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Altitude and Speed in COZY aircraft

Marc Zeitlin

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The question was asked "how high can a COZY go?". Jim Price has taken a normally aspirated 160 HP Long Eze to over 35,500 ft., and holds the altitude record for that weight class of single engine piston aircraft.


Vance Atkinson has taken his 160 HP COZY III to 23K ft and performed a stall series there. His TAS was 152 KT at full throttle.


Aerocad claims a Service Ceiling (100 fpm climb rate) of 25K ft. with a 200 HP engine. I have some vague recollection of Nat stating 26K ft. service ceiling for the COZY MKIV, although I can't find a reference.


With respect to top speeds, the Roncz canard is suspected (by John Roncz, the designer) of having a critical Mach # of about 0.5. At that point, the canard will Mach stall, and the plane will become an uncontrollable lawn dart (per discussions with Richard Riley - one of the former principals in Berkut Aviation, the fastest homebuilt canard).


0.5 Mach is 380 mph TAS at sea level (we don't ever have to worry about getting there). However, at the altitudes that some people are talking about, say 25K ft, 0.5 Mach is 346 mph. Notwithstanding flutter considerations (which a simple web search, left to the reader as an exercise, will show is dependent both on TAS and IAS), this critical Mach # is one limit that you do not want to get near. Since 200 mph IAS is ~300 TAS at 25K ft., that's not far away from 346 mph - a short dive to change altitude, or a distraction and small stick bump could easily lead to a large speed change in a short period of time in these slippery aircraft.


Mach stall is unrecoverable, unlike a short stint above Vne, or even a short stint with flutter. In either of the other two cases, throttle reduction and/or stick pressure will reduce speed and most likely eliminate the problem. Once Mach stall occurs, the nose drops, the speed builds, the Mach stall gets worse, and as RR has said, you tie the low altitude record while pointed straight down.


RR believes that no aircraft using the Roncz canard should use a number higher than 300 mph TAS for Vne. Since no COZY's now exist that can reach this #, the 220 mph IAS Vne has proven acceptable - if people put substantially more speed capability into their aircraft, such as Greg Richter, or some of the posters here, Mach stall will become a large factor in their considerations.


Just thought this might help some people in evaluating their options.

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Uh, wow...


Not sure what triggered that tidbit of info Marc, but that's one of the most intriguing tidbits I've seen on this forum. Thanks.


Not that I ever plan on going that high or fast, but that's an aspect of the canard design I had never considered, nor was I aware of...

Evan Kisbey

Cozy Mk IV plans # 1114

"There may not be any stupid questions, but I've seen LOTS of curious idiots..."

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Thanks alot for that Marc,

I'm enjoying this, as I have been working through my "Mechanics of Flight" by A.C. Kermode ( I recommend it to anyone, my bible) and this discussion brings me to chapter 11, Flight at Transonic speed.

Only two chapters left, Flight at Supersonic speed, and space flight.

I guess that will be left up to Marble and myself to conquer:D (only joking)

You are correct that Mach 0.5 is 300 knots at 25,000 feet on a standard day. Slower if it is colder.

I understood that Mcrit is the speed at which the local velocity of the air begins to turn supersonic and a shock wave may occur. So the Mach stall will occurr above this point.

Mcrit is the point where drag increases massively, so horsepower required will increase proportionaly, thus I think it will be very hard to actualy enter a Mach stall. As you say though, extreme caution must be taken.

You are right that it is a very important consideration, but one that makes me only more eager to try it, (I am in no way suggesting any one else try it).

Would you suggest some sweep in the canard, or perhaps a different canard aerofoil section?

Thanks for the info, 300 mph TAS sounds good to me anyway.


The Coconut King

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Originally posted by Marc Zeitlin

Roncz canard is suspected ... of having a critical Mach # of about 0.5 ... canard will Mach stall, and the plane will become an uncontrollable lawn dart... unlike a short stint above Vne, or even a short stint with flutter ... throttle reduction and/or stick pressure will reduce speed ... Once Mach stall occurs, the nose drops, the speed builds, the Mach stall gets worse

I have only heard vague references to Mach stall (unless there is another label that I might be more familiar with) and you have piqued my curiosity. How about some details how exactly it works ....

<... Once Mach stall occurs, the nose drops, the speed builds, the Mach stall gets worse ...>

I would have guessed that the transonic drag rise would inhibit acceleration. But then again, if the nose drops uncontrolably, acceleration is no longer necessary ... you don't need to be more uncontrollable - just maintain the speed/uncontrollability you've got.:( In any event, I'm trying to visualize how a nacient shock wave at the peak of the canard airfoil would stall the canard as I don't recall experiencing that on the main wing of planes that I have flown (in my very limited experience) transonically. My recollection involves a little nose tuck and then recovery as you move on to supersonic. Does it have to do with the shock wave blanking and neutralizing the elevator?


I'm out of guesses. Help me out please ....

...Destiny's Plaything...

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Hallo Jim,

I only know this from books, not experience.

What you describe I was told is called Mach Tuck. A nose down tendency caused by the Centre of pressure moving rearwards, requiring big trim changes.


My understanding of a shock stall is that there is a sudden and considerable increase in the density of air at the shock wave, a rise in pressure and temperature, and flow breaks away from the surface. The sudden and considerable increase in drag, combined with the loss of lift and change in the centre of pressure and pitching moment causes severe buffeting, and hence the name Shock stall. I thought it was more of a transonic problem.


I am not sure this will occurr very close to Mcrit, but then I'm not sure otherwise, lets just say I'm not sure eh?:D


However from my limited knowledge, I know we have Mcrit, where some supersonic flow begins; Mdet (Mach detachment) where the bow wave attaches to the leading edge; and Mcdr (critical drag rise) where there is a significant rise in drag.


I was led to believe these all happened before the shock stall.

However, the canard is very short in depth, so Mach tuck may be very severe. 300 mph TAS sounds a very reasonable Vmo to work around. Most turbo props and jetliners sit at Mcrit in the cruise.



The Coconut King

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Originally posted by No4

Be good fun finding out anyway. :D


Waitaminute-- finding out if a cozy can become a $50k lawn dart from 25k feet is your concept of fun? :eek:

Evan Kisbey

Cozy Mk IV plans # 1114

"There may not be any stupid questions, but I've seen LOTS of curious idiots..."

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Well Evan,

I think we are agreed that not everything on this site should be taken as gospel. Marc's opinion I regard higher than most. But in this instance I believe 300 true is possible, and I disagree that the plane will suddenly take a nosedive. Marc in fact quotes Vne ( I would call it Vmo, max operating) to be 300 True. 300 is a common cruise speed in a long line of proppeller driven aircraft, many of which are far less aerodynamic than a Cozy. I have stated elsewhere many times that the Mach drag on the prop will cause immense drag above 300 knots. Have any Lancair's "Shock Stalled"? I doubt it.


Even by building a Cozy, you are at the cutting edge of aircraft design, only now are composites considered for commercial aircraft, and the superior canards fail to reach production because of conservative purchasing.


I have spent 12 months researching my project, concurrently with studying for my ATPL license. By the time I finish building this plane I will hopefuly be working as a pilot, spending several hours a day at 300 mph or faster.


I do not enter into this project lightly, and for anyone else I suggest to stick to the plans. I am discussing with a flight test engineer a full test program, and I look forward to it immensely.


So yes, it will be good fun finding out anyway.:D

The Coconut King

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I suppose the first question should be whether or not a propeller-driven Cozy can even reach 346 TAS at 25k feet. He mentioned that the Cozy III was running flat out at 152 TAS at 23k, and the III isn't a whole lot slower than the IV...


Anyhow, I've stated before I'm no engineer, and I'm not trying to cut anyone down, but it sounds to me like something to explore through CFD or extensive wind tunnel testing, rather than in actual flight. This isn't something to find out experimentally unless you've bought yourself a good parachute.


Unless I misunderstood Marc, and assuming he's correct (he usually is), here's how I read it:


Your first indication of the type of stall he's describing would be a sudden loss of altitude and an increase in speed, associated with a downward pitching movement. By that time it's probably too late to recover, as the pitch and altitude change only accelerate the aircraft and contribute to the cause of the stall. I suspect you'd lose the canard from flutter and lord knows what else long before you hit the ground. Perhaps if you were super-observant you could cut the throttle, put out your belly board, and open up the rudders to shed some speed if you thought you were on the verge of the stall, but from Marc's description it doesn't sound like you get much in the way of warning.


I wonder if there's someone here with access to the simlation tools to demonstrate this effect short of actually trying it?

Evan Kisbey

Cozy Mk IV plans # 1114

"There may not be any stupid questions, but I've seen LOTS of curious idiots..."

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The Cozy's mentioned had IO-360 engines, they have insufficient horsepower to go above 200 mph at 10,000 feet. At 25 they would be down to 50 hp, as quoted by dust.


Marc is very knowledgeable, he is an aeronautical engineer, a pilot, and a succesful Cozy builder. Hats off. However, in my last three discussions with him, he has admitted to errors in his posts.


I disagree that a shock stall will occurr anywhere near Mcrit. At Mcrit, an already large amount of drag (Indicated Air Speed) will start to increase rapidly from the onset of supersonic flow. The proppeller will now be less and less efficient. So to accelerate into even a Mach tuck condition will require a massive increase in power, which simply will not be available. At any time cutting the power will turn the prop into a huge brake.


It would be great if some one could run it through a simulation tool, but as to how accurate it would be to a real Cozy would be equaly as debatable. :D

The Coconut King

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Whether or not a Lancair, or any other prop driven plane, has experienced mach stall at 300 mph is immaterial. The overall aerodynamic cleanliness doesn't really matter either.


Those other aircraft are using airfoils designed for higher speeds - not a fat Roncz like the Cozy canard. You simply can't make a meaningful comparison between an airfoil with Mcr of .5 with ones on the order of .7 (or better) at this speed.


I wouldn't count on the drag rise to save you. The canard is small, so the overall drag rise may not be as dramatic as the forward loss of lift. At low angles of attack that drag rise might be delayed just enough that you don't experience it until after it's too late. Once the nose is pitched down by as little as 10 degrees, gravity is providing more push than your engine was. By 15-20 degrees the gravity "thrust" has doubled. Are you sure you can recover before then?


Also note that the pitching moment of your main wing may be starting to rise dramatically (another mach effect), further loading the canard.


I'm not saying you are wrong about the ability of the Cozy to fly safely at 300 mph, but I sure wouldn't be using the Lancair or any other aircraft designed for high speed as evidence to that ability.



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Hang on a minute!

just to quote Marc again..

"RR believes that no aircraft using the Roncz canard should use a number higher than 300 mph TAS for Vne".


Sorry but I thought that was what I was planning all along.



If anybody can give me anything constructive on flutter I'd be grateful.


The Coconut King

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Originally posted by No4

.... But in this instance I believe 300 true is possible, and I disagree that the plane will suddenly take a nosedive.


If you read my statement carefully, I said that the designer of the airfoil BELIEVES that the critical Mach # for that airfoil will be around 0.5, or 346 mph at 25K ft. It could be higher, it could be lower. It's doubtful that it would be as low as 300 mph, and I never said that there's a problem at 300 mph TAS.


Originally posted by No4

Marc in fact quotes Vne ( I would call it Vmo, max operating) to be 300 True.

No, what I said was that for the sake of having some safety margin, Richard Riley __recommended__ that 300 TAS be Vne or Vmo, whichever you'd like to call it. Berkut claims a top TAS of about 300 mph - no Berkut will INDICATE 300 mph - more likely about 260 - 275 mph, but that's a guess. I do not know exactly what airfoil the Berkut uses, either for the canard or the main wing - I do know that they do NOT use the same airfoil for the main wing as the L.E. and COZY.


Originally posted by No4

... Have any Lancair's "Shock Stalled"? I doubt it.

The issue here is with the specific Roncz airfoil used for the canard of the COZY aircraft. Please list all the other certified/homebuilts that use this airfoil for flight over 345 mph before making such cavalier statements about safety issues......


Jim, others, See:


for more information on this phenomena. I thank Richard Riley for pointing me to this reference.


With respect to other questions, sweep in the canard, or a different airfoil MIGHT ameliorate the situation to some extent, but the factors that make this a good airfoil for a canard aircraft make it bad for "Mach stall", and the factors that give other airfoils a better "Mach stall" number make them poor performers as a canard airfoil in our type aircraft. Everything's a tradeoff. New airfoil or sweep - it's not even close to being a COZY anymore, and you better be an aerodynamicist.....


With respect to Jim Price's L.E. - the fact that he went to 35.5K ft. with a 135 HP engine makes it even more amazing :-). Realize, of course, that he was not at gross weight, and that service ceilings are determined (at least in certificated aircraft) at maximum gross. I don't know if the AeroCad/COZY service ceilings were determined at gross weight, but that's how they SHOULD be quoted.


To Evan's question, a normally aspirated COZY MKIV with a fixed pitch prop won't get close to 200 mph, much less 346 mph TAS at 25K ft. Some are proposing substantial power increases by turbocharging and/or substantially larger engines, and using CS props, and then the answer seems to be that 300 mph at that altitude should be achievable.


Hence the warning.

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Hopefully adding to the discussion,


For a discussion of IAS vs CAS vs flutter etc, do a google "groups" search using keywords "canard performance flutter equivalent". Read the post by highflyer dated Jan 4, 1999. Among other things, note the issue of localized transonic shockwaves, as Marc has pointed out.


Another unhappy point for the future test pilots. During flight testing for flutter at 320 knots, a catastrophic failure may result in a non-survivable, or at least incapacitating g forces. Wearing a chute may not be of value if one isn't awake to use it. I think the first BD-10 loss was at 380 knots when the tail let go and the pilot's neck snapped.


BTW, I'm pretty sure Jim's engine was nowhere near stock (ie it wasn't a 135hp engine).

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From: highflyer (highflyer@alt.net)


View: Complete Thread (8 articles)

Original Format

Newsgroups: rec.aviation.homebuilt

Date: 1999/01/04


Stephen J Beaver wrote:


> In article <769j78$cos@dfw-ixnews3.ix.netcom.com>,

> Carl R. Denk <cdenk@ix.netcom.com> wrote:

> >In article <19981222221241.18678.00000316@ng107.aol.com>,

> > hornetball@aol.com (HornetBall) wrote:

> >

> > The airframe only knows IAS, including flutter.


> Sorry but I believe that is simply not the case. The fact is that as alitude

> increases and the air becomes less dense, it exerts less of a damping effect

> on the control surfaces (they can move more freely in the thinner air)

> Althought the IAS decrease, the propensity for flutter increases. The onset

> of control flutter, therefore, is proportional to TAS, NOT IAS.


> Steve Beaver


This is a very old argument. The question is usually stated

"Does flutter depend on Indicated Air Speed or TRUE Air Speed."


Unfortunately, the CORRECT answer is NEITHER. Flutter is triggered

by many things and does NOT have a simple "This causes flutter."

statement possible. That being said, I can try to give SOME insight

into the causes and cures for airframe flutter.


Generally speaking flutter results from the interaction of vibration

modes in at least two degrees of freedom. The fundamental frequencies

or selected harmonics of the fundamental frequencies have to be close

enough to allow a positive feedback beteen the two vibrating systems.

This is usually dependant on a certain air loading that is a function

of both speed and density. This density corrected airspeed is called,

by those engineers who work with this stuff, EQUIVALENT Airspeed.


Typically, in the real world, EQUIVALENT airspeed is slightly less than

CALIBRATED airspeed, virtually all of the time. The exception is

when some part of the airframe approaches MACH 1. Then the pressure

distributions change and the flutter excitement speeds change also.

Unfortunately, you do NOT have to be traveling at the speed of sound

to experience local airspeeds near MACH 1. For example, with the

very common 23015 airfoil, you can experience transonic shockwaves

on the upper surface of the airfoil at speed as low as MACH 0.6.

At high altitudes that can be well under 300 mph indicated airspeed.


Since Equivalent airspeed is almost always a bit less than Calibrated

airspeed, and since Calibrated airspeed is usually very close to the

Indicated airspeed at the speeds where flutter usually becomes a

problem, it is generally safe to use the IAS as a limit to avoid

flutter as long as the IAS is less than about 250 mph and your

density altitudes are 20000 feet or less.


That would mean, that for most homebuilts with a properly installed

airspeed indicator, IAS can be safely used to avoid the flutter

margin. However, be aware that the flutter margin may be much less

that you think. Certainly, exceeding Vne puts you into a "flutter

test pilot" mode. In my experience flight testing new airplanes

and homebuilts, airborne flutter testing is one of the scariest

things you can do as a test pilot. Usually, decreasing the airspeed

will cause the flutter to decrease or stop, if the airspeed is decreased

quickly enough.


Because of this characteristic, flutter testing is usually done in a

climb and it is excited with a sharp blow to the suspect control.

Usually this works. Sometimes it doesn't, and a flutter mode, once

triggered, will NOT stop when the aircraft slows down. Sometimes

also a flutter mode, once triggered, will increase to structural

failure so quickly that slowing down is not a viable option. I ALWAYS

wear a parachute when doing flutter investigation in the air.


Flutter modes are changed or eliminated by altering the characteristic

vibration frequencies of the contributing structural or control

components. This is usually done by stiffening the structure, raising

the natural frequency or by decoupling the positive feedback

mechanisms by things like 100% balancing control surfaces.


The whole subject of aeroelasticity if extremely important, and

becomes more so rapidly as speeds grow beyond the 100 mph marker.

Unfortunately, it is not a subject that I can treat adequately in

a news group posting.



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Wow! Am I relieved!


I have always hated the "Mach Stall", lost the index finger on my left hand during the first one (picking my nose). You would have thought I would have learned my lesson, but during a trial run of my J3 Cub (modified) "Mach stall" claimed my left earlobe (ruined my soccer career), I should have never mounted that 450 BHP engine and modified the drag component of the wing!


By now I could have learned my lesson, and I might have, but now I am building a Cozy with a 200 HP engine and I was worried that during re-entry I might miscalculate the angle and bounce off the stratosphere and into oblivion!




I feel better now.

Back to building... #618 Cozy MK IV


My Cozy web pages, courtesy: Rick Maddy... :cool: WN9G :rolleyes:

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