How strong is the M20 wing?

[mooniac15u]

Isn't maneuvering speed the point where maximum deflection of the flight controls will not lead to over stress of the airframe?

Clarence

The published maneuvering speed is supposed to ensure that the plane will stall and unload the wing before you over stress the airframe. The key aspect is that it is full deflection of ONE control ONE time. The FAA published some clarification of that not too long ago.

[Shadrach]

Is flying a C in the bottom of the yellow while in mildly bumpy air hard on the airplane like driving an old IROC Z just over the speed limit on a bumpy road? I assume it is.

Negative... The IROC Z is more prone to cowl shake, body flex, panel rattle and it has a weak rear end that can explode like a bomb.  The C model is of much higher quality construction and has the best rear end in the business. The IROC does have an ever so slightly larger rear seat...

[rbridges]

Here's an M20K that broke up due to flutter. It sounds like the folks at Mooney predicted the onset of flutter to occur at 241 knots.

http://aviation-safety.net/wikibase/wiki.php?id=36780

 

someone told me that mooney has never had a structural in flight failure that led to a crash.  I guess they were wrong.  Thanks for the link.

[Shadrach]

On 9/2/2015 at 11:48 PM, carusoam said:

I'm not a believer in the pull hard theory, CNOE...

 

 

I agree whole heartedly, but I think it's mostly semantics.  I should hope that no one here thinks that literally pulling hard on the yoke is the way to scrub speed when approaching Vne.  I think the term "hard" is used to describe the G loading that occurs during the pull.  Let's say the plane is traveling at Vno or above at 1G, doubling or even tripling the G load is not going to require that much in terms of actual yoke force and it certainly isn't full up elevator. At indicated airspeeds approaching 200MPH, an immediate and sustained full up deflection would likely bend or break the airplane, the good news is that the pilot would not likely be awake to witness it. 

 

Think about the fact that in cruise a 1200FPM climb can be initiated with one finger. 

Edited August 17, 2021 by Shadrach

[cnoe]

Guys... I truly appreciate all the discussion and input and am in no way offended by such.

 

I imagine that I should qualify the term "pull hard" as "pull firmly and smoothly to in excess of 4 G's". The problem is that I don't know by instinct what 4+ G's feels like. From all accounts the majority of aerobatic students who are instructed to pull 4+ G's only  manage to reach about 2.5 before backing off. And If I'm looking at dirt when I start the pull then it might as well be 6+.

 

My take from all this is that even 10 G's isn't going to shear a wing but "flutter" can ruin your day (and that's caused by gross excessive speed).

 

What's tough is that the advice runs from "give 'er a slight push, to let the trim do it's thing, to give 'er a firm pull". Nobody can yet confirm from experience which is best in a mid-body M20 accelerating through 225 mia in a steep dive. But what I have learned is that horizontal stabilizers will and have departed the aircraft at 275 mia (240 kts).

 

The best preparation may be to get a good aerobatics instructor and practice some 45-60 degree banked descents and determine first-hand what it takes to arrest the acceleration (after rolling wings level of course). Having a G-meter on board to see what 4 G's feels like might be beneficial as well. This is exactly what was being proposed on the Beech forum and it sounds like good advice.

 

Thoughts???

[RobertGary1]

In my experience the average non-aerobatic pilot says "holy cow" beyond about +1.5G and -0.5G. If you weigh 180 pounds you're 360 at 2G's. That's a shock for the uninitiated.

[Shadrach]

3Gs feels like a lot to someone unaccustomed to it, which is likely why most folks don't pull as hard as they think they are. I don't worry much about the mooney wing. I have to say that when one looks at the tail fasteners and the bolts that hold the engine mount to the fire wall, overbuilt is not the first thing that comes to mind. The wing may very well remain intact the whole way to the crash site.

The moral of the story here is don't do anything abruptly. Be smooth and and deliberate. Nothing needs to be a hammer blow.

I can conceive of very few situations were the advisable thing to do would be to yank full aft elevator with all of your might. If you've screwed the pooch that badly it will likely only influence what angle the aircraft hits and whether it's flying or stalled. You're still likely to end up a smoking hole in the ground either way.

[MyNameIsNobody]

In the scenario where a pilot died they overstressed the airframe when they were still thousands of feet in the air. Not a high time pilot. Spacial disorientation, ice or a combination. He was not in control and did the human thing, panicked, pulled hard enough to break the plane apart.

I like the reduce power, roll level and recover as necessary based on altitude. He was done before he was done. I am not IFR, so "don't get into this" is my prevention, not "how hard to pull" or "trim" when I am in full panic mode.

For the record I hope to never be in full panic mode in my plane. This and mid-air collisions are likely to get you to full PM.

Thanks for clarification on the callback vs. there on his altitude and the Camaro backseat. I hope no one reading this ever has to try out the required recovery at 240 knots...

[Houman]

The 240 knots speed is Indicated speed, right?, not ground Speed, I have been as fast as 237 knots in ground speed in my Rocket.

 

It was shilling reading the full report, as a Rocket Owner, I know how damn fast this plane goes, and that is why I always start my power reduction several minutes before starting my descent, if not it is really easy to be in the yellow arc and even tugged against the VNE without much effort.

 

Most of the time in cruise I'm very close to the yellow arc as you can see on the picture below, this is going with a 70% engine power setting, if I dial it down to 65%, then I'm about 5 knots or a bit more from the yellow arc...

 

I think getting more transition training time and maybe an IR rating would have potentially helped save this pilot's life, something I'm doing now..

[Shadrach]

Ground speed means nothing in terms of flutter!   Keep working that Rocket Houman, my normally aspirated F has done better than 240kts across the ground in level flight.

 

While TAS is the how the onset of flutter is measured, Vne is a KIAS number.  

 

[cnoe]

240 KTAS. True airspeed is what matters as far as flutter is concerned. This is why you high altitude turbo drivers need to be most concerned about it.

 

 

Ground speed means nothing in terms of flutter!   Keep working that 231 Houman, my normally aspirated F has done better than 240kts across the ground in level flight.

 

While TAS is the how the onset of flutter is measured, Vne is a KIAS number.  

 

M20J Vspeeds per TCDS.png

 

The TAS part is what I have a hard time grasping. Why is "true" the concern? Isn't "indicated" what the control surfaces are experiencing?

 

I have no formal aerodynamic study in my background so it just seems like how many air molecules are beating against the aluminum that would prompt the surfaces to begin oscillation.

 

Sorry for this follow-up but my curiosity has been piqued.

[Shadrach]

240 KTAS. True airspeed is what matters as far as flutter is concerned. This is why you high altitude turbo drivers need to be most concerned about it.

 

You are mistaken sir. None of us know where the M20 Airframe flutters, only the factory knows.  It's rumored to have been dive tested in excess of 300MIAS.

 

You are correct that flutter occurs at a certain TAS range.  However, that number is not Vne, it's likely not even close to Vne.  Even for Turbo drivers Vne is an IAS number. Keep it under redline and there should be no issue all other things being equal.

 

 

[Houman]

Ground speed means nothing in terms of flutter!   Keep working that 231 Houman, my normally aspirated F has done better than 240kts across the ground in level flight.

 

While TAS is the how the onset of flutter is measured, Vne is a KIAS number.  

 

M20J Vspeeds per TCDS.png

 

Yep, Doing my IFR to be able to go higher and really try this thing, but still within limits...

 

Vne is in Indicated since it is on the dial and true is relative to your altitude and tempeature, so obviously KTAS with a KIAS of 195 KIAS at 5000 feet is very very different from one at FL240.... 

[Shadrach]

The TAS part is what I have a hard time grasping. Why is "true" the concern? Isn't "indicated" what the control surfaces are experiencing?

 

I have no formal aerodynamic study in my background so it just seems like how many air molecules are beating against the aluminum that would prompt the surfaces to begin oscillation.

 

Sorry for this follow-up but my curiosity has been piqued.

 

It has to do with harmonics.  Flutter is caused caused by the speed of the air molecules traveling over the airfoil, not the volume of air.  While the air at 22K is much thinner and for sure there are fewer molecule traveling over the airfoil, the speed of those molecule is the important factor, which is why flutter occurs in a range of true air speeds.

[Shadrach]

Yep, Doing my IFR to be able to go higher and really try this thing, but still within limits...

 

Vne is in Indicated since it is on the dial and true is relative to your altitude and tempeature, so obviously KTAS with a TIAS of 195 KIAS at 5000 feet is very very different from one at FL240.... 

 

It is, but the factory has determined that 195KIAS or less at any altitude represents an adequate under whatever TAS causes flutter.  I am sure there are other considerations as well.  

[mooniac15u]

You are mistaken sir. None of us know where the M20 Airframe flutters, only the factory knows. It's rumored to have been dive tested in excess of 300MIAS.

M20K Vspeeds per TCDS.png

Did you read the full narrative on the M20K break up? Mooney said they predicted the onset of flutter at ~240 knots for that airframe. I suspect that 300 mph dive is just an OWT.

[cnoe]

It has to do with harmonics.  Flutter is caused caused by the speed of the air molecules traveling over the airfoil, not the volume of air.  While the air at 22K is much thinner and for sure there are fewer molecule traveling over the airfoil, the speed of those molecule is the important factor, which is why flutter occurs in a range of true air speeds.

 

Thanks. Your explanation is simple enough for my feeble mind to grasp. 

[PTK]

The TAS part is what I have a hard time grasping. Why is "true" the concern? Isn't "indicated" what the control surfaces are experiencing?

 

I have no formal aerodynamic study in my background so it just seems like how many air molecules are beating against the aluminum that would prompt the surfaces to begin oscillation.

 

Sorry for this follow-up but my curiosity has been piqued.

 

Yes, you're absolutely correct...when speaking about the ability of the airframe to withstand structural stress.

 

There are two distinct phenomena. One is structural stress of the airframe and the other is flutter.

 

If you look at Vne as a limiting speed for the airframe to withstand aerodynamic loads then you need to look at IAS because it is sheer number of air molecules hitting it that matter. Therefore IAS dependent. The pitot tube measures airspeed through sensing air pressure. As altitude increases air density decreases. To sustain lift speed has to increase. The equation that relates these is q=1/2ρv^2. q is dynamic pressure the pitot tube sees, ρ ιs air density and v is speed.

The pitot tube measures this pressure as an IAS and constant pressure means a constant IAS. A given AOA will predictably produce a certain amount of lift.

 

If you look at Vne as a limiting speed for flutter then need to look at TAS. The number of air molecules striking the airframe don't play a significant role. It's the harmonics and the oscillations. Flutter is invited more easily up high where thin air doesn't dampen the oscillations well as thicker air down low would.

 

 In order to not exceed a given TAS the Vne IAS has to decrease as altitude increases.

 

It is possible to be below Vne and, although safe from structural stresses, can still experience flutter. It depends on TAS. This is the danger of being in a high speed descent from high altitude and thin air. The ASI lies. It underreports and you think you're fine being below Vne. But excessive TAS will tear the airplane apart.

When I get around to it I'm going to plot on excel altitude as a function of IAS and come up with a constant TAS plot that shows this. 

 

[Houman]

There are two distinct phenomena. One is structural stress of the airframe and the other is flutter. The former depends on IAS, the latter on TAS.

 

The pitot tube measures airspeed through sensing air pressure. As altitude increases air density decreases. To sustain lift speed has to increase. The equation that relates these is q=1/2ρv^2. q is dynamic pressure the pitot tube sees, ρ ιs air density and v is speed.

The pitot tube measures this pressure as an IAS and constant pressure means a constant IAS. A given AOA will predictably produce a certain amount of lift.

Flutter is different.  It is based on TAS. In order to not exceed a given TAS the Vne IAS has to decrease as altitude increases.

 

It is possible to be below Vne and still experience flutter. It depends on TAS. This is the danger of being in a high speed descent from high altitude and thin air. The ASI lies. It underreports and you think you're fine being below Vne. But excessive TAS will tear the airplane apart.

When I get around to it I'm going to plot on excel altitude as a function of IAS and come up with a constant TAS plot that shows this. 

 

 

I would be very interested in that. I usually calculate my TAS during cruise using my E6B, it's part of my routine. What I usually don't do is check my TAS during descent.

 

Am I wrong to think that as long as I'm in the green Arc which is under 175 KIAS for me, in a normal descent ( I usually aim for 500 fpm, max 1000 fpm ), I should be well below any TAS excess danger levels ? I will try to calculate my TAS during decent next time and see !!!

 

I always watch my speed and engine temps during descent, to make sure I fly within the numbers, if I really have to, I can always bring out the speed brakes to slow down while keeping the engine warm... but usually at 500 fpm, it is very smooth and never had problems with this...

 

I appreciate all the knowledge here, reading this accident report hit close to home and is a good learning experience of what not to do...

[aviatoreb]

There are two things going on here - one is an engineering term and one is a legal term.  In engineering terms, flutter is a function of true air speed, TAS.  Every wing has a critical TAS beyond which it will flutter.  And for a constant IAS, TAS increases with altitude.  So this seems as if we loose our safety margin as we go higher.  Yes we do if we are test pilots.  But this is where the legal comes in.  A test pilot already did the work for us in our certified airplanes.  Vne is supposed to be free of flutter (and a safety margin too) in the entire flight envelope.  The flight envelope of my specific airplane is to FL24.  So at FL24, Vne=196IAS is supposed to be free of flutter - yes, 196IAS->283TAS on a standard day at FL24 but as a dumb pilot I don't need to know the TAS conversion - as a dumb pilot all I need to know regarding flutter is that my plane has been certified as free from flutter in its entire flight envelope, not just at sea level, in terms of the indicated airspeed, marked as Vne=196IAS.

 

This is my understanding of the legaleeze behind certification as it pertains to IAS and how to interpet my Vne marked as an indicated airspeed.

 

If you are a test pilot, aka an experimental airplane builder, then this is a really good read on the topic: http://www.vansaircraft.com/pdf/hp_limts.pdf

[aviatoreb]

I read at some point that there was an M20C that broke up in flight, I think on its way to the bahamas, and severe corrosion was named as the culprit together with strong but not crazy turbulence.  I was unable to refind just now the report.

[PTK]

The beauty of certificated airplanes is that if we keep within the envelope we're fine. We can't forget the envelope includes density altitude!

[Shadrach]

Whatever happened in that accident and whatever the Mooney rep said to that investigator is one thing. I can't see the report right now, but did it specify whether it he was suggesting TAS, IAS or CAS? Whatever was quoted in the report is not regulatory. For all we know it's a best guess based on a number of factors.

Those of you suggesting that Vne should reflect TAS are absolutely mistaken. Look at your TCDS. The speed is given in IAS if CAS or TAS were to be used, it would be listed as such.

Yes flutter is dependent on TAS, but that does not mean that an IAS that will give adequate margins at all altitudes cannot be assigned. The Factory has done just that with Vne.

By all means do the math, create the spreadsheets but do understand it's a mental exercise and nothing more. Vne is an indicated airspeed. Turbo guys need to mind Vne just like the rest of us.

[Shadrach]

The beauty of certificated airplanes is that if we keep within the envelope we're fine. We can't forget the envelope includes density altitude!

DA is not relevant to Vne for the purposes of this discussion. One can safely descend from 25000 ft to sea level within the speed envelope with nothing more than an ASI. DA need not be known.

[mooniac15u]

Those of you suggesting that Vne should reflect TAS are absolutely mistaken. Look at your TCDS. The speed is given in IAS if CAS or TAS were to be used, it would be listed as such.

 

 

Who suggested that Vne reflected TAS?