IAS and TAS and VNE

[Tumbleweed]

I fly a 2007 CTSW with 100Hp Rotax.  My Dynon 100 EFIS shows IAS.  At what point, in altitude or otherwise,  do I need to convert to using estimated TAS in regards to VNE, or any other V speeds?

Thanks!

Robin

 

 

 

[Ed Cesnalis]

Let the Dynon do it for you continuously and display TAS

[Anticept]

You don't use TAS for V speeds, you use calibrated (or indicated if no calibrated).

TAS is to correct for thinner hotter air, and fuel and travel calculations. TAS calculations for a long cross country can make a few gallons difference in a CT because you'll be moving faster through the thinner air.

On the other hand, airframe stresses, lift, and your pitot static system react to the cumulative effects of your speed and air density considered together: The lower the air density, the higher your true airspeed has to be in order to "feel" the same effects as slower airspeed and more dense air. It effectively cancels out, so just use your CAS/IAS.

[Ed Cesnalis]

TAS is a good choice for Vne assuming flutter is a consideration.

[Jim Meade]

The CTSW POH lists Vne as CAS.  Page 4-2, paragraph 4.4

[Tom Baker]

8 hours ago, Jim Meade said:

The CTSW POH lists Vne as CAS.  Page 4-2, paragraph 4.4

I suspect that number is for sea level under standard conditions. In this case true and calibrated will be the same. 

[Tom Baker]

As you go up in altitude the delta between calibrated and true airspeed increases. It is my understanding from my glider training that on high altitude flights that you should use true airspeed for VNE.

[Anticept]

CAS and TAS diverge because of the thinner air. You gain 2% of true airspeed per 1000 feet. This is because the thinner air does not create as much drag for a given TAS vs denser air, so if you continue to generate the same amount of power, you will have a surplus, and your true airspeed increases. This true airspeed is only for the purpose of ground reference, aka, how fast you are going between two points, and wind correction is the next step.

Calibrated airspeed, however, is indicated airspeed adjusted for instrument installation error. No instrument is perfect, no pitot system is perfect. In general, IAS and CAS are almost the same at cruise speeds, while they will diverge a little at higher AoA and slower speeds.

Dynamic pressure is what we care about though when it comes to how the airplane will behave, its structural integrity, etc. This is measured with Calibrated Air Speed in small aircraft. Comprehensibility effects become significant at high speed, that's why fast aircraft used mach number. But let's just stay focused on CAS.

As an aircraft flies at low altitude, it is flying at 100 knots through thick, dense air, with a lot of mass, it is going to strike a number of air molecules per second. If that airplane were suddenly teleported 20,000 feet higher, there would be a lot less mass, so all forces on the body of the airplane would be cut significantly. It would experience a sudden loss of lift, a dramatic decrease on the airspeed indicator, and quite possibly stall if it's already close to it.

During the fall, it picks up airspeed, and the pilot recovers and holds steady. Let's say it's got a turbocharger, so it keeps the same power development. Instead of traveling (to a ground observer) at 100 knots before, now it's traveling at 135 knots. That is its True Airspeed.

However, the airplane and structure still acts like its 100 knots. Controls still feel like they're at 100 knots. That's because the dynamic pressure equation components have changed. The air molecules are spaced further apart, so if the airplane must increase in speed through the air (TAS) in order to maintain the same dynamic pressure. It needs to hit more molecules per second at a higher velocity to have the same aerodynamic forces that it experienced when it was at a lower altitude.

This also translates to VNE. VNE is about overstressing the airplane, in reaction to all that force generated from hitting air molecules per second. Fewer molecules in an air parcel means the airplane must move faster to experience that same force.

Your airspeed indicator, just like the airplane, experiences forces from the air just like the wings. If the air is more dense, you don't have to move as fast to get the same reading that you do at higher altitude. High altitude, less dense air, fewer particles being struck, and so less pressure will register on the sensor unless your airplane is moving faster.

So, in effect, your airspeed indicator is almost a direct measurement of energy being imparted on the aircraft structure and lifting components. Correct for calibration, and it's about as close as you're going to reasonably get, so use CAS for VNE.

Something to try: climb as high as you can and then try to fly at a couple knots above stall speed using TAS. Can't be done, you would stall every time if it's calibrated properly. Try it with CAS, and now it will act normally.

PS. If VNE cared about TAS, it would be in the regs and in your AFMs. It only lists CAS/IAS becauss V speeds are a calibrated measurement.

[Anticept]

PS 2: here's also a good thought.

The atmosphere extends for hundreds of miles, well into the exosphere. The international space station is in low earth orbit, but technically is also still in the atmosphere. It is travelling at 4.76 miles PER SECOND, yet doesn't tear itself apart. It is smacking air molecules so fast, but there are so few of them, that the net result is barely notable.

If it were to suddenly appear at 10,000 feet going that speed, it would break up instantly into a giant streaking multi-piece fireball.

It also has to occasionally fire the thrusters because it still experiences a tiny amount of atmospheric drag.

[Tumbleweed]

Thanks Everyone.  Your answers helped a lot.

 I know that CAS is listed in the CTSW POH for V-speeds, but, how do I determine the difference between my IAS shown and the CAS?  Is there a formula to calculate the difference between IAS and CAS, while in flight?

Robin

[Tom Baker]

40 minutes ago, Tumbleweed said:

Thanks Everyone.  Your answers helped a lot.

 I know that CAS is listed in the CTSW POH for V-speeds, but, how do I determine the difference between my IAS shown and the CAS?  Is there a formula to calculate the difference between IAS and CAS, while in flight?

Robin

There should be a conversion chart in the POH. It will likely be off at slow speeds and close to indicated at cruise.

[Tom Baker]

Corey, I went back and looked at my notes. You are correct that the VNE doesn't change, in that the structural aerodynamic load is the same. The problem is that the flutter speeds seem to follow the true airspeed. Because of this there could be a possible flutter issue at or below VNE. With glider flying in wave this can be an issue, especially with their long and somewhat flexible wings. I would still be cautious of going near to VNE at altitude with a CT.

[Tom Baker]

20 hours ago, Tumbleweed said:

I fly a 2007 CTSW with 100Hp Rotax.  My Dynon 100 EFIS shows IAS.  At what point, in altitude or otherwise,  do I need to convert to using estimated TAS in regards to VNE, or any other V speeds?

Thanks!

Robin

Since you have the Dynon D100, if you also have the OAT probe you should be able to display true airspeed and density altitude in the upper left corner. That is my preferred way of setting the display up.

[Anticept]

I wasn't sure why flutter kept being brought up, until I realized you guys might be talking about the results from the aformentioned compressibility issues.

That compressibility can do strange things to an aircraft as they approach mach 1, and sometimes even very well before it.

In an ideal gas, and even the major component of a real gas, the speed of sound changes with temperature. That is mach 1.

At sea level, standard temperature, mach 1 is 661 knots. At 20,000 feet, it is 614 knots. Not much change, and you won't be able to climb this high without a turbo.

You're not getting anywhere close enough in a CT to see flutter caused by mach effects. If you have a flutter issue, it's going to be because of a structural issue, not a mach one. Maybe maybe maybe in an edge case, but really though, we are so low in airspeed and power that I doubt an aero engineer would write anything but 0 in the equasion for any mach effects.

[Warmi]

VNE does very much change with altitude …

Here is that famous Vans paper addressing this very issue..

https://vansaircraft.com/wp-content/uploads/2019/01/hp_limts.pdf

[Anticept]

You posted that just after I got done, read through it, and glad to see it talks about the same things I just did :p.

All in all, we don't go fast enough, or don't have enough power once we potentially get high enough, that any of this is an issue.

This is high powered airplane territory or those with really long skinny wings like the glider boys!

[Tom Baker]

16 minutes ago, Anticept said:

I wasn't sure why flutter kept being brought up, until I realized you guys might be talking about the results from the aformentioned compressibility issues.

That compressibility can do strange things to an aircraft as they approach mach 1, and sometimes even very well before it.

In an ideal gas, and even the major component of a real gas, the speed of sound changes with temperature. That is mach 1.

At sea level, standard temperature, mach 1 is 661 knots. At 20,000 feet, it is 614 knots. Not much change, and you won't be able to climb this high without a turbo.

You're not getting anywhere close enough in a CT to see flutter caused by mach effects. If you have a flutter issue, it's going to be because of a structural issue, not a mach one. Maybe maybe maybe in an edge case, but really though, we are so low in airspeed and power that I doubt an aero engineer would write anything but 0 in the equasion for any mach effects.

It is an issue for sailplanes when flying in the wave, and their speed is nowhere close to Mach.

[Ed Cesnalis]

7 minutes ago, Tom Baker said:

It is an issue for sailplanes when flying in the wave, and their speed is nowhere close to Mach.

Right it's about a TAS where airframe harmonics might initiate flutter and it could be below Vne @ IAS / CAS

15 years ago there were many things in the POH that weren't consistent or presented well.  Using TAS for Vne just cover's another base.

 

[GlennM]

I believe the design requirements for most powered aircraft certifications require any flutter issues to be outside of the never exceed speed by a certain margin. Not sure on gliders.

[207WF]

Our airspeed indicators are so inaccurate that IAS vs CAS is relatively trivial.

I only know that because I have two airspeed indicators.

They can differ by 10% at times (although, of course, they agree at zero).

WF

[Anticept]

If they're differing by that much, you need to get them checked. That's not normal.

Rarely have I seen working airspeed indicator differ by more than a couple knots when tied into the same pitot static system, and in every case, it was a gauge problem.

[207WF]

Same pitot static, and I have checked for leaks. It's the FD analog gauge that is so optimistic and it gets worse with age.  The dynon D10 is only three knots too high in cruise. 

 Nice to have two.

WF

[Anticept]

I still have my original and it's still accurate. But yes with age, those aneroids don't work like they used to.

[Ken]

Upcoming webinar on VNE and altitude.

 

 

https://www.faasafety.gov/SPANS/event_details.aspx?eid=109631&caller=/SPANS/events/EventList.aspx