Vne adjusted for altitude?

[MIm20c]

I came across an article for Vans aircraft that said the IAS limitations could not be used at higher altitudes because the Vne is always decreasing as the air get thinner.  I’m curious about this because I have been using middle of the yellow in smooth air when coming down. Should I be concerned with a let down from higher altitudes 16-18k?  

Vans Vne

 

[PTK]

34 minutes ago, bluehighwayflyer said:

That article is only pointing out that by flying higher via turbo charging or installing a more powerful engine than an airplane was designed  for you risk flutter when flying at high altitude even when operating under Vne. This is because flutter is a function of true airspeed, not indicated airspeed.  Yet Vne, which is what we use to protect ourselves from flutter, is obviously a function of indicated airspeed, so there is an incongruity there.

In certified airplanes we don’t have to worry about these things because the certification standards protect us, but I do think that it is an important distinction to understand.  I wonder sometimes what Rocket Enginnering had to demonstrate to the FAA vis-a-vis their flutter margins when they certified the Rocket and even more so the Liquid Rocket, and if any modifications were required. 

Jim

The Rocket conversion does not change the 196 KIAS Vne. There was an inflight breakup of a Rocket Mooney due to exceeding Vne. See N231BY.

[David Lloyd]

Many EFIS displays will display VNE as either a true or indicated airspeed.  Have my Dynon Skyview set for true.

[BDPetersen]

I’m pretty sure that flutter is a function of true airspeed ( as is RPM of a runaway propellor). “ Go down, slow down” was the mantra we were taught for either event. In other words, reduce TAS. At least that’s the way it was ‘ splained in college aerodynamics and flight engineer classes. 

[David Lloyd]

I think some certified sailplanes also have true airspeed VNEs.  Don't know of any GA piston aircraft that have true airspeed limitations.  Thanks.

[L. Trotter]

This topic is confusingly interesting. The Acclaim has a indicated Vne of 194 kts. In the flight levels, I routinely cruse at TAS > 220 kts and never felt "flutter". Newer Acclaims with the same Vne boast TAS >240 kts.

Just for completeness, stall speed is based on indicated air speed not TAS.

Is it being suggested I should slow down? I have never heard anything about this concern.

[jlunseth]

Flutter is a function of TAS, the velocity of the air particles striking the airfoil.  IAS is a pressure measurement converted to an airspeed.  A pressure measurement is obviously important because it determines airfoil performance and speeds such as stall or Vna.  Vne is set on your airspeed indicator as an IAS so that within the normal flight envelope, the aircraft will not experience flutter below that speed, but that does not mean that flutter is a function of IAS.

[carusoam]

When it comes to Mooneys...

1) there are going to be some important vibrations that you may never feel... prop/engine dynamics, the reason we have yellow arcs on our tachs....

2) By the time you feel flutter of an airfoil, have no fear, your personal parachute will still work...  only test pilots go this far to the other side of the marked Vne... it is pretty far from the published Vne.

3) Basically this is the reason some people prefer the factory built airplanes that are fully proven throughout a giant operational envelope...

4) Vne is probably an indicated Airspeed, because that is how it is indicated on my ASI that has a TAS display.   Look it up, it is simply an FAA technical definition... easy to understand... tough detail to remember.  Not that critical for what it really represents...

 

5) Things that are important to remember...

• Staying lower than Vne will keep the wings on in smooth air...
• In rough air, maneuvering speed requires knowing your actual weight at the time, or erring to the slower side... yellow arcs  on the asi

6) What takes the fun out of flying near Vne is you never know when you are going to run into bumpy air.  Vne is pretty far away from Maneuvering speed.  Crossing paths with large planes at altitude is a strange place for fearing wake turbulence...

7) It is really hard to Accidently exceed Vne in level Mooney flight...  On descent, it is pretty easy to exceed Vne in any Mooney... especially with 310hp driving you forward...

8) Other important things to know and be familiar with are...

• CAS
• TAS
• IAS
• Maneuvering speeds
• stall speeds
• bottom of the white arc and flap positions, don’t forget that gear position is important there too... also in the definitions...
• there are also known Airspeed errors that are documented in the modern POHs...

 

Pp thoughts only about the ASI and what they mean, and how they are measured on a Mooney ASI.  

The modern POH literally covers this detail over a few pages.

@MIm20c why are you using the middle of the yellow arc on the ASI while descending?

 

let me know if I missed something, or misstated something, or any other errors...

Best regards,

-a-

[Hank]

IAS to CAS at cruise is simple on my C. CAS = IAS - 4mph. Yellow > 175 mph; Vne = 200.

Below 120 mph, they are closer together. 

[rbridges]

As Mooney models became faster and vne increased, were any changes made to the airframe? Or were they just tested to higher speeds?

[jlunseth]

Some, maybe most of the speeds on your ASI or in the POH, are purely mathematical calculations done to meet test certification standards.  That does not mean they are unsafe numbers.  The mathematical calculation set up by the certification standards is done to ensure that the particular speed is definitely going to be safe, with a good margin of safety.  One of those we have had a couple of threads on recently is max crosswind.  Max crosswind says nothing about what the airplane can do.  It is .2 Vso, period.  The aircraft has to be demonstrated to be capable of landing with normal effort in the landing configuration at a crosswind component of .2 Vso, but the aircraft may actually be capable of landing in a higher crosswind component.

Vne is similar.  It is numerical computation from another speed, Vd or design dive speed, not on your dial. For certification, the aircraft must be flutter free at Vd.  Vne is .9 times Vd, and that number is displayed for the pilot’s use as an IAS, but IAS is not what causes flutter.  Here is an article: https://www.flyingmag.com/technique/proficiency/technicalities-are-you-feeling-lucky . Also try Flying High Performance Singles and Twins, by Ecklabar, pp. 221-222.  Some aircraft have reductions of Vne in their POH for higher altitude flight, some actually have indicators to show the reduction. Vd is itself not necessarily a “max flutter free speed.”  It is a speed computed from another number, Vc or design cruising speed.  During testing, the aircraft must be shown to be flutter free to Vd, if it is not, then a different number, Vdf is used and Vne is .9 times that number.

Here is the main point.  Lots of pilots and lots of instructors assume that because a speed is shown as an IAS on the dial, or is stated in the manual as an IAS, that the phenomenon the speed is set for (such as Vne to prevent flutter) is an IAS.  That is not necessarily so.  The testing standards in FAR Part 23 and in the CARs are what sets the number in many cases, the number has a built in safety margin, and the number is displayed on your ASI in a way that you can readily use it, i.e. you don’t have to know Vd, or Vdf, for a particular altitude, you can just look at your Vne on your ASI and be safe.

Edited April 30, 2018 by jlunseth

[jlunseth]

I just want to say one other thing.  I am not a test pilot or an aerodynamic engineer, I am just a damn lawyer with a head for math. So don’t anyone go out now and think, that’s cool, I can run my aircraft up to +10% of Vne.  For one thing, from what I understand from my friends who actually do know this stuff, the certification standards have built in tolerances for things like wear and tear.  So your 35 year old Mooney may not have the same flutter tolerance it did when it came off the line.  For another, you can’t necessarily infer from one airframe to another.  Didn’t we just have a thread a couple of months ago where someone said that the Rockets, for example, had to have extra reinforcement added to the airfoils?  Test pilots get things like quick exit doors and parachutes, and instrumentation to measure the onset of flutter, which, by the way, is a sudden onset event, a change of 3 or 4 knots is enough to tear your airfoil off.  These are all good things to know, if I found myself with a fire and needed to get down from, say, 19k, I might fudge Vne a little given the other choices, but just please don’t let becoming smart about something make you stupid.

[MIm20c]

9 hours ago, carusoam said:

 

@MIm20c why are you using the middle of the yellow arc on the ASI while descending?

 

let me know if I missed something, or misstated something, or any other errors...

Best regards,

-a-

Good question. I know the middle of the yellow ~150 kts IAS is well below the 174 kts beginning of the yellow for you. However, I figure it provides plenty of safety margins for the bumps that sneak up on me. My Vne is around 165kts IAS. 

My question for the thread was it’s very easy for me to exceed 165kts TAS in level flight let alone pointing the nose down. Just wondered if that was a problem. 

[Hank]

23 minutes ago, MIm20c said:

Good question. I know the middle of the yellow ~150 kts IAS is well below the 174 kts beginning of the yellow for you. However, I figure it provides plenty of safety margins for the bumps that sneak up on me. My Vne is around 165kts IAS. 

My question for the thread was it’s very easy for me to exceed 165kts TAS in level flight let alone pointing the nose down. Just wondered if that was a problem. 

I thought older Cs had the yellow beginning at 150 mph; mine starts at 175 mph. I thought the corresponding change in Vne was more than 11 mph (165 knots = 189 mph; my Vne = 200 mph). The only thing I use knots for in my C is groundspeed from the Garmin, and now TAS since I learned how to get that from it (after converting OAT from my displayed °F into °C).

[mike_elliott]

1 hour ago, jlunseth said:

I just want to say one other thing.  I am not a test pilot or an aerodynamic engineer,

Well here is your chance to become one. Mooney is looking for a test pilot (tempting but that is a young mans game)

https://recruiting.myapps.paychex.com/appone/MainInfoReq.asp?R_ID=1893063&B_ID=91&fid=1&Adid=0&ssbgcolor=17143A&SearchScreenID=7751&CountryID=3&LanguageID=2

 

[jlunseth]

That would be really cool if I were like, oh, 40 years younger.  When I got my CPL I thought for sure Delta would call the following Monday.  I had at least six months left to mandatory retirement and they needed pilots real bad.  They must have lost my number...

[EricJ]

I recall an article in some aviation magazine many decades ago talking about what drove Vne in various aircraft;  in one the windshield caved in, in another the side windows blew out, etc.   Does flutter drive it in most airplanes?   Does anybody know what actually drives Vne in Mooneys?   Is there actually a flutter inducement, and if so, which surface?

 

[rbridges]

We had a local Bonanza v-tail pilot lose his life when his tail departed from the fuselage. That's always stuck with me. 

[Rick Junkin]

Mr Kromer is speaking at the Mooney Summit this year about maximizing the performance of our Mooneys, and this would be a great topic to bring up during the Q&A. Although my guess is that he will say it would require another extended session to cover in detail.

Forgive me for a word of caution out of concern for my Mooney brothers and sisters.

Please don't take this discussion on Vne and its relation to flutter margins as anything other than educational, and stay within the published limits for your airplane. I believe I'm being the Master of the Obvious in stating this but you never know. The regulatory mathematics quoted by @jlunseth are the minimum requirements for aircraft certification and result in driving aircraft designs that can at a minimum meet those requirements as demonstrated during flight testing. The limit parameters of that testing get translated to KIAS for GA aircraft so that they are easily employed by the average pilot, although the numbers on your ASI may be different than the actual demonstrated aircraft limits to make the ASI numbers stay within the regulatory mathematics. But we have no way of knowing where the deltas are or how large/small they are, if indeed they exist, without seeing and understanding the flight test data. And it is not cost effective for a manufacturer to test beyond the required demonstration limits, not to mention the associated risk. 

Flutter is nothing to mess with unless you have the proper training and test support, and even then it can kill you very quickly. As @carusoam stated earlier, by the time most of us would recognize there was a problem the game would already be over. The "heart of the envelope" is a really good place to be, and staying within the published limits in a well-maintained aircraft will keep you safe.

Cheers,
Rick

[Yooper Rocketman]

This was a good subject to bring up.  Although VNE in "Certified Aircraft" DOES account for flight up high, especially in the turbo Mooneys, your margin from flutter clearly gets much less the higher you get if flying close to VNE using  IAS.  I'm sure our ATP's can jump in, but airplanes flying in the mid to upper flight levels have Mach limitations to accommodate for the lower pressure the airspeed indicator will report.  The picture below displays how a VNE in the "breathable altitudes" on my Lancair of 274 knots drops to 225 knots at 26,000' (reference the barber pole area of the air speed tape).  It continues dropping the higher you get, now being limited by Mach number.  

This whole conversation changes my perspective of flying near VNE during the early part of my descent from the flight levels in my Rocket.

Tom

[MIm20c]

6 hours ago, Hank said:

I thought older Cs had the yellow beginning at 150 mph; mine starts at 175 mph. I thought the corresponding change in Vne was more than 11 mph (165 knots = 189 mph; my Vne = 200 mph). The only thing I use knots for in my C is groundspeed from the Garmin, and now TAS since I learned how to get that from it (after converting OAT from my displayed °F into °C).

Here is the PoH for the older C’s. I’ve been using the 165-170 mph for coming down lately.  

I’m glad to hear certified planes have a Vne that can be referenced at higher altitudes. However, similar to @Yooper Rocketman I’m going to think twice about pushing the yellow too much at higher altitudes as I start flying 15k plus. 

[kortopates]

To my recollection there has been at least 3 K model break ups. Too long ago since I looked at the data but I recall all being Rockets, but at least one was attributed to over stress from penetrating a thunderstorm. 

Mooney's flutter analysis in the accident report on the Rocket left me with more questions than answers. But long time Mooney engineers like Bill Wheat and Bob Kromer have always referred to our Mooneys as being very safe from flutter during dive certification testing. When Mooney increased the max gross weight of the K model for Encore, they increased the weight of all the control balance weights, doubling the weight on the elevators, based on their computer modeling of flutter analysis to maintain their flutter reserve (or so I've heard from them as the explanation for the changes years ago).

To my knowledge Rocket Engineering's only enhancements to the airframe where to support the 2 batteries in the rear for the required ballast with the bigger engine up front. No change in control balance weights was done.

[Jerry 5TJ]

9 hours ago, jlunseth said:

.  ......Max crosswind says nothing about what the airplane can do.  It is 1.2 Vso, period.  The aircraft has to be demonstrated to be capable of landing with normal effort in the landing configuration at a crosswind component of 1.2 Vso, but the aircraft may actually be capable of landing in a higher crosswind component.....

What?  

So if Vso is 50 knots, you claim the plane must demonstrate a landing with a crosswind component of 60 knots?  

[aviatoreb]

3 hours ago, kortopates said:

To my recollection there has been at least 3 K model break ups. Too long ago since I looked at the data but I recall all being Rockets, but at least one was attributed to over stress from penetrating a thunderstorm. 

To my knowledge Rocket Engineering's only enhancements to the airframe where to support the 2 batteries in the rear for the required ballast with the bigger engine up front. No change in control balance weights was done.

The "normal" common rocket does not have any structural strengthening so it must stay within the original M20K speeds.  There were 5 "liquid rockets" with 350hp tsioL engines and those had gussets added to the wing-airframe joins to strengthen the structures.

I do not know the certification standards but I know quite well about flutter. Flutter is a function of TAS but also of a few other things, like loading (meaning therefore how much weight you are carrying) ad then other factors you cannot control - such as shape (long skinny wings are more prone to flutter all things being equal) and strength (how stiffened the wing is - again something you cannot control from the pilot's seat) and of course TAS is a function of IAS and altitude (on a given day).  SO not knowing the certification standards, I would think that the FAA would certify a bullet proof standard - meaning suppose 196IAS is certified as safe in an M20K certified to 28,000ft.  Then whatever that TAS works out to (303TAS - yikes that's fast....) would be certified as "safely" below the actual flutter speed by some pre-specified margin.  But obviously that margin would be lower than the same IAS at a lower altitude - and a lower weight for that matter.

Not just the main wing can flutter, but the tail structures horizontal and vertical - and that's where a lot of v-tails used to fail.  But even the entire fuselage could flutter - which is not likely in a little airplane - but as I understand it some of those spectacular rocket mishap explosions if you may remember film from the 1960s space race era were the body of the rocket going through flutter and then loosing it.

Edited April 30, 2018 by aviatoreb

[jlunseth]

2 hours ago, Jerry 5TJ said:

What?  

So if Vso is 50 knots, you claim the plane must demonstrate a landing with a crosswind component of 60 knots?  

No, sorry, typo, I meant .2 Vso.  10 knots.

Edited April 30, 2018 by jlunseth