Mach Tuck

[Dust]

Originally Posted by MarbleTurtle

Then I caught up on a few emails... read the critical mach discussion and realized, I can't use full power at 25k feet anyway because critical mach (calculated for the canard) appears to be 0.55 which is roughly 200kts indicated at 25k. Uh oh... Dust is in trouble!

I hear is is between .55 and .75 - unknown

Originally Posted by waiter

During Phase I flight testing, one of the objectives is to test the aircraft to establish its Never Exceed speed (Vne). This allows a safety margin that the aircraft has been provin. I normally perform the Vne Establishment flight in conjunction with the Flutter test.

This test should be performed while wearing a parachute. The test will require a dive to build up speed. I can usually get several data points during each dive sequence, especially during the first two or three dives.

I start out at 10,000 ft and take the plane to say 150 kts, Give it a "pulsed" i.e. bump the controls in all three axis in an attempt to induce flutter. IF no flutter occurs, add 5 kts and do it again. Continue this process until you have reached 10% over your proposed Vne. i.e If I plan on set my Vne at 230kts, Then I must test the aircraft at 230 + 23 = 253 kts.

Flutter will start as a "buzz" in the controls. The flutter will get worst, very fast, so it must be stopped. I load up the controls, i.e. gentle pull back on the stick with a gentle left turn and a light pressure on both rudders. Sometimes you can see a light flutter before you can feel it, so look at the surfaces as your performing these maneuvers.

Stop all testing and fix the flutter.

1.1 Vne will require what seems like a dive STRAIGHT DOWN. I establish a lower deck (5,000 AGL) Recover must begin at or above this altitude. (believe me, you'll need it) .

ESTABLISH Vne - Take the highest speed you achieved in the flutter test, Multiply this by 0.9. This is your new Vne.

Flutter testing and Vne Establishment must be done during Phase I while wearing a parachute, NOT while doing that first "Low Approach" to show off the new airplane.

My LongEZ is placarded with a Vne of 235 kts. It has been tested at 2G's at 260kts.

Waiter

http://canardaviationforum.dmt.net/s...+tested+waiter

post number 5

Originally Posted by Richard Riley

Quote:
Originally Posted by John Slade
Richard, where does Mach Tuck come into play? (or is that just a name for what you do on the way down )


He he.

Mach Tuck (or in our case, shock stalling the canard) will happen somewhere between .55 and .75 mach. No one knows for sure.

And mach speed (the speed of sound) varies greatly with temperature. The colder it is, the slow the speed of sound. So the higher you go, the slower the speed of sound. At sea level (on a standard day) mach is 761 mph. At 30,000 feet, it's almost 100 mph slower - 678.

So lets assume that mach tuck will hit at .55 - that's pessimistic, but let's go with it for the moment. And you're unpressurized, so you're limiting your altitude to 20,000. Your limit is 335 knots TRUE.

Going further (not your intent, I know) the problem isn't speed straight and level, it's in a decent. Especially if you are running a jet engine, like a couple of canards are now, so you don't have a prop. Pull back on the engine, it's still producing a little thrust. Push the nose down, you don't have a windmilling prop slowing you down. So if you were at 220 knots at 20k feet, and you don't watch your speed on the way down, you may be through .6 or .65 mach before you know it.

Then you do the maneuver you so eloquently described.

http://canardaviationforum.dmt.net/s...8&page=1&pp=15

Originally Posted by riley

Quote:
Originally Posted by phoenix221

So the entire wing/canard of the Berkut is made of carbon fiber only?


I'm not going to get into a discussion on flutter analysis, but I'll give you the basic facts.

The early Berkuts were foam core wings, winglets, canard and elevators. The canard and wing spar caps were carbon, the main wing skin (including aileron skins) were carbon. The canard, elevators and winglet skins were E-glass, the same layup schedule as the LEZ

Later Berkuts were molded canards, wings, winglets, and ailerons. The ailerons, canard and mainwing skins, canard and mainwing sparcaps were carbon. Elevators and winglets were glass. All 2 skin sandwitch over a 5 lb PVC and SAN cores, with ribs made of Eglass over 8 lb urethane core (urethane only for the low cost)

This is the last time I will answer this question on this forum.

thread
http://canardaviationforum.dmt.net/showthread.php?t=633

berkut construction

This is what i know on mach tuck

[mplafleur]

A number for critical mach was calculated recently by Todd Parker on the mail list. Here's a snippet.

This idea that the Vne of the Cozy falls at or near 190 kts, due to the critical mach number being reached, has intrigued me. So, I did a little research last night to see if it could be true. I dug out my Abbot and von Doenhoff and read pages 256-261 again, which have various empirical methods of determining critical mach for an airfoil, the most conservative being the Karman-Tsien method. I then ran the Roncz R1145MS (the canard airfoil) in a 2-d modeler, X-foil, to get the Cp (pressure coefficient) values. According to the Karman-Tsien method the critical mach is determined by the maximum Cp value for the wing. Assuming a speed of 250 kts, the coefficient of lift for the canard is about .25 and should be flying at about -2 degrees AoA.

The maximum Cp for the airfoil at this AoA is about -1.2 and occurs on the bottom surface. From the Karman-Tsien method this would indicate a critical mach of .55, which is well above the .35 mach of the assumed airspeed. Interestingly enough, the bottom of the wing is the first surface to reach critical mach, where the norm would be for the top surfaces to reach it first. At higher altitudes the AoA increases and the critical mach actually increases slightly over reasonable lift values.

Based on this analysis it would appear the Vne of the Cozy has nothing to do with the critical mach number of the canard. The wing would be less susceptible to this.

But what does this mean? More to come...

[mplafleur]

A response by Marc Z:

At sea level standard conditions, M=1 is equivalent to about 660
kts. Therefore, 250 kts IAS = 250 kts TAS, and M=.38.

However, if you now climb to 20,000 ft., 250 kts. IAS will be ~350 kts TAS,
and M=1 will have dropped to about 625 kts, giving M=.56.

So at 20k feet and 250 kts, you are critically close to mach tuck.

I hpoe you don't mind my quoting you Marc. I can remove if you so desire.

[Dust]

MMm, interesting computations, but, as Señior Riley is closer to the subject, i would tend to put more weight in his calcs and experience

[MSires]

For certified aircraft Mach tuck affects have to be considered starting at .42mach. There is an interesting article on it in the latest Flying magazine. It discusses why the Columbia 400 has a lower VNE than some less capable certified airplanes. They went over mach .42 at 25,000, and had to redo all their speeds....

Mark

[John Slade]

Waiter said: Flutter will start as a "buzz" in the controls. The flutter will get worst, very fast, so it must be stopped. I load up the controls, i.e. gentle pull back on the stick with a gentle left turn and a light pressure on both rudders. Sometimes you can see a light flutter before you can feel it, so look at the surfaces as your performing these maneuvers.

I read this once before but didn't notice the inference that Waiter actually encountered flutter, presumably at the 260 kts max IAS that he tested if not at lower speeds. Is this correct, Waiter?

You also made another interesting comment

Stop all testing and fix the flutter.

Am I right in assuming that the fix referred to here is rebalancing the control surfaces. Anything else?

Sorry - I know this thread is on mach tuck, but they're related subjects.

[Spodman]

From Cozy Newsletter #5, a more innocent time, with a GU canard:

In one letter we were asked what was the not-to-exceed speed, above which the Cozy would come apart? I said I didn't know and I hoped I never found out. From the last Canard Pusher I read that that has been determined for the Long EZ. Its somewhere over 400 kt. I think you have to go straight down to go that fast!

[Marc Zeitlin]

Originally Posted by Spodman

From Cozy Newsletter #5, a more innocent time, with a GU canard:

What you quoted was Nat's interpretation of the text in CP-39, from January, 1984. Nat's statement that "It's somewhere over 400 kts" is NOT what the CP said.

Let's go to the horse's mouth:

"A California VariEze, travelling through Arizona was destroyed in a weather related accident. The pilot and passenger were both fatally injured. The weather was reported at 500 foot ceiling, poor visibility in sleet and freezing rain.

The aircraft totally disintegrated in the air. Very little damage was due to the impact with the ground. We spent a lot of time looking for possible causes, and we carefully examined all of the pieces which were found. The wreckage was spread down wind for over two miles. The damage showed signs of extreme high speed flutter, rather than overload due to excessive g. This accident was probably caused by the pilot pushing on into bad weather, or possibly trying to climb over bad weather. He may have become disoriented or overcome by hypoxia, the aircraft probably ended up straight down at very high speed. Finally it reached a speed beyond anything intended for this design, when it literally experienced flutter over the entire airframe.

The important point to note is that there was no evidence of a massive 'g' overload, such as would be expected if the pilot tried to pull out of a high speed dive, was found. All of the evidence points to total catastrophic failure due to high frequency, divergent flutter. The damage could only have resulted from and extreme overspeed condition possibly in the region of 400 knots plus."

So, the number 400 kts is merely "possible". It could possibly have been 300 Kts, or 450 kts, or any other number greater than whatever speed V.E.'s have been tested to.

Plus, since the V.E. structure is substantially different (at least in the wings) than the COZY/L.E. structure, I'm not at all sure that I would use this guess as an indication of anything in particular.

[Spodman]

I'm not at all sure that I would use this guess as an indication of anything in particular

I certainly wasn't going to try it out

[Dust]

mmm looks like someone else is aimin to go high and fast, in this issue of CSA news letter marc warns that he knows someone that thinks they can go 210 Knots IAS at 25,000 with a turbo canard, and that may be the speed that mach tuck can occur.

Wish i thought i had a chance of going that fast, if i make it to 200 MPH IAS at 25000 I'll be happy, overjoyed. 210 Knots is 241 MPH, but if i can cruise at 230 MPH IAS at 25000, I'll hold it at that! heh heh heh

just a little math

200 mph IAS at 25000 is roughly 300 MPH TAS
210 knots IAS at 25000 is roughly 315 Knots TAS or 362 MPH TAS !