Flight Design insolvency

[Anticept]

You're on the right track, but I need to make some corrections. While it is true that more lift typically means more drag, high aspect ratio wings are insanely efficient. Making the wingspan wider and making the chord shorter will dramatically reduce induced drag for the same amount of lift. It does increase form drag, which can be compensated by reducing thickness. The ETA glider has some huge wings and glides at something completely rediculonk, 70:1 or something nuts like that.

 

The drawback to high aspect ratio is a reduced speed, reduced manuverability, higher torque bending stresses, and difficulty finding a place to park!

 

Anyways, the virus sw has slightly longer wings, but what I can tell, they are indeed thinner. They have a slight taper too which also imparts an aerodynamic advantage. It's still a little crazy that it pulls off thise marketing points, but I want to actually see one to compare the engineering to get an idea of what gives it those stats.

[deckofficer]

Let me rephrase.  I'm not quite calling "pants on fire", but IMO the specs pointed out by Corey in the POH are simply not possible.  Like the "better, faster, cheaper" saying, you can pick two for an airplane using a Rotax 912 with a fixed-pitch prop:

 

117kt

5300rpm

795nm range on 24.5 gallons. 

 

You are also VERY unlikely to be doing 115kt @ 3.2gph in any airplane that will make the LSA stall requirements.

 

Also, any airplane can dive at the runway for a fast past.  I have run mine to 140kt in similar circumstances, the only reason I couldn't do 160kt is that it's above Vne.  Meaning I could have, but shouldn't have, so didn't. 

 

Physics are physics.

 

Andy,

 

  This is where I think you nailed it and I agree with you. Even though on paper the stall specs look close, it is hard for me to believe that claim too. There is some wiggle room on giving stall specs, because unlike older aircraft, the new ones don't drop a wing due to stall but rather mush along. Add this to the inaccuracy of IAS at higher AoA, fudged numbers can slip through on self certifying, since in LSA it is the manufacturer that does the testing and generates the numbers and not a government body.

 

  Pipistrel's entire line is based on gliders, self launch gliders with retractable pods, motor gliders, and then the 35' short wing Virus and Alpha Trainer. To core a thermal you need the ability to fly slow in steep banks but don't need to have an artificially slow stall to satisfy LSA requirements. As you said "physics are physics" and a fast, efficient aircraft is going to have a corresponding higher stall speed. Pipistrel just might be guilty of massaging stall numbers.   

[FlyingMonkey]

Andy,

 

  This is where I think you nailed it and I agree with you. Even though on paper the stall specs look close, it is hard for me to believe that claim too. There is some wiggle room on giving stall specs, because unlike older aircraft, the new ones don't drop a wing due to stall but rather mush along. Add this to the inaccuracy of IAS at higher AoA, fudged numbers can slip through on self certifying, since in LSA it is the manufacturer that does the testing and generates the numbers and not a government body.

 

  Pipistrel's entire line is based on gliders, self launch gliders with retractable pods, motor gliders, and then the 35' short wing Virus and Alpha Trainer. To core a thermal you need the ability to fly slow in steep banks but don't need to have an artificially slow stall to satisfy LSA requirements. As you said "physics are physics" and a fast, efficient aircraft is going to have a corresponding higher stall speed. Pipistrel just might be guilty of massaging stall numbers.   

 

And before I come across as a sour grapes "it can't possibly be better than a CT" kind of guy, I'm just looking at the specs and comparing to all other LSA out there, not just the CT.  Also since the Pip and the CT both use the same engine, we have some common ground for comparison.

 

Now what *might* have happened here is that even for the LSA version, the POH lists only stats for the non-LSA (and best performing) version.  That's not ideal, but they would not be the first company to list a single set of stats for multiple versions.

 

Again, I don't know enough about adjustable props in general, and this one specifically, to comment on performance numbers.  I feel somewhat confident in saying that a Rotax 912ULS cannot make 5300rpm at 3.2gph.  the engine turning a specific RPM will burn a certain amount of fuel (varying somewhat with altitude, since higher = less gas in mixture).  But I don't think you could get that engine below 5gph at that RPM, at least not below 10,000ft.  Maybe that 3.2gph number is for the 80hp model, but even then I find it a tough number to fathom, and certainly not at 117kt (again, for a fixed pitch LSA version).

 

The taildragger will lower drag even more, and will certainly be faster.

 

If the numbers are "market massaged", they are not the only ones who do that.  I think the CTSW POH lists max range at ~900nm...but to do that you'd have to fly at something like 94 knots and 4300rpm.  Nobody wants to do that for 9 hours!!!  

[deckofficer]

In my post #170, I included a link to the specs of both SW and SW LSA. Big difference, a fixed pitch prop for LSA is like being stuck in 2nd gear in a car.

 

Turning a given rpm does not dictate by itself fuel consumption. The overall drag of the airframe will dictate throttle setting to achieve a given rpm and that is where the Pip gets its low fuel burn. 

[Ed Cesnalis]

Again, I don't know enough about adjustable props in general, and this one specifically, to comment on performance numbers.  I feel somewhat confident in saying that a Rotax 912ULS cannot make 5300rpm at 3.2gph.  the engine turning a specific RPM will burn a certain amount of fuel (varying somewhat with altitude, since higher = less gas in mixture).  But I don't think you could get that engine below 5gph at that RPM, at least not below 10,000ft.  Maybe that 3.2gph number is for the 80hp model, but even then I find it a tough number to fathom, and certainly not at 117kt (again, for a fixed pitch LSA version).

 

Andy,

 

You are treating RPM alone as a metric that determines power or by extension fuel consumption.  A 912ULS could easily make 5300 on a 3.2gph burn, just reduce the load.  What if the Pip cruised at the prop's take off setting and throttled back to 5300? That could easily be a 500RPM reduction.

 

If the adjustable prop has 5 settings then 5300 can be 5 different power settings depending on the prop setting.

[FlyingMonkey]

Andy,

 

You are treating RPM alone as a metric that determines power or by extension fuel consumption.  A 912ULS could easily make 5300 on a 3.2gph burn, just reduce the load.  What if the Pip cruised at the prop's take off setting and throttled back to 5300? That could easily be a 500RPM reduction.

 

If the adjustable prop has 5 settings then 5300 can be 5 different power settings depending on the prop setting.

 

With a fixed pitch prop in level flight?  That's what we're discussing, I don't think it can be done and still make LSA stall standards.  Maybe with a little super low drag stub wing airplane.

[Ed Cesnalis]

With a fixed pitch prop in level flight?  That's what we're discussing, I don't think it can be done and still make LSA stall standards.  Maybe with a little super low drag stub wing airplane.

 

Even with a fixed pitch (ground adjustable) prop in level flight the load is not fixed it changes with altitude.  RPM alone will always be a poor way to describe a power setting. 

[deckofficer]

Andy,

 

It is all about drag, lets forget the in-flight adjustable prop. A good example just came to me, notice my avatar? That is a 1923 Ford Track-T, and unlike a T-bucket hot rod, mine doesn't have the tall vertical windshield but instead a very short and raked windshield. Instead of the drag of an open engine, mine is aerodynamically clean in comparison. I've run 3 different engines and all have given much lower fuel burn than the same engine in a T-bucket.  

 

[FastEddieB]

Maybe with a little super low drag stub wing airplane.

I think you still have that backwards.

 

Stubby wings have higher drag, not lower.

 

First Google hit:

 

http://www.pilotfriend.com/training/flight_training/aero/drag_red.htm

[deckofficer]

I think you still have that backwards.

 

Stubby wings have higher drag, not lower.

 

First Google hit:

 

http://www.pilotfriend.com/training/flight_training/aero/drag_red.htm

 

Good link, I've read it in the past. Of all the specs we get from aircraft manufacturers, too bad Cd isn't one of them. 

[FlyingMonkey]

I think you still have that backwards.

 

Stubby wings have higher drag, not lower.

 

First Google hit:

 

http://www.pilotfriend.com/training/flight_training/aero/drag_red.htm

 

I think if you take two wings with the same profile, one 10ft long and one 20ft long, the shorter wing has less drag.  Do you disagree?

 

The example in the link you shared are two different wing profiles:

 

"The magnitude of induced drag depends on the amount of lift being generated by the wing and on the shape and size of the wing. Long, thin (chord wise) wings have low induced drag; short wings with a large chord have high induced drag."

[deckofficer]

Andy, I think you answered your own question.

 

If two wings share the same chord, and one is 28' and the other is 35', then drag would be higher on the longer wing. The problem is how much weight will the shorter wing support in flight with less lift of the longer wing. If thickness is changed to produce the same lift, then the longer, thinner wing would have less drag.

 

Competitive soaring where distance covered is the name of the game (along with altitude gains), it is almost counter intuitive. When your milking lift from a thermal, you are generally flying at the speed of minimum sink, coring the thermal. If the next possible thermal activity is a distance away you speed up which carries a faster sink rate, but overall you cover a greater distance per altitude lost.

[Ed Cesnalis]

Are you focused on drag when L/D is a better focus?  Won't the 10' wing have less drag but a poorer L/D? 

[FlyingMonkey]

Bob, there is a reason that more highly loaded, short wing airplanes tend to be faster.  The shorter wing makes less drag.  Now if you change the profile of the wing, everything changes.  But the wing profile that makes a great motorglider rarely makes for a great high speed cruiser.

 

Which is part of what calls into question some of Pipestel's performance claims.  They *seem* to be claiming near ideal glide performance, near ideal cruise performance, and near ideal fuel economy.  Seems a tough bill to fill, particularly with fixed pitched props and LSA performance requirements (primarily stall).

[FlyingMonkey]

Are you focused on drag when L/D is a better focus?  Won't the 10' wing have less drag but a poorer L/D? 

 

Of course, but the shorter, lower drag will enable higher cruise.  The lower L/D does not come into play except at low speed.  Sure the longer wing will stall slower, but that's not the portion of the envelope the longer wing is optimized to.  If you keep the profile the same, that Pipestrel would be faster with a shorter wing.  It will stall at higher speed as the trade off.

 

Nothing is free, which was my original point with the Pip's performance claims.

[FastEddieB]

I think if you take two wings with the same profile, one 10ft long and one 20ft long, the shorter wing has less drag.  Do you disagree?

 

 

I think your focusing on form drag, one type of parasitic drag.

 

I think we're talking mainly about induced drag.

 

Imagine your wings were each only 1' long. Now imagine the speed and angle of attack you'd have to use to support your plane's weight a the induced drag would be HUUUGE - as Donald Trump would say.

[Tom Baker]

I know threads on this forum can take on a life of their own, but maybe a mod could take all of this chatter that is not relevant to Flight Designs money issues and move it to a new thread.

[FlyingMonkey]

I think your focusing on form drag, one type of parasitic drag.

 

I think we're talking mainly about induced drag.

 

Imagine your wings were each only 1' long. Now imagine the speed and angle of attack you'd have to use to support your plane's weight a the induced drag would be HUUUGE - as Donald Trump would say.

 

But we're not talking about that, we're talking about airplanes with wings in the 20-40' range.  I'm not saying hack off half a wing and watch you need 500hp to stay out of the stall.  I'm talking about reasonable changes to wing profile and length to optimize for LSA flight (i.e. Pipestrel v. Flight Design).

 

We can take any design argument to the extreme and get silly results.  I'm not suggesting that.  Are you disagreeing that if we kept the shape the same and took 2' off of each side of the Pipestrel (or CT) wing, that it would not be a faster airplane?  That's pretty much exactly what they did going from the CT2K to the CTSW; they took a foot off the wing on each side, and the airplane stall speed went up and speed increased.   

[FastEddieB]

EFFECT OF ASPECT RATIO. The effect of aspect ratio on the induced drag is the principal effect of the wing planform. The relationship for induced drag coefIicient emphasizes the need of a high aspect ratio for the airplane which is continually operated at high lift coefficients. In other words, airplane configurations designed to operate at high lift coefficients during the major portion of their flight (sailplanes, cargo, transport; patrol, and antisubmarine types) demand a high aspect ratio wing to minimize the induced drag. While the high aspect ratio wing will minimize induced drag, long, thin wings increase structural weight and have relatively poor stiffness characteristics. This fact will temper the preference for a very high aspect ratio. Airplane configurations which are developed for very high speed flight (especially supersonic flight) operate at relatively low lift coefficients and demand great aerodynamic cleanness. These configurations of airplanes do not have the same preference for high aspect ratio as the airplanes which op- erate continually at high lift coefficients. This usually results in the development of low aspect ratio planforms for these airplane configurations.

 

Aerodynamics For Naval Aviators, p. 71 (available online for free)

[Znurtdog]

I know threads on this forum can take on a life of their own, but maybe a mod could take all of this chatter that is not relevant to Flight Designs money issues and move it to a new thread.

I agree

[FlyingMonkey]

EFFECT OF ASPECT RATIO. The effect of aspect ratio on the induced drag is the principal effect of the wing planform. The relationship for induced drag coefIicient emphasizes the need of a high aspect ratio for the airplane which is continually operated at high lift coefficients. In other words, airplane configurations designed to operate at high lift coefficients during the major portion of their flight (sailplanes, cargo, transport; patrol, and antisubmarine types) demand a high aspect ratio wing to minimize the induced drag. While the high aspect ratio wing will minimize induced drag, long, thin wings increase structural weight and have relatively poor stiffness characteristics. This fact will temper the preference for a very high aspect ratio. Airplane configurations which are developed for very high speed flight (especially supersonic flight) operate at relatively low lift coefficients and demand great aerodynamic cleanness. These configurations of airplanes do not have the same preference for high aspect ratio as the airplanes which op- erate continually at high lift coefficients. This usually results in the development of low aspect ratio planforms for these airplane configurations.

 

Aerodynamics For Naval Aviators, p. 71 (available online for free)

 

If you have an argument to make on this, please state it and then quote sources, I don't want to have to read a textbook and infer what you mean.  

 

You SEEM to be agreeing with me that wings best for soaring are not best for high speed flight.

 

EDIT:  If you hack off a foot of wing you have reduced its aspect ratio, and made it better for higher speed flight.  Correct?

[FastEddieB]

If you have an argument to make on this, please state it and then quote sources, I don't want to have to read a textbook and infer what you mean.  

No thanks.

[deckofficer]

I'm sorry. I started this thread drift so if an admin wants to bundle it up and move it, might I suggest the topic header as "Flight Design vs. Pipistrel".

 

When I do a google search wanting to read comparisons between aircraft X and aircraft Y, that is how I input the search, i.e. "Ch-701 vs Savannah".  

[FlyingMonkey]

No thanks.

 

Um...okay then!

[Anticept]

No thanks.

 

 

Um...okay then!

 

 

LOL that made me laugh. He actually did quote his source... "Aerodynamics For Naval Aviators, p. 71", if you're not going to read it, that's your problem!

 

The ideal wing for flying would be one that is paper thin, long, and a short cord. If you can invent a mystery material that can achieve this and still have a lot of strength, you could also go fast and be maneuverable.

 

The realistic wing for flying fast and maneuverable, is a low aspect ratio wing. This is due to material limitations.

 

That said, a high aspect wing on the Pipistrel is not necessarily an indication of speed itself! That's why I specifically mentioned how thin it is too.