WW2-fighter and critical Mach speed

[GregP]

Critical Mach does NOT cause airplanes to break. All that happen at critical Mach number is you get local supersonic flow and shock waves form that can and do blanket control durfaces, making control impossible until things slow down a bit.

Flutter will cause breakage in any aircraft.

Dynamic pressure would be a tough sell to me, especially in a fighter plane designed for typical fighter strength. If it's out of control due to shock waves, there will be little in the way of pitch control moments ... that's why if dives relatively straight.

 

[drgondog]

Critical Mach does NOT cause airplanes to break. All that happen at critical Mach number is you get local supersonic flow and shock waves form that can and doo blanket control durfaces, making control impossible until things slow down a bit.

Flutter will cause breakage in any aircraft.

Dynamic pressure would be a tough sell to me, especially in a fighter plane designed for typical fighter strength. If it's out of control due to shock waves, there will be little in the way of pitch control moments ... that's why if dives relatively straight.

The abrupt vertical line out at ~ 505 mph for the P-51 is an example of 'bend/break the airplane threshold' on the V-n diagram and represents the dynamic pressure predicted limit at 1 G.

 

[GregP]

Not to argue with you, Bill, but the abrupt outer vertical line in my textbooks represents Vne, which is 90% of Vd, safe dive speed, beyond which the control surfaces and even flying surfaces will or may experience flutter.

The inner vertical line should be Vno. Operating above Vno and below Vne could cause structural failure if you encounter a 35-knot vertical gust, which is the bottom of the definition of extreme turbulence. This will add to (or subtract from) the flight load and could exceed design ± structure limits.

Then again, these are just basic aerodynamics texts. So they merely define and explain the terms and do not launch into excursions of structural statics theory or aerodynamics beyond what is needed for normal design or analysis. I'll take your word for the dynamic pressure, but I think Max Q is much more important for missiles than for WWII fighter aircraft. I could be wrong there.

I think the planes I am aware of will flutter destructively, at least in smooth air, before they get to Max Q. Maybe not. The Bell X-1A experienced control surface blanking and/or inertia coupling at Mach 2.44, and still didn't lose it's wings to dynamic pressure or sttructural overload even when it spun out of control supersonically, and Yeager survived it. Of course it was a VERY strong airplane.

Last time I flew RC, we had a little impromptu pylon race with unlimited aerobatic planes and I experienced control surface flutter going faster than I should have ... and lost an aileron. Can't miss that sound! I was lucky to get down and the servo had stripped gears to boot. I was lucky it was a plane with 2 aileron servos ... it was a Jerry's Big Boy. A GREAT aerobatic plane, but not a speedster. Mine was/is overpowered with an old MVVS 77 and a tuned pipe. Prior to that flight I always used the power for vertical maneuvers, not speed. Guess I have to go back to aerobatic flying with it .. after I fix the aileron. I think I'll add some counterbalance weight to it, too, since it DID flutter. If one aileron did, the other surfaces can't have been far behind.

 

[stona]

That's the Drag Divergence Mach Number.

Critical Mach Number isn't really even a limit, if considering the whole airframe. The airflow can reach supersonic at points other than the wing (over a canopy is one place, or the hump on a 747), which would be the Crit Mach Number for that airframe.

Or that's my understanding, anyway.

Nearly. It's the lowest Mach number at which the airflow reaches but does not exceed the speed of sound over some point of the air frame. I've just read the entire thread discussing critical Mach numbers and at no point, unless I missed it, did anyone define what it is

Cheers

Steve

 

[drgondog]

Nearly. It's the lowest Mach number at which the airflow reaches but does not exceed the speed of sound over some point of the air frame. I've just read the entire thread discussing critical Mach numbers and at no point, unless I missed it, did anyone define what it is

Cheers

Steve

Greg defined it well enough. Critical Mach is the Velocity over a surface (usually wing) at which point the shock wave forms and remains 'in state'.

Drag divergence is a definition which points to that velocity at which the Total Profile drag increases 'x' percent (usually 5-10%)

 

[drgondog]

Not to argue with you, Bill, but the abrupt outer vertical line in my textbooks represents Vne, which is 90% of Vd, safe dive speed, beyond which the control surfaces and even flying surfaces will or may experience flutter.

It has zero to do with flutter as primary cause of the Do Not Exceed Q Load - but flutter is influenced by velocity forces to the degree that it introduced both torsion and bending of an aerodynamic structure (elevator, aileron, wing, etc.)to the point where stability of the restoring force is inadequate to prevent the oscillation - which by the way can occur well below a Q limit load. The P-38D, prior to installation of the improved wing fuselage filet experienced significant flutter due to a Combination of Bending and Torsion loads while immersed in the turbulent votices in the wake behind the inner wing. The solution is to stiffen the offending control surface so that the natural frequency is experienced at a higher velocity than V-Ne, or change the mass distribution (like a control horn)

The inner vertical line should be Vno. Operating above Vno and below Vne could cause structural failure if you encounter a 35-knot vertical gust, which is the bottom of the definition of extreme turbulence. This will add to (or subtract from) the flight load and could exceed design ± structure limits.

Almost - Vne is the boundary which combines G (positive and negative) loads plus indicial Gust loads for the A to B (positive G), A to E (negative G)points and thence from B to C and E to D points on the V-n diagram. At the far right the vertical limit C to D is the maximum Q which is 1/2*Rho*V^^2.

All of those boundaries in the V-n diagram are LIMIT loads, not Ultimate. A 'rule' of thumb is that the Q load limit is 1.2 to 1.5 max LEVEL speed, but the Definition is Q which obviously varies with altitude and equally obviously is only reached in a dive for conventional (non jet) a/c.

Then again, these are just basic aerodynamics texts. So they merely define and explain the terms and do not launch into excursions of structural statics theory or aerodynamics beyond what is needed for normal design or analysis. I'll take your word for the dynamic pressure, but I think Max Q is much more important for missiles than for WWII fighter aircraft. I could be wrong there.

Greg, Dynamic Pressure Q is the force that tore up the Spit in the dive, and caused structural damage to the Mark IV Mustang at .84M for example, but the Max Q for the Diagram (LIMIT LOAD threshold) for the Mustang was for dynamic pressure resulting from combination of Velocity and density somewhere between .75 and .82

I think the planes I am aware of will flutter destructively, at least in smooth air, before they get to Max Q. Maybe not. The Bell X-1A experienced control surface blanking and/or inertia coupling at Mach 2.44, and still didn't lose it's wings to dynamic pressure or sttructural overload even when it spun out of control supersonically, and Yeager survived it. Of course it was a VERY strong airplane.

Last time I flew RC, we had a little impromptu pylon race with unlimited aerobatic planes and I experienced control surface flutter going faster than I should have ... and lost an aileron. Can't miss that sound! I was lucky to get down and the servo had stripped gears to boot. I was lucky it was a plane with 2 aileron servos ... it was a Jerry's Big Boy. A GREAT aerobatic plane, but not a speedster. Mine was/is overpowered with an old MVVS 77 and a tuned pipe. Prior to that flight I always used the power for vertical maneuvers, not speed. Guess I have to go back to aerobatic flying with it .. after I fix the aileron. I think I'll add some counterbalance weight to it, too, since it DID flutter. If one aileron did, the other surfaces can't have been far behind.

Solving flutter is ultimately beyond adding a control horn but the speed envelope you are operating in, it may work

 

[GregP]

If it has zero to do with flutter, then my aerodynamics texts must be wrong. Stranger thongs have happened. I acutally have two math books with errors in them.

 

[pbehn]

I thought flutter was a type of resonance, a wing/control surface can flutter without being intrinsically weak or badly designed, or am I wrong again.

 

[GregP]

No, not really. Flutter happens like the back end of a flag in the wind. To help raise flutter speed, you need the control surfaces to be weighted so the leading edge is about 15% heavier than the trailing edge. If you check the ailerons on any US WWII fighter (and Spitfires, Hurricanes, Sea Furies, and Bf 109s), you'll find the leading edges of the ailerons, elevators, and rudder to be filled with lead enough to make them front end heavy by about 15%. Same for the A6M6 Model 52 Zero, the Pilatus P-3, and the Fw 190 Flugwerk replica we have displayed and occasionally fly at the Museum. It is owned by Rudy Frasca out of Illinois.

Weighting the front end raises the flutter speed of the surface but dones not eliminate flutter. In the P-51, they also added a bob-weight on a bellcrank about where the end of the canopy is to reduce thebtendency to load-up in turns. Removing the bob weight is one of contributors to the death of Jimmy Leeward at Reno in 2011 (coupled with monkeying with the trim system and untilately losing a trim tab).

For my model, I'll add some counter weight to leading edge extensions and decline to race it again at high power. It is, after all, an aerobatic model, not a high-speed model. Might as well use it for aerobatics.

ALL surfaces will flutter at SOME speed.

 

[drgondog]

If it has zero to do with flutter, then my aerodynamics texts must be wrong. Stranger thongs have happened. I acutally have two math books with errors in them.

I took exception to your statement "Not to argue with you, Bill, but the abrupt outer vertical line in my textbooks represents Vne, which is 90% of Vd, safe dive speed, beyond which the control surfaces and even flying surfaces will or may experience flutter."

Flutter often occurs well below V-ne line and represents an instability of motion and interaction between the distortion of a structure and the aerodynamic forces causing such distortion.

As long as the motion can be defined by a set of linear differential equations with constant coefficients (and most can), then the stability derivatives can be determined.

It is an aeroelastic phenomenum Greg. Doesn't appear in a rigid body. Can you have Flutter well below V-ne? Yes. Beyond V-ne? Yes. Near V-ne? Yes Is V-ne a predictable threshold? No.

If your Aero text says different - it is incorrect. What is the name and author of your aero source and pages you are referring to? maybe I can help you understand it?

 

[drgondog]

double post

 

[GregP]

First, I wasn't baiting you, no need to take exception. I simply have seen different explanations.

Because I have a different view sometimes is not due to me disagreeing because you said something; I have a good respect for your knowledge. But the V-G diagrams on Wiki, NASA, or almost any aerodynamic website where I look does not say what you said, true though your statements may be.

Here is just one example:
View: https://www.youtube.com/watch?v=S5_P7VvOMu0

And I apprciate the offer to help. I'll probably take you up on that down the pike, via PM if that's OK with you. I am loathe to turn this into an aerodynamics forum, but would probably pursue it outside of the forum. What I'd really like is to come up with an easy-to-use aerodunamic spreadhseet for generally accurate calculations for these what-ifs and basic design typoe things. I already have several done and want to consolidate them into one sheet for quick reference.

Last time I offered to share some data with you I had the same problem I have now ... a 20Mb limit. So, if I / we come up with anything bigger, I say let's use smail mail and CD/DVDs like I do with my brother. It works but is a bit slower than PMs.

 

[drgondog]

First, I wasn't baiting you, no need to take exception. I simply have seen different explanations.

Because I have a different view sometimes is not due to me disagreeing because you said something; I have a good respect for your knowledge. But the V-G diagrams on Wiki, NASA, or almost any aerodynamic website where I look does not say what you said, true though your statements may be.


Here is just one example:
View: https://www.youtube.com/watch?v=S5_P7VvOMu0

And I apprciate the offer to help. I'll probably take you up on that down the pike, via PM if that's OK with you. I am loathe to turn this into an aerodynamics forum, but would probably pursue it outside of the forum. What I'd really like is to come up with an easy-to-use aerodunamic spreadhseet for generally accurate calculations for these what-ifs and basic design typoe things. I already have several done and want to consolidate them into one sheet for quick reference.

I am fine with that.

Last time I offered to share some data with you I had the same problem I have now ... a 20Mb limit. So, if I / we come up with anything bigger, I say let's use smail mail and CD/DVDs like I do with my brother. It works but is a bit slower than PMs.

Thanks - doing fine so far.

Summary - V-ne is ALL about the boundary for Limit G loads as translated into Yield (Limit Load) STRESS. the boundary on the left is pure structural due to Limit Load Stresses for maneuvers - positive and negative but define a boundary between Stall and No Stall. The boundaries above and below are for Limit Load STRESSES due to maximum permissible positive and negative G caused STRESSES for symmetric flight such asd Pull out or push over. The Boundary to the right is for the attainment of LIMIT STRESS (Yield) in some condition - usually Dive - in which the Dynamic (Q) PPRESSURE Load causes a Limit Stress on some component of the airframe in that State.

Flutter has nothing to do with LIMIT LOADS except by accident. It IS related to the natural frequencies initiated by free stream or turbulent flow, or even engine harmonics - if the velocities at that point cause the bending/torsional coupling in an elastic way that diverges from stability rather than damp out... or aircraft stability issues caused by oscillating aileron or elevator.

V-ne is IMPLIED on a velocity gauge because the pitot tube should in fact record the combination of density and velocity to yield Dynamic Pressure - which relates directly to the vertical line on the right of the V-n diagram. Note that the placard Do Not Exceed dive velocity should be to the Left (slower) than that vertical line. Other issues including compressibility, divergence and aileron reversal should occur at dynamic pressures to the Right of the Do not Exceed Velocity/Q

 

[pbehn]

Gents please bear with me I have an abstract question.
I have no experience of piloting a plane but flutter seems to me like a wheel wobble or "tank slapper" on a motorcycle. Normal opinion would be that a "tank slapper" is bad, very bad. However the bike I started racing (Suzuki X7) was always on the edge of a tank slapper when you got over about 85MPH in a curve. That was the down side the up side was its steering was lightning quick and control under braking and acceleration was best in class, so good it could embarrass an open class racer with anything but a great rider on board. To race it you had to stay low on the tank, ride smooth and never change the throttle in the curve just ride through the weaves on the edge of control.

Making a bike completely stable is actually very easy, the only drawback it will not go around any sort of corner at any sort of speed.

From this I have a question. Did WW2 aircraft design take aircraft to the limit of having flutter and instability because that is an an advantage in other areas of maneuverability? In the same way as A/C such as the Eurofighter Typhoon are unstable without a computer to keep them in the air.

 

[GregP]

Well Bill, I guess you can forget corresponding. With answers like above I don't care to parcipate and you are certainly convinced of your "superior" knowledge. I can show you pics of the same style V-G envelope with alternate explanations, but I suppose they are "bovine fecal matter" too, huh? I see some fecal matter, too, but it isn't quite the same.

One of the books I was using is The Illustrated Guide to Aerodynamics by Skip Smith and I don;t need to take a picture of it. Others include Hoerner Fluid Dynamics - Lift and Drag.

Here's an AOPA website that explains their viewpoint, wrtten by aeronautical engineers, probably more current than you.:

Operating Within the Envelope - Part 1 The Airplane's V-g Diagram - Flight Training

Here is another pdf from the US Navy.: http://www.faa.gov/regulations_policies/handbooks_manuals/aviation/media/00-80t-80.pdf

See page 337. The Navy has more aeronautial engineers and more flight time than you and your father put togther with all his friends and cohorts. They don't even mention too much lift or dynamic pressure at Vne, though it is a factor. I do see flutter and gust load in there, though, as I expected.

Go bully someone else and leave me alone.

I guess ignoring each other is, after all, the correct move. So, cheers and let's stay away from discussion with each other for good, if that's OK with you. Sorry I tried to interact again. It doesn't seem worth it. I am not going to calculate something every time we interact in here, but some of your musings are less than in line with the US Navy who has the best-flying airplanes in the fighter world up to and including today.

 

[GregP]

Hi Pbehn,

I rode Observed Trals for 20 years and raced both pavement and open desert in Arizona, so I can relate to your post.

The X7 probably had a beer-can grade steel frame and very probably needed a strong steering dampner and a stiff swingarm coupled with good swingarm bushings. I surmise the trail may have been a bit short. Our trials bikes were always twitchy on the trail between the sections due to short trail, but you could usually feel the max speed on the loose decomposed granite sand and rock easily. Get too fast and it was tank slappers.

Flutter is an aerodynamic phemomenon that manifests itself by the control surface doing a "tank slapper" up and down.

Here is a video on it:


View: https://www.youtube.com/watch?v=_oIYiFyyGC4

If it continues without amplification there is little danger unless you speed up. If it increases, it can be and usually is fatal. It is NOT something you want to experience though I have in RC models (lost 2 and landed / repaired 4) and once in a real plane. I got a Cessna 150 to flutter accidentally when I looked down at a lake while banking in a descending turn and got a bit fast becasue I wasn't paying attention. Never did that again even when a topless babe was floating about. Too dangerous. I throttled back and pulled up very slightly and the flutter desisted.

Go do a stall in a Piper Tomahawk and look at the tail through the stall. You probably won't fly one again.

I'd never do that again today, but it isn't all that difficult, even in a Bonanza if you lose focus. It cruises just short of redline, if you want to burn the gas. Allow it to descend slightly at cruise power and you are at redline ... and so are a new baby test pilot.

 

[drgondog]

One of the books I was using is The Illustrated Guide to Aerodynamics by Skip Smith and I don;t need to take a picture of it. Others include Hoerner Fluid Dynamics - Lift and Drag.

Page what in Hoerner's Fluid Dynamic Drag? I couldn't find any reference to the V-n diagram - and didn't expect to.

Here's an AOPA website that explains their viewpoint, wrtten by aeronautical engineers, probably more current than you.:

Interesting swipe. So, you did notice that the very nice Navy tutorial below the AOPA link on applied aerodynamics and aeronautical engineering Pre-dates my Degree in Aero (1968), my Masters in Aero (1972) and my last work experience in 1984? Does that make H.H. Hurt, Jr. less capable to produce this textbook in 1965 when it was published? As to the below AOPA discussion of V-n, do you suppose that it borders on advanced state of the art theory?

Operating Within the Envelope - Part 1 The Airplane's V-g Diagram - Flight Training

Good discussion until he poses Flutter as The primary illustrated condition that sets the Velocity/Q load threshold - see your USN tutorial below to help you understand the multitude of expected issues when exceeding V-ne


Here is another pdf from the US Navy.: http://www.faa.gov/regulations_policies/handbooks_manuals/aviation/media/00-80t-80.pdf

This is a far better resource, Greg. Note that beginning on page 360, the Navy Textbook goes deeply into the various issues of exceeding the dynamic pressure limits characterized by the 'do not exceed velocity' - they may even refresh your understanding of why altitude and density are important factors to contrast the 'velocity discussion'

See page 337. The Navy has more aeronautial engineers and more flight time than you and your father put togther with all his friends and cohorts. They don't even mention too much lift or dynamic pressure at Vne, though it is a factor. I do see flutter and gust load in there, though, as I expected.

Let me help you out and direct your attention to the section beginning on page 360. You will see a series of discussions ranging from aileron reversal to Divergence to Buffet to Flutter 'at high dynamic pressure including issues with compressibility - all of which, when understood in flight test, define the velocity-Q Load limit.

EDIT NOTE. IF FLUTTER encountered at or below a mission desired velocity limit, that limits the top speed/velocity to that below a point deemed necessary to accomplish a mission or operate within a desired envelope - the problem is addressed and FIXED so that the issue may or may not arise again at a velocity/Q threshold BEYOND V-ne. This is why Flutter is not THE major discussion within V-ne. You can easily fix flutter most of the time, but can't do much about compressibility, aero elastic torsion and aileron reversal, buffeting, stability and control divergence - without major redesign of wing and or wing/body interface.

I am not going to calculate something every time we interact in here, but some of your musings are less than in line with the US Navy who has the best-flying airplanes in the fighter world up to and including today.

Ah, no - they are not 'less than in line' Greg. They are Exactly In Line with the discussions I have had with you - not only for the fabric of the theory and construction of the V-n Diagram but also the discussion I advanced about the question I had regarding airframe stress approach of the A6M - at least from the US engineer perspective.

As to the swipe regarding the USN flight time versus my 'father and all his friends' - if you include USN vs USAF as the comparison you would be wrong - again. You may freely flail away at me Greg, but don't you think you are petty to include my father in the discussion?

 

[VALENGO]

I have read long before that the limit speed was given for the onset of supersonic speed on airscrew tips and not for the airplane itself. Not sure if someone´s already wrote this down.

 

[pbehn]

Hi Pbehn,
The X7 probably had a beer-can grade steel frame and very probably needed a strong steering dampner and a stiff swingarm coupled with good swingarm bushings. I surmise the trail may have been a bit short.



If it increases, it can be and usually is fatal. It is NOT something you want to experience though I have in RC models (lost 2 and landed / repaired 4) and once in a real plane. I got a Cessna 150 to flutter accidentally when I looked down at a lake while banking in a descending turn and got a bit fast becasue I wasn't paying attention. Never did that again even when a topless babe was floating about. Too dangerous. I throttled back and pulled up very slightly and the flutter desisted.

Go do a stall in a Piper Tomahawk and look at the tail through the stall. You probably won't fly one again.

The X7 had a tube spine frame with the engine as a stressed member, racing allowed addition of a steering damper, they had such a hard time that I replaced them every month. The frame would flex around the steering head which you could feel and in some cases see as a spectator. That is not the point I was making however. As a design it was short wheelbase and the rake/trail gave light fast steering and high cornering speed at the expense of stability. To race it I had it tuned which made an extra 10/15MPH and fitted wider racing tyres which were great for grip but decreased stability.

My question was about basic design and the philosophy behind it. Do some aileron/wing types give high performance or better feel and control but a greater risk of flutter?

 

[GregP]

There are several aileron typesm each of which has positive and negative things about them. Most, at least on warbirds, are chosen for perfoemance in the upper end of the flight envelope, though many were optimized for best perrormance in the nid-range.

For instance, the Mitsubishi Zero was a very good roller at low to medium airspeeds, but lost roll authority above 300 mph, where they fought moat of the last 2 years of the war. Likewise the Bf 109 was superb at 180 - 280 mph. By the time it hit 320 mph, the controls were less than woinderful. Lack of rudder trim didn't help.

If you read up on some aerodynamic design topics, you'll find ailerons can be almost amything the designer wants them to be. Aerobatic planes like teh Zivko Edge 540 have WONDERFUL ailerons ... but they would be almost useless at 800+ knots. We went into WWII flying rag-wing biplanes and came out flying jets, some of which still had fabric-covered control surfaces.

I'd Google the term "pb/2v" coupled with "aircraft roll" and look at that.