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Cozy's Vne?


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Hi,

 

Why the Vne of Cozy is 220 mph?

If someone would use a jet engine in Cozy instead of

piston engine, this could become a serious limitation.

What detail in the airframe limits it to 220 mph?

Could it be modified to allow higher speeds somehow,

for example by adding more strength in certain parts?

Many Lancairs have higher Vne than Cozy. What makes

it dangerous to fly Cozy faster than 220 mph, is it expected

that the airframe can not handle greater speeds and tears

apart?

 

What I have tried with X-plane is that the virtual Cozy can

fly a lot faster than 220 mph. But what about the real Cozy,

what happens when flying faster than the 220 mph limit,

for example in the case of jet engine?

 

Best Wishes,

Karoliina

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I'm completely talking out of my butt here, but from what I've read in a few places, there are two things that limit the Vne to 220 mph. First, as a manufacturer, you can set the Vne to anything you want, as long as the plane still holds together and is controllable in flight when you slightly exceed this speed, while fully loaded. It's probable that this is a speed that Nat felt comfortable with in his Lycoming-powered Cozy. He certainly pushed the envelope while testing, and probably reigned this in for safety's sake.

 

The second reason I've seen listed for this Vne is fear of catastrophic failure due to flutter. I've read a few accounts that state that above this speed, flutter tends to occur, with the canard flapping like hummingbird wings. Likewise, there are a few arguments about approaching Mcrit for the Roncz airfoil at higher altitudes, if you go much above this speed.

 

I'm sure that flutter can and does occur at a speed higher than 220 mph, but where does it start? Are there other factors like build quality or material used that can move this speed point?

 

I've asked about using carbon fiber and kevlar in certain areas to improve strength with similar or less weight, but have been discouraged by others. At the same time, the Berkut people use these materials in a very similar design, and don't experience flutter up to their Vne, which is in excess of 300 mph. I fail to see why carbon fiber or kevlar could be used judiciously in a Cozy design to raise Vne while reducing the specter of flutter.

 

By the way, since you mentioned jet power, Greg Richter retrofitted a jet engine in his Cozy III. His webpages (address escapes me right now, but he is the CEO of Blue Mountain Avionics) state that he is really worried about the Mcrit issue, as the jet pushes him through an aerodynamic region that is not understood. He's mostly worried not about hitting high speeds, but how the plane handles slowing down from them.

 

-- Len

-- Len Evansic, Cozy Mk. IV Plans #1283

Do you need a Flightline Chair, or other embroidered aviation accessory?

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You really hit the 'nail on the head' about Vne. Nat has always appeared very protective of his machine and wanted safeguards that would allow for some sloppiness in building without getting someone killed. It seems whatever the airplane's true Vne will be a well-kept secret, albeit to protect the plane's reputation and pilots lives too.

 

It would be interesting to find determine if yo went to kevlar or carbon fiber could raise the Vne and by how much?

Nathan Gifford

Tickfaw, LA USA

Cozy Mk IV Plans Set 1330

Better still --> Now at CH 9

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Hi,

 

So a solution could be the following:

- change the wing and canard airfoils to a such that handles better high speeds, I think the modified Eppler may not be good for

high speeds. Some NASA laminar flow airfoil could be used instead of the Roncz/Eppler combination?

- Use stronger materials in canard construction such as

carbon fiber.

 

But now what - what we get: kind-of Infinity-1 (allthough a four

place version)...?

 

Yes, I have visited Greg's web page, that is where the idea

came from (I had been thinking that already by myself,

but did not think it would be possible until I saw Greg's page)

- Cozy can be used with a jet engine. Now how

fast it can go with a jet engine is the question and how to

determine how to modify it to make it better suitable for

the jet engine.

 

Best Wishes,

Karoliina

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Definately a NASA profile would be the best for the canard elevator in my opinion. I can't remember the profile I have used as generic template in my designs, but it was something like NASA 9206 (totally guessing here now), it was a relatively thick profile with semi-neutral end point stabilization. Well, I prefer to keep the depth of canard wings rather short, and utilize the uplift of the profile at low speeds.

 

Does anyone know of a website with the list and graphics of all NASA profiles?

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I asked about Velocity's profiles from Velocity. They did not know it in details something like "There is no number for them, they were designed by a NASA engineer". Starship is also using some

(laminar flow?) NASA profiles.

 

Mika, did I understand it correctly

that you are either designing or going to

design own canard aircraft or otherwise interested design

a such thing?

 

I would be very interested doing an own design (I have

had that dream very very long time, even before I became

aware that it could be possible, for many years I thought

it would not be possible for an individual with limited

amount of money and resources) allthough

there is are several big problems:

before that would be feasible, it is

quite much needed to do some other design first to understand

what an aircraft consists of and that is going to make several

years to accomplish. And secondly think I need to study

some more mathematics as the mathematics present in the

book Aerodynamics for Engineers goes beyond my understanding

with my current level of understanding in higher math.

And thirdly I need to do first some radio controlled scale down

models and try to figure out in depth how X-plane's aircraft editor

works - to try things with the sim.

 

Best Wishes,

Karoliina Salminen

(Espoo/Finland)

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I am sure going to design an or multiple own aircrafts, but this will be more actual when I can dedicate more time into this, and that is not in near future, alas.

 

However, I have done several radio controlled canard designs, which could be possibly used as basis for 1:1 scale aircrafts too. Mostly I have just done experimental aircrafts, with different size and shape of canard wings, one of my planes had even a negative V-angle canard, and it was an very interesting concept.

 

I don't think money is really an issue, I mean this one crazy finnish guy on the country side designed an conventional radio controlled airplane, then he made a bigger version it, and finally he made the exact same design as a 1:1 man carrying aircraft, and flew with it. The local police confiscated his airplane, but then he made Tiira II, and flew again, and it was confiscated again. Nowadays you can find Tiira II in a finnish aircraft museum.

 

But of course with canards the design on an 1:1 scale airplane is much more complicated, and even professional aircraft companies have failed in calculating the relations correctly - the swedish military's Saab 37 Viggen (not to confuse with Rutan's Vari-Viggen) was a sad example of it.

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  • 3 years later...

Back to the original question - I looked up flight envelope speeds in Airplane Aerodynamics and Performance, Roskam & Lan 1997.

 

Based on FAR 23 & FAR 25 certified airplanes,

 

the Design Diving speed V_D must satisfy the relationship V_D >= 1.25 Cruise Speed, and must be specified by the designer. I believe V_NE is the same as the dive speed to the design engineer, so if nat chose that speed it probably satisfies that relationship but it on the low end to be conservative for safety reasons.

 

Design limit load factors are also selected by the designer but must meet the following condition for FAR 23 and FAR 25 certification:

 

n_lim_pos = 2.1 + (24,000/(W+10000))

 

however (FAR 23) n_lim_pos need not be greater than 3.8,

n_lim_pos = 4.4 for utility category and n_lim_pos = 6 for acrobatic category.

 

and (FAR 25) n_lim_pos may not be less than 2.5.

 

there are a few other caveats I won;t take the time to write down, but all this has a direct relationship to the V-n diagram.

 

http://www.cozybuilders.org/performance/VN_Normal_2050_SL.html

 

Per FAR 23, V_C is the design cruising speed, which must satisfy the relationship:

 

V_C = k_c SQRT(W/S)

 

where W is the flight design gross weight, or the max design takeoff weight.

 

k_c is a constant, k_c = 33 for normal and utility =category airplanes with wing loadings up to W/S = 20 psf. k_c varies linearly from 33 to 28.6 as the wing loading varies from 20 to 100 psf.

 

V_C also must not exceed 0.9 V_H, where V_H is the max level speed obtained with max power.

 

the V_B is related to the lift curve slope and the wing loading, and is referred to as the gust line. The gust lines are defined by the equation

 

n_lim = 1 +- (K_G* U_de * V * C_L_alpha)/(498(W/S)

 

I'm not going to get into the details here though.

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