M20K Rocket Airspeed Limitations

[Houman]

Hi, this might sound very long and already answered question, but I just want to check with other Rocket owners to confirm my numbers, here is my question :

 

I'm going through my newly purchased Rocket's POH page by page as I always do with a plane I'm not completely familiar with, on page 2-3 of the original POH, there is the Airspeed limitations page, for all the V speeds, my question in particular is the VNE and VNO.

 

The VNE on this page is 196 Kias, and VNO at 170 KIAS, now on the top of the page, it clearly says : See Supplement for Rocket 305 STC.

 

When looking at the Rocket STC, on the very first page "Flight Tips", it says the Maximum Structural cruising speed (VNO) has been reduced for increased gross weights above 2900 lbs. ( My understanding is that the original M20K 252 had a GW of 2900, but the Rocket 305 STC bring it up to 3200 lbs, but all weight about 2900 lbs has to be fuel in the wings ).

 

Ok, so going back to my question, the VNO is now reduced to 166 KIAS when between 3018 to 3200 LBS, but no mention of the VNE.

 

Also, on the Rocket Engineering site performance chart and there attached excel sheet performance settings: http://www.rocketengineering.com/content/rocket-performance, the max power continuous at 12000ft or less is at 210 Ktas and at 24000ft at 235 ktas.

 

Is this just the difference between IAS and TAS ( Indicated airspeed compared to true airspeed with margins increasing with altitude and OAT) or am I missing something.

 

If I use my normal formula to calculate the TAS via the IAS : TAS= (IAS* .02)* (Altitude/1,000)+ (IAS), that 196 KIAS VNE would give me about 290 KTAS at 24000 ft, confirmed by using : http://www.hochwarth.com/misc/AviationCalculator.html

 

And also the 235 KTAS at 24000 feet give about 162 KIAS, so still below the 196 KIAS...

 

My understanding of the VNE is that it's not attached to the engine, but the aircraft structure limitations, so the Rocket 305 STC should not have changed that, right ???

 

What kind of indicated airspeed do you guys usually see on the Rocket ?, I want to modify the Excel sheet to include the KIAS, not just KIAS, or maybe that is not really worth much ?

 

Thanks and sorry for the long winded question !!!

[jlunseth]

I don't have a Rocket, so take what I have to say with a grain of salt and I can't tell you what typical Rocket speeds are.  What I know about V speeds, and particularly Vne are my distillation of Eckalbar's excellent book, Flying High Performance Singles and Twins, and I am going from memory so I hope I am not slaughtering Eckalbar's excellent math. 

 

Vna (Design Maneuvering Speed) and Vno(Maximum Structural Cruising Speed) are based on similar measures (both are Indicated Airspeeds) and serve similar purposes.  Vna is the speed at which the aircraft will stall before exceeding the airframe design limit of 3.8 G's.  Vna assumes no limit on the degree of turbulence that the aircraft is experiencing to cause lift to rise to 3.8, rather it is the speed at which lift can never exceed 3.8 because the wing will stall first.  Vno serves a similar purpose, that is, it attempts to help the pilot stay under the 3.8 G design limit of the airframe, but in the case of Vno, the degree of turbulence is limited to gusts of 15-20 inches per second as I recall.  It essentially answers the question, what speed can I go in clear air without exceeding the airframe design capacity so that I am safe if I hit limited, moderate turbulence.  The important point is that both Vna and Vno are measures purely of pressure and lift, in other words they are IAS's.  Our ASI's, of course, measure the difference between ambient pressure and ram air pressure and convert that to an IAS.  So the reduction in Vno at certain weights, would be to protect the airframe from the results of lift over weight ratios that would exceed 3.8G's if the aircraft hits vertical gusts of an assumed amount. 

 

Vne is an Indicated Airspeed, but it is actually intended to protect against flutter, and flutter is a function not only of pressure, but of the velocity of the particles of air striking the airfoil regardless of lift or pressure.  TAS is a better measure of the Never Exceed speed of an aircraft than is IAS, because TAS is a measure of the actual velocity of the airframe through the surrounding airmass.  Vne is an IAS that has a sufficient engineered safety margin designed in such that, if the aircraft is flown within its specified design envelope, flutter will not occur.  Flutter depends mainly on the ability of the airfoils and control surfaces to resist flutter at a given velocity.  Putting a heavier (viz. a Rocket) engine on the airframe does not change those characteristics, so to answer one of your questions, no, the Rocket STC should not change the Vne.  Theoretically, it might be possible to put a great big turbine engine on the same airframe, take it to a high altitude where there is very little ambient pressure, and drive the airframe at a sufficiently high TAS that flutter could occur even without exceeding the Vne Indicated Airspeed that is on your ASI, but that is just theory and Rocket has apparently done the testing and calculations necessary to assure that, within the specified flight envelope of your aircraft, Vne expressed as an Indicated Airspeed, will protect against flutter. 

 

An airspeed expressed in KTAS is just that, it is a True Airspeed.  A TAS is an IAS (actually a CAS, but small difference) adjusted for temperature and pressure (altitude).  It isn't Indicated Airspeed compared to True Airspeed, rather, it is True Airspeed.  When the performance charts provide a KTAS at a given altitude at a particular power setting, they are telling you that if you make that power setting at that altitude at standard temp., you can expect to get that TAS.  That is all they are saying. 

 

It is unlikely that, at level cruise or in a climb, you will ever exceed Vne.  The time to watch out for Vne is in a descent, esp. one that is more like a "dive" than a descent.

 

I looked at the hochwarth link and did not see a TAS calculator.  The formula you have provided is the "quick formula," that is, the "rule of thumb" measure that assumes that the difference between IAS and TAS increases by about 2% per thousand feet.  However, it does not account for temperature, and I have seen days when temps are signficantly nonstandard and make a difference, and I have also read material that the actual difference is about 1.5% per thousan feet.

 

Don't you have a True Airspeed Indicator?  The kind where you dial in an OAT and an Alt., and the indicator then provides an IAS on one ring and a TAS on the other ring?  That would do it.  If you want to put numbers in your chart, I would either use an E6B or electronic equivalent because they correct for nonstandard temp., but you will be making assumptions about temp. that may or may not be valid on a given day.

[Houman]

Thanks, that was quite a good explanation, I think the IAS indicator on the rocket has a TAS outer ring with a OAT and altitude adjustement, so I guess I should be ok with that. I'm just doing my mental preparation before I get my real training in the aircraft. I find the more I read and understand, the more confidence that I have when I'm behinde the yoke of knowing my stuff...

 

Thanks !!!

[jlunseth]

You could actually sit in the aircraft and use the TAS ASI as a sort of calculator.  Put in an altitude of "x" and a temp of "y" and the TAS's and IAS's for that temp and altitude will then correspond.  Pick a TAS for the altitude and you will have the IAS.

 

I have a 231 so don't have quite your power and speed, but I have not felt any need to prepare a chart like the one you are contemplating.  In real life I think you will find that Vno is not very useful.  In my aircraft I have to be really cranking to ever exceed it, although your engine might put you over that.  But at the altitudes where you would have the high cruising speeds that could exceed Vno, say from 12k up into the flight levels, the air is typically dead still and no turbulence unless you are penetrating weather or clouds, in which case you should probably slow to Vna anyway.  We both have the advantage of being able to get above most of the clouds, and it is a very rare trip for me when that does not mean still air.

 

I think what you will find is that the training will focus on maneuvers, takeoffs and landings.  You will have to learn about cruise for yourself, and if you have been flying an NA aircraft you are in for a real treat.  High, fast and smooth is the normal regime.

[aviatoreb]

It is very easy in a rocket to penetrate Vno in even a mild descent so it is something to pay attention to and moderate power when coming down.  It is actually possible to inch very close to Vne at a bit steeper but not that steep descent with high cruise power.  So it is necessary to stay awake about tipping the nose down and pulling back on power to keep within designed parameters.

[David Mazer]

Erik is very correct.  Vno and Vne are definitely on my mind during descents.  I try to keep my IAS below 175 kts even in clear air just in case I need to slow down quickly and that is why I believe the speed brakes aren't an option but a requirement on the Rocket.  I've also had problems with ATC when I request deviations around even small cumulous to avoid the need for dramatic speed reductions (once they asked if I was really IFR as I tried to weave around clouds during a descent).  The Rocket isn't a plane you sit in for a ride.  You need to really fly it since you are operating near limitations on a regular basis and you need to be aware and prepared.

[aviatoreb]

BTW - Yes, Vne is a TAS concept - flutter is due to Hopf bifurcation by the way (something in my home field specialty of dynamical systems) and it is a TAS that induces flutter which is an aeroelastic phenomenon, meaning it is a nonlinear interaction of the mechanical structure with the aerodynamics.  What I do not know is how TAS is set relative to flutter speed.  I read somewhere that TAS is meant to be at least 30% below flutter speed.  Who knows?

 

And TAS is a function of IAS as well as altitude and temp and pressure (density altitude). 

 

So, what I would PRESUME (meaning guess) is that Vne would be marked on an airplane as follows during certification test flights.  a) determine what will be the certified altitude.  In the case of my airplane that is 24,000.  Then determine the flutter TAS at 24,000ft. Then convert that TAS to IAS assuming a standard day at 24,000ft - which lowers the number as TAS will be higher than IAS. That way a pilot can instantly interpret free of flutter (with  safety margin) right off the IAS on a standard day....and the pilot does not need to think or do engineering-math in his/her head - just follow the rules and keep IAS within design parameters.

[Houman]

Thanks to all Rocket owner's advice, since this beast is very fast, I want to make sure to know my stuff before while getting my insurance required training on my new bird.

[jlunseth]

If you want a better understanding of how the speed limitations are derived and can tolerate reading some math, I would highly recommend Eckalbar's book.  Read Ch. 11 .  Basically, all the speed limitations are based on limits (such as the 3.8 G positive load limit for Normal category aircraft) and design standards that were set some time ago.  For our aircraft, they would be from CAR 3, which no longer exists except for "grandfathering" of aircraft designed under the CAR's (now we have FAR Part 23).  There is even some material in the Chapter about high powered, high altitude aircraft that have their Vno reduced at higher altitudes (such as your Rocket).  After looking through the material again, I see that Vno also is intended (in addition to protecting against vertical gusts), in high altitude flight, to protect against flutter and "control reversal," where you turn the yoke left, but the movement of the control surfaces causes the wing to twist and the plane turns right.  Better you read the stuff from someone who understands it, if you really want to know.