a small but substantial market for such a plane and many pilots are looking for something like that.
That makes a lot of commercial sense but I'd say the performance envelope is a little (lot?) wide unless there was some very substantial wing geometry/area change capability as well as the use of lift enhancement devices which would add complexity, weight and cost that might put it way beyond the acceptable market price.

There aren't many designs that will cruise at more than 2.5x the flapped stall speed especially with that sort of engine power. It might be more realistic to aim at the 40kt stall with simple flaps and a cruise of 90-95kts.

I'm sure folks will come up with all sorts of arguments including X-plane modelling to demonstrate that the plane could actually stall at under 30kts and cruise at 199kts with 12.2hp and so of course it can be done, but the reality doesn't support that. If it did there would be more than a few examples out there to confirm it.

Good comparisons are the Corby Starlet and the Jodel D9, they're pretty, small, light, clean and 65-80hp. In the real world they stall at around 42-45kts flapped, and cruise at around 90-100kts. I haven't come across many simple (i.e. not aerodynamically and structurally highly complex) designs that exceed that performance envelope by anything significant.

Just to put this into perspective let's keep in mind that the WORLD SPEED RECORD for aircraft under 300kg, and here we're talking about 180hp or so IIRC was set by Prof Iscold's University Aeronautical project at 329km/h (177kts) over 15km course, 327km/h (176kts) over 100km course and 360km/h (194kts) over 3km course. This is with a state-of-the-art and tiny-winged aircraft designed specifically for the FAI world speed records so I don't think a simple plane that will stall at 35-45kts and cruise at 150kts on 40-60hp is at all possible.

Another example - a few years ago, before he moved to the USA, I went for a fly with an old friend of mine called Mike O'Sullivan. He's the owner of the Supermarine Company which produces the Supermarine Mk26 Spitfire. We flew in the first or second of the two seat versions, in fact there wasn't even a seat in the back, just a couple of cushions and a harness. It had the Isuzu V6 engine which was producing about 260-300hp I think and it is a small plane, I think it's 80% scale. When you see those things screaming through the sky they appear to be really going fast - the truth on this day proved to be 165kts indicated, WOT, straight and level. From the pilot's perspective it does feel fast, at that speed the air feels quick thick and control response is solid and every ripple in the air is as hard as the corrugations on a dirt road.

165kts may not sound very fast but at that speed, unless you are flying very high, you don't want to be in an airframe that is anything but 'built like a brick outhouse', and you're not going to have that and still get off the ground with a 40-60hp powerplant.
 

autoreply

Well-Known Member
Joined
Jul 7, 2009
Messages
10,753
Location
Rotterdam, Netherlands
There aren't many designs that will cruise at more than 2.5x the flapped stall speed especially with that sort of engine power. It might be more realistic to aim at the 40kt stall with simple flaps and a cruise of 90-95kts.
There's a dozen European MLA's that meet the 35 knot stall speed, have a 912S and cruise at the 140-160 kts range.
That makes a lot of commercial sense but I'd say the performance envelope is a little (lot?) wide unless there was some very substantial wing geometry/area change capability as well as the use of lift enhancement devices which would add complexity, weight and cost that might put it way beyond the acceptable market price.

I'm sure folks will come up with all sorts of arguments including X-plane modelling to demonstrate that the plane could actually stall at under 30kts and cruise at 199kts with 12.2hp and so of course it can be done, but the reality doesn't support that. If it did there would be more than a few examples out there to confirm it.

Good comparisons are the Corby Starlet and the Jodel D9, they're pretty, small, light, clean and 65-80hp. In the real world they stall at around 42-45kts flapped, and cruise at around 90-100kts. I haven't come across many simple (i.e. not aerodynamically and structurally highly complex) designs that exceed that performance envelope by anything significant.
A fair point. Not one I agree with though. Many planes are aerodynamically fairly horrible. Enormous cooling drag, complete flow separation at the wing root, that kind of stuff.

That's where aviation suffers from all too often, extrapolation from substandard aircraft will make you way too conservative.

Assuming 60 hp and decent prop efficiency, you have an EFPA of 1.40 Thats andandahalf times that of the AR5 and roughly the same as the Quickie 2, the Delaminator, GP5, Nemesis NXT and Determinator.

Note that several of those are two-seaters with enormous engines (and size), a small engine in a single seater is much easier to make low-drag.
Also note that this is over twice the drag other homebuilts like sailplanes have. No need for whizz-bang complex stuff, the mentioned numbers are downright easy to achieve.

See here for some more data:
http://www.homebuiltairplanes.com/forums/general-experimental-aviation-questions/8762-aircraft-efficiency-tool-list-equivalent-flat-plate-areas-various-planes.html


Just to put this into perspective let's keep in mind that the WORLD SPEED RECORD for aircraft under 300kg, and here we're talking about 180hp or so IIRC was set by Prof Iscold's University Aeronautical project at 329km/h (177kts) over 15km course, 327km/h (176kts) over 100km course and 360km/h (194kts) over 3km course. This is with a state-of-the-art and tiny-winged aircraft designed specifically for the FAI world speed records so I don't think a simple plane that will stall at 35-45kts and cruise at 150kts on 40-60hp is at all possible.
It had eighty, not 180 hp. If we remember the power cube rule, (177/150)^3, it can sustain 150 kts at 60% power.
 

Head in the clouds

Well-Known Member
Joined
Mar 11, 2012
Messages
1,983
Location
Gold Coast, East Coast of Australia
There's a dozen European MLA's that meet the 35 knot stall speed, have a 912S and cruise at the 140-160 kts range.
I'm amazed, could you name them please?



It had eighty, not 180 hp. If we remember the power cube rule, (177/150)^3, it can sustain 150 kts at 60% power.
OK, as I said I couldn't remember for sure, I thought it was more hp than that. What engine did it utilise? Nonetheless it was/is a highly developed airframe at the technological leading edge. Do you really expect homebuilders to come anywhere close to this?

AND - it's 80hp, not 40-60hp.

As for the cube rule, that's theory that doesn't play out in practice. Next time you fly something powered have a look at the rpm/power graph and try throttling back to 60% power and see if you can maintain 85% airspeed. It just doesn't happen! There are other factors at play, including constant speed props, is that on the agenda for this simple aircraft too?
 

autoreply

Well-Known Member
Joined
Jul 7, 2009
Messages
10,753
Location
Rotterdam, Netherlands
I'm amazed, could you name them please?
VL3, Millenium Master, Blackwood Prime, MCR01, Fascination, Virus SW, Shark. I'm sure you can google the other half. All have a (NA) cruise speed with WOT in excess of 150 kts, two fixed gear.

Do you really expect homebuilders to come anywhere close to this?
Anybody who puts in a little effort can easily come within 50% from those kind of optimized aircraft, that's the range (with 60 hp) where we're talking about.
As for the cube rule, that's theory that doesn't play out in practice. Next time you fly something powered have a look at the rpm/power graph and try throttling back to 60% power and see if you can maintain 85% airspeed. It just doesn't happen! There are other factors at play, including constant speed props, is that on the agenda for this simple aircraft too?
It'd be pretty stupid to put a prop on a new design that's optimized for the wrong cruise speed. So yes, the cube rule does exactly play out, in the real world, unless one is dumb enough to put the wrong prop on.
 

BJC

Well-Known Member
HBA Supporter
Joined
Oct 7, 2013
Messages
11,538
Location
97FL, Florida, USA
VL3, Millenium Master, Blackwood Prime, MCR01, Fascination, Virus SW, Shark. I'm sure you can google the other half. All have a (NA) cruise speed with WOT in excess of 150 kts, two fixed gear.
There are some impressive performance claims on their web sites. Is there any independent validation of those claims?

(I wish that the CAFE organization was still conducting flight and performance evaluations. I trusted their information. Brian Seely, are you listening?)


BJC
 

autoreply

Well-Known Member
Joined
Jul 7, 2009
Messages
10,753
Location
Rotterdam, Netherlands
There are some impressive performance claims on their web sites. Is there any independent validation of those claims?

(I wish that the CAFE organization was still conducting flight and performance evaluations. I trusted their information. Brian Seely, are you listening?)


BJC
I have verified (well, slightly lower, but noticeably above 150 kts) two of those planes. From at least four of them I've seen the GPS traces (two opposite tracks in the same time frame and altitude) to believe the cruise numbers, N/A, so not with the 914. Plenty of world- and local records as well.

If I'm not mistaken, at least the Virus SW was tested by CAFE.

Much of this I presume is just the "not invented here" syndrome and why people keep referring back to 70 year old technology to show that it can't be done. ;-)
 

BJC

Well-Known Member
HBA Supporter
Joined
Oct 7, 2013
Messages
11,538
Location
97FL, Florida, USA
Much of this I presume is just the "not invented here" syndrome and why people keep referring back to 70 year old technology to show that it can't be done. ;-)
No NIH syndrome here, just years of experience in reading performance claims here that are not true. Plus, I haven't had much experience with aircraft from outside of the USA. Therefore my question.

I neither made reference to any technology, of any age, nor did I attempt to show that it could not be done.

I enjoy reading your views and preferences, so lighten up, don't get defensive, and stop shooting with a shotgun ("Much of this I presume ... "), you hit innocent bystanders that way. :)


BJC

Edit. CAFE aircraft performance reports are available here: http://cafefoundation.org/v2/research_aprs.php
 

Head in the clouds

Well-Known Member
Joined
Mar 11, 2012
Messages
1,983
Location
Gold Coast, East Coast of Australia
VL3, Millenium Master, Blackwood Prime, MCR01, Fascination, Virus SW, Shark. I'm sure you can google the other half. All have a (NA) cruise speed with WOT in excess of 150 kts, two fixed gear.
No, I couldn't find any others, could you provide some links?

Did you note that the claimed cruise speeds are not indicated/CAS at ISA sea level? The Dynaero is the only one to be honest enough to actually admit it and they state their claimed performance is actually true airspeed at FL110 which would bring it down from 140kts (not 150kts mind you) to 115kts at sea level, it's not quite so impressive now ... and I bet their claimed 34kts stall speed isn't at a Flight level DA ... and there's a Virus SW at the Pipi dealers at our local club, the demo pilot says it cruises at 110-115kts comfortably. Mind you, here they talk about cruise in practical everyday operational conditions, 25C, 2500ft DA, not 15,000ft as Pipi allude to in their blurb. We're not allowed as high as flight levels here, in any case.


Anybody who puts in a little effort can easily come within 50% from those kind of optimized aircraft, that's the range (with 60 hp) where we're talking about.
It'd take more than 'a little effort'.

Not one of those optimised aircraft meet the criteria I described in post #384. I said "I haven't come across many simple (i.e. not aerodynamically and structurally highly complex) designs that exceed that performance envelope (2.5x flapped stall speed) by anything significant." The key word is 'simple', those aircraft are all vacuum infused and oven cured carbon fibre airframes, developed and optimised with the use of complex CAD FD modelling and even so are structurally at the very bottom limit of acceptability for airworthiness.

Even with their high tech materials and construction methodology their strength is marginal in order to meet the weight limitations. This was amply demonstrated when they grounded much of the fleet of those types here in Oz because they were being flown under our (then) 544kg weight limit instead of the factory 450kg limit and showing signs of structural damage in normal operations. The factory(s) were approached to provide an STC for the increased weight and at the time they wouldn't agree to a single kg let alone a hundred.

Crashes of those types also demonstrate their very poor occupant protection, much of the airframe shattering into a cloud of splinters in many instances.

But -I think this is rather irrelevant to rtfm's case, to reach anything like those performance figures (in post #383 you said he should aim for "35-45 kts stall range, 40-60 hp, 150-ish kts WOT cruise") which is way ahead of anything those high tech Virus/Shark/Blackshapes claim to achieve, even with their 100hp ...
 

autoreply

Well-Known Member
Joined
Jul 7, 2009
Messages
10,753
Location
Rotterdam, Netherlands
No, I couldn't find any others, could you provide some links?

Did you note that the claimed cruise speeds are not indicated/CAS at ISA sea level? The Dynaero is the only one to be honest enough to actually admit it and they state their claimed performance is actually true airspeed at FL110 which would bring it down from 140kts (not 150kts mind you) to 115kts at sea level
Nonsense.
With a normally aspirated engine (and a perfect VP prop), cruise speed in this speed range will very slightly increase once you fly lower.
it's not quite so impressive now ... and I bet their claimed 34kts stall speed isn't at a Flight level DA ... and there's a Virus SW at the Pipi dealers at our local club, the demo pilot says it cruises at 110-115kts comfortably. Mind you, here they talk about cruise in practical everyday operational conditions, 25C, 2500ft DA, not 15,000ft as Pipi allude to in their blurb. We're not allowed as high as flight levels here, in any case.
Stall is always measured in IAS or EAS as you should know if you're a pilot.

Of course the Virus SW doesn't cruise faster than 120 if it's the LSA version...

All of those planes are at (or above) the mentioned speeds with WOT at close to sealevel. I'd rather trust independently verified records by NASA and CAFE, thrustworthy journalists who know what they're doing and my own observations (GPS tracks) than some 3rd hand old wives tales, too absurd to take seriously.
But -I think this is rather irrelevant to rtfm's case, to reach anything like those performance figures (in post #383 you said he should aim for "35-45 kts stall range, 40-60 hp, 150-ish kts WOT cruise") which is way ahead of anything those high tech Virus/Shark/Blackshapes claim to achieve, even with their 100hp ...
As we've established it's comparable to those planes. Going from a reasonably big 2-seater with 100 hp to a small 1-seater, 60 HP sounds about right for that kind of performance=>smaller airframe. 40 hp and 130 kts of course would also be fine.

The construction techniques have little to do with the performance of the noted planes but - as you note - a lot with meeting the rather limited weight constraints.
There's no reason a Rutan-style composite can't have the exact same performance and in fact several planes in that style have been built. (AR5/6, Arletty Racer)

Maybe you should just contact an Ozzie dealer for one of the mentioned airframes, bring a GPS and witness yourself...
Especially the Dyn'Aero and the Pipistrel have a significant presence down under, so it should be fairly simple to find one you could fly ;)
 

Head in the clouds

Well-Known Member
Joined
Mar 11, 2012
Messages
1,983
Location
Gold Coast, East Coast of Australia
Nonsense.
With a normally aspirated engine (and a perfect VP prop), cruise speed in this speed range will very slightly increase once you fly lower.

Stall is always measured in IAS or EAS as you should know if you're a pilot.

Of course the Virus SW doesn't cruise faster than 120 if it's the LSA version...

All of those planes are at (or above) the mentioned speeds with WOT at close to sealevel. I'd rather trust independently verified records by NASA and CAFE, thrustworthy journalists who know what they're doing and my own observations (GPS tracks) than some 3rd hand old wives tales, too absurd to take seriously.

As we've established it's comparable to those planes. Going from a reasonably big 2-seater with 100 hp to a small 1-seater, 60 HP sounds about right for that kind of performance=>smaller airframe. 40 hp and 130 kts of course would also be fine.

The construction techniques have little to do with the performance of the noted planes but - as you note - a lot with meeting the rather limited weight constraints.
There's no reason a Rutan-style composite can't have the exact same performance and in fact several planes in that style have been built. (AR5/6, Arletty Racer)

Maybe you should just contact an Ozzie dealer for one of the mentioned airframes, bring a GPS and witness yourself...
Especially the Dyn'Aero and the Pipistrel have a significant presence down under, so it should be fairly simple to find one you could fly ;)
OK, well, I just don't agree with your interpretation of their specs and I've little interest in it anyway, so we'll leave it at that.

BTW, we don't have any speed limit for LSA in Oz so there's no 'of course' about it, you're thinking USA rules ...

As others have mentioned auto, lighten up, there's no need to take everything so personally, it makes it unpleasant to discuss anything with you, you move the goalposts between one of your posts and the next.
 

DangerZone

Well-Known Member
HBA Supporter
Joined
Sep 5, 2011
Messages
2,110
Location
Zagreb HR
There are some impressive performance claims on their web sites. Is there any independent validation of those claims?

(I wish that the CAFE organization was still conducting flight and performance evaluations. I trusted their information. Brian Seely, are you listening?)


BJC
There is a possibility to fly these airplanes if one would want to buy them and see for himself.

The Tarragon Aircraft, Millenium Master and Blackshape Prime are based on the wooden homebuilt Asso X which is actually faster than their carbon fiber 'brothers'. With the same engine at same cruise power level the Asso X is slightly faster than they are simply because it was lighter at 285kg empty weight. The newer props with variable pitch and advanced Rotax 912 settings allow these airplanes to cruise at ober 300km/h (over 160kts) and stall at less than 64km/h (34kts).

TARRAGON

Millennium Master - Wikipedia, the free encyclopedia

Blackshape Prime - Wikipedia, the free encyclopedia

Vidor Asso X Jewel - Wikipedia, the free encyclopedia
 

autoreply

Well-Known Member
Joined
Jul 7, 2009
Messages
10,753
Location
Rotterdam, Netherlands
OK, well, I just don't agree with your interpretation of their specs and I've little interest in it anyway, so we'll leave it at that.
We're not talking about an "interpretation".

We're talking about accurate measurements by NASA, performance evaluations posted on flightaware etc, reports by several highly respected journalists and by the FAI.
As others have mentioned auto, lighten up, there's no need to take everything so personally, it makes it unpleasant to discuss anything with you, you move the goalposts between one of your posts and the next.
If you find it unpleasant, good. Stop arguing hard - verifiable - facts and countering them with your opinion, especially if you can't even get your basic numbers and/or facts right.

There's no shame on being wrong every now and then and admitting it. Trust me, I speak from experience. You'll learn a lot more than way ;)
 

Head in the clouds

Well-Known Member
Joined
Mar 11, 2012
Messages
1,983
Location
Gold Coast, East Coast of Australia
We're not talking about an "interpretation".

We're talking about accurate measurements by NASA, performance evaluations posted on flightaware etc, reports by several highly respected journalists and by the FAI.

If you find it unpleasant, good. Stop arguing hard - verifiable - facts and countering them with your opinion, especially if you can't even get your basic numbers and/or facts right.

There's no shame on being wrong every now and then and admitting it. Trust me, I speak from experience. You'll learn a lot more than way ;)
Fine, auto, I bow to your superiorness.

BTW, ref your #392 yes I am a (commercial) pilot, I probably have about 10,000 hrs more than you at a guess.

Yes I have designed and built more than a dozen successful aircraft, I understand you haven't designed or built any, so you speak from theory only?

I'm not as wrong as you claim, neither are your 'references' valid unless you supply them rather than just say they exist, please supply them. Also we're still missing six of your claimed 150kt/30kt aircraft, or was that an exaggeration?

While you're looking for references, we've been waiting more than two years for you to justify your regular statements that cantilever wings are lighter than strut braced, have you found any examples to justify that yet?

I'm keen to try the 'ignore' function, I've never used it before, it is permissible to put an irritable moderator on ignore?
 

WonderousMountain

Well-Known Member
Joined
Apr 10, 2010
Messages
2,028
Location
Clatsop, Or
:)Looked up virus sw, with 80 912rotax engine.
Most of those use that or 100hp variant.

Claimed performance 34.55 knots to 132 knot, a spread of 3.75, which seems right.

At 25C 2500 115 is 7/8th speed, what we don't know is engine RPM.

The other two look maybe a little faster, but expensive.
Actually the cf for composites is lower, except against bare bonded aluminium.

Da-5/6 could do it, but its not available because he preferred to continue building rather than kit out.
There are a few one off's might could hit those marks.

It is possible, maybe there's a 300kg racer with molds up for sale.
Problems get in the way like engine in front, windshield visibility, landing gear, engine cooling, weight increase with flaps.
Also the simplify specification, both helps and hurts the attainment of goals.

You don't want to know what my avatar does with 60 HP ;-)

Stop picking on downwind Jarno,
luPi
 

ragflyer

Well-Known Member
Joined
Apr 17, 2007
Messages
234
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: http://www.rotaxservice.com/documents/912Sperf.pdf

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.
 

BJC

Well-Known Member
HBA Supporter
Joined
Oct 7, 2013
Messages
11,538
Location
97FL, Florida, USA
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: http://www.rotaxservice.com/documents/912Sperf.pdf

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.

Steve did some neat things in his designs. An inverted V-8 in the Tailwind, which he said at one time was the best engine for it, his W-10 wing tips, and the wing spar-to-fuselage connection on the V-Whitt.

Anyone have a photo of that spar connection? I would love to see it discussed by the experts here.:)

The CAFE test airplane had a 160 HP O-320, but the W-8 flew nicely on 85 HP. 85 HP would perform even better in the W-10.


BJC
 
Q

qxev

Guest
[QUOTE = RTFM; 256164] Я собираюсь превратить в течение ночи, так вот возрождение ранее дизайна я рисовал с некоторое время назад. На этот раз я сделал немного больше работы по размещению крыла, фактическая компьютерная CG и играл с клапанами / лифтов и размещения элеронов. В настоящее время развертывания закрылки производит нос вниз момент, так что я немного больше работы, чтобы сделать. , Ох, и произведение немного shonky слишком я думаю, что это можно было бы на самом деле есть определенный смысл для двухместного (100 л.с.). Дункан [/ QUOTE] Добро пожаловать - надкрылье Французский разработчик демонстрирует макет одного сиденья личного твин-Jet - 6/19/2013 - Полет Global





View attachment 38007 View attachment 38008 View attachment 38009







http://www.youtube.com/watch?v=HAJXV7XK91k









 
Top