True airspeed color ranges

[skysailor]

I am flight testing an RV4 just completed. I have put all of the appropriate V speeds into the Skyview and the airspeed tapes accurately reflect the analog Vans airspeed indicator mounted as a backup. Cruising at 7500 feet and indicating 154 knots, the Skyview airspeed tape just turned yellow as you would expect. A glance at the true airspeed number below showed 178 knots true airspeed. Vans publishes Vne as 182 knots. While I understand the experimental category allows me to determine Vne, I prefer to use the number determined by the aeronautical engineers with a safe margin. Vne is measured in true airspeed (TAS). The number is dynamic with reference to indicated airspeed. Because the Dynon Skyview continuously computes and displays TAS we have the opportunity to add a layer of safety in this display.

The TAS display could stay in the same location where it currently resides. When the value enters a caution area set by the user either surround the value with a yellow box or turn the TAS reading yellow. I might use 10 knots prior for this. At Vne the TAS would go to a red background or the text might change to red. A yellow range for the TAS could generate a Caution message. Exceeding the value set into the red would give a Warning message.

My thought is this feature would be enabled by the user. If you prefer not to use this feature you could leave it disabled in the System Settings. Thoughts?

 

[nigelspeedy]

That's a good idea. I have flown a few helicopters that dynamically calculate VNE and it is a nice to have feature.

 

[Dynon]

We agree. This is already in our list of neat features to add.

 

[KRviator]

If you blokes need numbers to decide feature priorities, count me in as thinking this is a top idea and something I'd like in the next release.

At work, our locomotives have a glass-cockpit setup and some of the data boxes will change to a flashing-yellow to show a caution indication, with others flashing red as a warning. Coupled to an audible warning, this is a great feature.

 

[ckurz7000]

It is not at all clear to me that Vne changes with true airspeed. Just take the stall speed as a counterexample. The plane (in a specified configuration and level flight) will always stall at the same INDICATED stall speed regardless of what the true airspeed is.

Unless otherwise specified by the manufacturer, Vne is ALWAYS referenced to INDICATED airspeed. If, for a particular aircraft, this is otherwise, it will be stated in the POH and an appropriate table will be included.

Having green/yellow/red markings or alerts coupled to true airspeed can be very misleading!

-- Chris.

 

[skysailor]

Actually Chris, Vne is only in True Airspeed. You are correct about the stall being accurate with indicated but you will find Vne is defined differently and for good reason. Generally, it has to do with the velocity of the air vice density and the propogation of non damping ocillations in the structure. There is a very good explanation in the Vans website under the FAQ. It is about a page in length with good examples and explanations. There is a nice snippet posted there from a guy who got into the issue accidentally. I can send you a link if you like. The article is very well done.

 

[ckurz7000]

Thanks, Skysailor, for your thoughtful reply. I will certainly follow up on your suggested reading.  Meanwhile, however, permit me to elaborate on my understanding of the matter so far.

There are many different considerations why a manufacturer sets a specific Vne, only one of which is flutter. Another is structural integrity of the airframe, yet another may have to do with rotor dynamics (I fly gyroplanes, mostly), others might have to do with the stability of the aircraft, etc. Which factor, ultimately, turned out to be the decisive one is by no means the same for all aircraft categories, classes, types and models.

Certification requirements usually have you set Vne to the highest safe speed (including a prescribed safety factor) at which integrity, stability and controllability of the aircraft can be maintained within the bounds set forth by the syllabus. Whichever factor causes you to cross these bounds first determines what your Vne is.

Aerodynamic forces are proportional to air density, as is indicated airspeed. Therefore, all Vne restrictions based on aerodynamic forces should be referenced to indicated airspeed. I had the opportunity to be the test pilot for a novel gyroplane in which Vne was determined by structural integrity consideration. These factors scale with indicated airspeed and hence the Vne for our aircraft needs to be referenced to indicated airspeed. But, of course, you are correct in that a Vne which has been set because of incipient control flutter should be referenced to true airspeed.

How Vne changes under different operating conditions such as density altitude should be noted in the POH. Therefore the POH is the ultimate authority which tells you which speed to reference your Vne to.

Greetings, -- Chris.

 

[RVDan]

In certified airplanes where the service ceiling is high enough where the Vne factors that are influenced by TAS are limiting, they use a "barber pole" (Vmo) that moves based on altitude. As altitude goes up, Vmo comes down so that the it depicts the redline against IAS. Perhaps what is really needed here is the ability to set up a barber pole for Vmo or Mmo.