What do speed mods do to the yellow/ red arc / Vne?

[mooniac15u]

The point is this:

 

To the extent the reason for Vne is to avoid flutter we have to be talking TAS and not IAS.

 

As altitude increases flutter damping decreases, we go faster and the more the ASI lies. 

 

Trusting Vne as read on the ASI can make for a very bad day. It doesn't always mean that we're safe as long as we stay below red line. 

 

Bad things can happen way before red line on the ASI!

 

For flutter protection we need to know our TAS.

 

The beauty of certificated airplanes is that if we keep it inside its envelope we're fine as long as we're below redline.

 

Absolutely agree that you need to be aware of your TAS any time you are near Vne.

 

Ultimately we can thank the regulatory world for any confusion here.  Published airspeed limitations (including Vne) are IAS and are mandated by the FAA.  Actual structural limitations are mandated my mother nature.  You can try to ignore either one but only at your own peril.  The one is designed to protect you from the other but regulations can't actually help you fly the plane.

[mooniac15u]

Since we are discussing flutter in Mooney airframes this seems relevant:

 

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

 

Perhaps the most interesting part of this is:

 

"Engineering analysis and testing by the airframe manufacturer predicted the onset of flutter to occur at 241 knots."

 

This was an M20K airframe so presumably the flutter speed for any mid-body Mooney would be similar.

[mooniac15u]

Another interesting related item.  The first new Acclaim appears to boldly proclaim "242 KTAS" on its tail.

 

http://www.aopa.org/News-and-Video/All-News/2014/July/30/A-new-Mooney-delivers-its-first-new-Mooney?WT.mc_id=140801epilot&WT.mc_sect=osh

 

But the TCDS lists the the Vne as 195 Knots IAS. 

 

That's a pretty big gap any way you look at it.  I wonder what the predicted flutter speed is for the M20TN.

[jetdriven]

For the Aerostar it's over 700 MPH TAS. All calculated out of course.

[mcpilot]

With my former 66E. Most of the speed mods really didn't give me more speed. IMHO probably not worth the money spent

[Andy95W]

Another interesting related item.  The first new Acclaim appears to boldly proclaim "242 KTAS" on its tail.

 

http://www.aopa.org/News-and-Video/All-News/2014/July/30/A-new-Mooney-delivers-its-first-new-Mooney?WT.mc_id=140801epilot&WT.mc_sect=osh

 

But the TCDS lists the the Vne as 195 Knots IAS. 

 

That's a pretty big gap any way you look at it.  I wonder what the predicted flutter speed is for the M20TN.

Thats the difference between indicated airspeed and true. Indicated is what the airplane feels and true is how fast you get there.

[aviatoreb]

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.

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

One of these days I'm going to plot on excel of altitude as a function of IAS and come up with a constant TAS plot that shows this.

 

 

Well said Peter.  You are proposing to plot a level-curves (contour) plot for level TAS.

 

 

The point is this:

 

To the extent the reason for Vne is to avoid flutter we have to be talking TAS and not IAS.

 

As altitude increases flutter damping decreases, we go faster and the more the ASI lies. 

...

 

For flutter protection we need to know our TAS.

 

The beauty of certificated airplanes is that if we keep it inside its envelope we're fine as long as we're below redline.

 

 

Yes I agree - the beauty of certified is that the engineers and certification process determined our safe envelope for us. As pilots in theory we do not need to know any further than never exceed Vne - period.

 

As conscientious pilots, we should always ask why what and how - to educate ourselves as to what goes on inside our machines.  Flutter is a function of TAS (once the airplane is built, but it is also a function of the aspect ratio of a wing - how long and thin - and how strong the wing - how stuff, and so forth - so in design phase the flutter TAS can be increased by building either a shorter stubbier wing or a stiffer wing - although stiffer then could make a heavier wing...).

 

Repeating what others have said here...in my words...  :-)  ...  If my airplane is certified to 24,000 ft and I have a Vne of 196IAS then that converts to a TAS.  The certification process has to prove that my airframe is free of flutter, and with a sufficient dummy-proof margin, as that will be declared the certified envelope margin.   That was the design and certification process.  End of story from an operations stand point.  

 

If I were to be bent (double meaning intended) on exceeding flutter TAS I could do so in one of two ways.  I could climb past 24,000 - say to 30,000 and then the same 196IAS would no longer be positively separated from flutter TAS and my wing might become separated from my airplane.  Or I could go beyond 196IAS within the published altitude envelope of 0 to 24,000.  I read once but have not been able to find again, that the intent is that flutter airspeed should be 30% beyond this envelope TAS but actual flutter need not be demonstrated.

[aviatoreb]

By the way - as some of you know my day job is as a math professor and my specialty is dynamical systems which includes bifurcation theory as one of its areas.  I have done a good bit of research work in Hopf bifurcation including some in the flutter problem but not for airplanes rather for wind turbine blades - which are very long and thin so especially hard to design out flutter since weight there is also a premium.  

 

My other major hobby is bike riding - and Hopf shows up here too - as the shimmy a bike will make if you go to fast - it is a sudden onset of the steering suddenly shuddering.  It is very scary if you are going 50mph down a hill and suddenly your bike starts shaking uncontrollably.  Believe me - its scary - I have been there.  Actually I wrote a letter the editor on this topic in a bicycle magazine about a year ago- and I related the universality of the phenomenon back to airplanes.

 

http://velonews.competitor.com/2013/11/bikes-and-tech/technical-faq/technical-faq-bifurcation-and-high-speed-shimmy_309601

 

In airplanes you also see shimmy (officially called weave) in airplane landing gear - the nose gear is a lot like the fork wheel setup for a bicycle.  Folks in the airplane design industry have studied-the-heck out of this problem.

 

You also see weave (Hopf) in trains if you go to fast, and also in grocery story shopping carts front wheels. This last is the easiest and safest to observe.  If you are interested to try it: Go to the Piggly Wiggly near you - grab a shopping cart and start running!  Watch the front wheels start shuddering at a magic threshold speed.

 

You also see Hopf bifurcation appear in all sorts of other scenarios from electronics, to even population dynamics (with periodic limit cycles being the stable state) of populations of predators and prey.  And on and on - this universality is why I love math.

 

But back to our regularly scheduled program - as a pilot - all that is a must know about Vne is that it is designed to keep us safe from the bad phenom of flutter within our altitude certified envelope.  The designing engineer needs to know lots more.

 

The only airplane mod I ever heard of that changed the structural properties of the airframe is the Mooney Liquid Rocket conversion that is reported to have added strengthening gussets at the leading edges where wings and control surfaces meet the fuselage.  Is that true - can anyone confirm?  I have read reported cruise speeds as high as 263TAS on that airplane.  Since 196IAS at 24,000 on a standard day converts to 283TAS , that does not leave much margin.  So presumably the strengthening was to increase the Vne marking.

[DS1980]

I just want to know why the resistance to Vne being TAS. I stated a fact and got railed.

[sleepingsquirrel]

"the M20 airframe has been dived in excess of 330 mph and withstood static tests in excess of six G's without failure as well as withstanding the flutter test at over 200 mph." 

 

I find comfort in this statement. 

[aviatoreb]

I just want to know why the resistance to Vne being TAS. I stated a fact and got railed.

 

I am not sure what you mean by getting railed.  I think we have all been polite and conversational about the discussion.  So right or wrong I don't see if anyone has been railed.

 

I do believe that Vne is the number marked on your airspeed indicator and marked as the red line.  It is the IAS corresponding to the redline.  As many of as have said in our opinion it is designed with TAS issues in consideration but all of that is boiled down by the FAA certification process - a legal process with engineering considerations but nonetheless a legal process - to be packaged for presentation to pilots in the field as an Indicated Air Speed on your instrument.

[N201MKTurbo]

Here is an article that explains it all:

 

https://www.vansaircraft.com/pdf/hp_limts.pdf

[PTK]

I just want to know why the resistance to Vne being TAS. I stated a fact and got railed.

I don't think there's any resistance.

It's perfectly ok to be conservative and treat Vne as a TAS. As altitude increases IAS becomes gradually less than TAS.

So for a Vne of say 198KIAS, as in my airplane, if I were to treat that as KTAS my Vne would drop accordingly.

There is safety to being conservative.

[DS1980]

Here is an article that explains it all:

 

https://www.vansaircraft.com/pdf/hp_limts.pdf

See post #16. Beat ya!

[DS1980]

I don't think there's any resistance.

Posts # 14, 20, 25, 44, 47 and 52. All arguing that Vne is an IAS.

[DS1980]

 as well as withstanding the flutter test at over 200 mph." 

 

I find comfort in this statement. 

Not good news for anyone with a 201 or faster. 

[sleepingsquirrel]

Not good news for anyone with a 201 or faster. 

Well, they will just have to hire their own test pilots  and collect their own data points!

[N201MKTurbo]

See post #16. Beat ya!

 

Just goes to show that I'm too lazy to read all the posts.

 

So why are people still arguing?

[aviatoreb]

 

So why are people still arguing?

 

Cuz its the internet - and this is a forum.  :-O

[DS1980]

Nailed it aviatoreb

[mooniac15u]

Just goes to show that I'm too lazy to read all the posts.

 

So why are people still arguing?

Because that article is addressing why Vne will not protect you from flutter if you let your TAS get too high by mounting a bigger engine on an existing airframe. Flutter speed is TAS. We all agree on that. You might note that nowhere in the article does it actually say that Vne is TAS. Only that Vne will not always protect you if your TAS gets too high.

On the other hand the FAA and the airframe manufacturers both clearly publish Vne as IAS (see my previous posts for relevant links).

If you choose to ignore all of the evidence I can't change that. But please look at one of my more recent posts about the Acclaim and give it careful thought. If Vne is TAS then how does Mooney advocate flying an Acclaim at 242 KTAS when the Vne is published at 195 knots?

[mooniac15u]

Also on page 2 of the linked article take a look at the chart of Vne for the Pipistrel Sinus. It is clearly labeled KIAS. And on page 3 the the 3rd paragraph discussion of Part 23 certification of Vne as IAS.

[DS1980]

Because the FAA and manufacturers think Vne is an IAS. They are incorrect. Just like Lycoming is incorrect to suggest running their engines at 50 degrees ROP.
A Vne of 195 KIAS at 25,000 feet (service ceiling) is 292 KTAS.

Mooney says the normal cruise speed of the Type S is 230 KTAS.

[mooniac15u]

Because the FAA and manufacturers think Vne is an IAS. They are incorrect. Just like Lycoming is incorrect to suggest running their engines at 50 degrees ROP.

Vne is an FAA term used in certifying aircraft. They pretty much get to define it as it relates to certificated aircraft.

Once again I will restate, we all agree that flutter speed is TAS. Vne is defined by the FAA and it is IAS. Vne is there to protect us from flutter speed but it is an oversimplification so it is imperfect. It is not wise to approach Vne without being aware of your TAS.

[mooniac15u]

Vne is also there to protect us from overloading the airframe in the event of a sudden gust. That's why it's derived from the wing loading.