T

tonyjohnson

Well Known Member
What are you using for true airspeed indication?

There is an airspeed indicator made by Falcon that allows you to set the OAT and read the true airspeed in a window. I am not a Falcon fan. The face of a true airspeed indicator looks "busier" that I would like with all the windows etc.

I viewed the web pages for dynon and GRT sport, and did not see that either of those will supply true airspeed.

I think in important to know your true airspeed. Those of you who read the post of Smokey Ray about almost overspeeding his RV4 because of a high true airspeed will appreciate my concern.

Your thoughts and experiences will be appreciated.
 
I must have missed a great post...because to me "overspeed" implies faster than Vne. Isn't that a line on the airspeed indicator? Meaning it's in IAS, not TAS?

I want to know TAS because it tells you what performance you're getting out of the airplane, and it helps calculate wind. Other than that, why do I care about TAS?

Cheers,
Martin
 
I believe flutter depends on actual speed of the air molecules rushing over the plane. For this reason, Vne is TAS and does change as we climb.

Steve Zicree
 
TAS and VNE

I don't remember where I read the article by Smokey Ray, it may have been in an RVater. It was outstanding. Even Smokey Ray, an F16 pilot, fell into the TAS v IAS trap and wrote about it for the benefit of the rest of us.

At high altitude in his RV4 he was crusing along at a speed under indicated VNE when the plane began to shake. He slowed down. He then realized that his true airspeed was well above VNE although his IAS was less.

It is my understanding, and I am FAR from an expert, some V speeds Ie Vs are determined by IAS. VNE however is determined by TAS. It is a real "gotcha" than can ruin your day.
 
VNE Revisited

I too understood that critical airspeeds were all indicated. In an article by Ken Kreuger in the 6th issue 2004 discussing the reason not to turbonormalize an RV-10, the statement, "flutter does not depend on Indicated Airspeed/dynamic pressure. It is directly related to true airspeed - the velocity of the air passing by the airframe."

In the 1st issue of 2005, Rob Ray describes a flight in his RV-4 during which he was dodging some thunderstorms and saw a hole for his let down. He started a descending 360 turn and felt a very high frequency vibration. He recovered and after landing did some research on flutter.

The two articles changed my understanding of the phenoma.
 
Like many things in aeronautical engineering, it's not as simple as it at first looks. My understanding is this...please correct me if I'm wrong:

Flutter is dependent on both dynamic pressure (EAS) and the speed of the air actually moving past the aircraft (TAS). The reason for the EAS dependency is that it takes a force to deflect the structure, and that force is provided by dynamic pressure.

The reason for the TAS dependency is damping. As the TAS increases, a given deflection rate (inches/sec or whatever) of the structure causes a smaller local increase in angle of attack. That local increase in angle of attack opposes the motion of the structure, and thus tends to damp the motion. However, at high TAS, the angle of attack change due to a given deflecton rate of the structure is less, so there's less damping.

That causes a structure which might not flutter at a particular EAS to be more prone to flutter a the same EAS but higher TAS.

The design speeds in 23.377, however, are in EAS. The above TAS dependency though suggests that expanding the envelope with caution is a good idea...that's why professional flight test organizations have lots of guys staring at strip charts when a pilot is exploring a new regime.
 
I find this very strange. I don't understand why Ken Krueger would say flutter depends on TAS while at the same time Van's is selling an airspeed indicator with the Vne for the RV-7 (200kts/230mph) marked on it with a red tick line. This is the indicator I have in my RV-7 and it's IAS only.

Also, imagine taking off in a rocket powered RV-7. As you climb maintain a constant 200kts over the ground or 200kts TAS (no wind example). Near sea level your IAS should be around 200kts but climb up to say 90,000ft, at which point your TAS is still 200kts, but your IAS is zero(or very close to zero). No air molecules, no flutter. So, even if Ken is correct, he's not correct for every situation and they need to change the airspeed indicator they sell.

Tobin
 
more complicated than it needs to be...

REDLINE (and flutter) is based on IAS (and CAS, but they are mostly the same).
Equivelant airspeed (EAS) is indicated airspeed (IAS) corrected for compressibility effects and indicator position error (*).
"compressibility effects" (think Mach) are dynamic pressure and air density dependent - this is the point where most people start to confuse EAS with true airspeed (TAS). For aircraft the speed of our RV's (and most faster piston aircraft) and the altitudes we normally operate (less than 18K-ish), EAS and IAS are generally the same (although are different enough for aero and flutter calculations). Think of EAS as IAS with a correction for 1) Mach speed and 2) instrument position error. There is more of an effect the closer to the speed of sound you go. As mentioned, EAS is usually negligible for these types of airplanes and is mostly important for flutter analysis at max airpseed and altitude. This is why your E6B seems to ignore EAS correction (I lost my and don't remember, but pretty sure).

True airspeed (TAS) is indicated airspeed EAS corrected for density (temperature and altitude). But since the difference between EAS and IAS is negligible, TAS is mostly IAS corrected for density.

*CAS has omitted from this explanation for clarity. I skipped this because it seems to always add to the confusion. CAS is IAS corrected for error.

Now everyone should be effectivly confused...
 
Both Dynon and GRT have TAS

tonyjohnson said:
I viewed the web pages for dynon and GRT sport, and did not see that either of those will supply true airspeed.
Click to expand...

The Dynon has TAS. You have to have the OAT option of course to complete the equation. I always have it up on my D10 to do a quick wind check.

The GRT has TAS also.
 
RV4 flutter

This past weekend, while flying with my S8 and a friends 4, he encountered a similar situation. In a dive (trying to loose me), under G, a low frewuency shake which he imediately attributed to flutter. Subsequent tests show it was oil canning that he does not get unless he is at red line and under positive G.

Just another mans experience. And yes he lost me. His little 4 and his flight experience can shake my fat cat 8 in about 10 seconds every time. ARGH!

Kahuna
 
Vne & Vd

Found this on the internet, maybe it'll help a little, or maybe cause more confusion :) .

Flight at airspeeds outside the envelope (or at inappropriate speeds in turbulent conditions or when applying inappropriate control loads in a high-speed descent or, indeed, at any time) is risky and can lead to airframe failure. Vne is the IAS which should never be intentionally exceeded in a descent or other manoeuvre and is normally set at 90% of Vd, the 'design diving speed'. For a normal category aircraft, Vd is required to be 1.4 times Vno and, to receive certification, it must be demonstrated, possibly by analytical methods, that the propeller, engine, engine mount, and airframe will be free from overspeeding, severe vibration, buffeting, flutter, control reversal and divergence. To provide some safety margin, Vne is then set at 90% of the lower of Vd or Vdf. Vdf is a diving speed which has been demonstrated without problem in test flights and which must be lower than, or equal to, Vd.

I still believe Vne has to be related to IAS, again as in my earlier example, fly high enough and your indicated goes to near zero (fringes of outspace) while your TAS could still be 200kts(obvioulsy you'd need a rocket motor to test this). There is no way you are going to get flutter up there. I would like to read the article mentioned on a prior post written by Ken Krueger and see where he's coming up with his conclusion.

Tobin
 
TAS and IAS and airspeed markings...

I got this out of the RVator. May help / may add confusion, but I think it might add a datapoint to the discussion here:

"RV's are designed presuming the installation of naturally aspirated engines. Van's flutter analysis is conservative, but not so conservative as to allow for the true airspeeds that might occur using an engine that can develop 75% of rated power up to altitudes of 20,000 feet or more."
​
Stated another way...airspeed indicators are marked presuming the installation of certain engine(s), if/when a builder elects to install an engine different than that for which Van's designed the aircraft, the airspeed indicator markings may change.

Back to work (ugh),
 
A complex subject

As other posts have suggested this is a complex subject, and the simple story breaks down under a little questioning. The best way to understand what is going on is to read an air data and structural dynamics text book.

To try to do it justice in a few lines, flutter is a complex phenomenon and usually instantly catastrophic, control surface flutter (or "buzz") is different and sometimes survivable. Flutter is caused by the wing structure twisting under load, rapidly reversing due to the torsional stiffness of a typical wing (stiffer at the front than the back), reversing again and so on until the structure fails - it usually happens very quickly. The only hope of survival is if you are slowing down when it starts (usually not the case, and might not help). Control surface buzz is often caused by control rods/cables bending and allow the control surface to vibrate; well balanced surfaces are more resistant. Flutter & buzz are more likely as speed increases.

As noted before usually all relevant speeds are IAS (well actually CAS/EAS, but CAS/EAS gauges are expensive and usually IAS will do, especially at the speeds we fly at and at the altitudes a normally aspirated engine can get to). The exception is the "flutter speed", which is related to your TAS (also as noted before). Ken Kruger probably has an estimate of the "flutter speed" for a typical RV, but I doubt anyone has done any testing. It must be well in excess of the published Vne as we don't see many RVs falling out of the sky. Calculating the equivalent IAS for your TAS Vne (for a standard day, using a spin wheel), at sea level they are the same (surprise, surprise!) at 10K' 230 mph TAS = 198 MIAS, at 15K' 230 MTAS = 184 MIAS (@ 10K 210 MTAS = 182 IAS & @15K = 168MIAS). So flying at 230 mph IAS at 10,000' means your TAS is around 266 mph. How much margin remains before you are in danger of flutter? Who knows?

If you were to fly straight and level at 1g to a speed well in excess of Vne (IAS or TAS), and slow down, probably not much would happen. Its only when you hit some turbulence that you are likely to "excite" these flutter modes. The problem is that you can't usually get well past Vne in level flight - you have to dive to exceed Vne, then most people pull to recover. Pulling will usually makes things worse (but often it is an instinctive reaction). Slowing down with the smallest increase in loading on the airplane is probably best (Disclaimer - this is all theortically based as I am no test pilot). Proper flutter testing is very expensive and is done by flexing the structure in a controlled way, at a speed well below the predicted flutter speed, and measuring the response of the structure. Wind tunnel data can also help.

As I said at the top its a rather complex topic, difficult to cover well here. Hope this helps rather than confuses. Pete
 
You must log in or register to reply here.