Why is the VNE less on -6 versus -7 ??????????

[RV8JD]

Originally Posted by Northernliving

This report is worth a read for those that haven't seen it.
http://www.vansairforce.net/safety/G-GNDY.pdf

Originally Posted by jask

The Canadian report is a report on a 7 that came apart due to rudder flutter.

That's not quite correct. The report states that while it's certainly possible that the airplane came apart due to flutter, they could not rule out that the vertical stabilizer and rudder separated as a result of overstress due to being well past Maneuvering Speed.

From the report:

"At the time of separation, the airspeed exceeded the 124 knot manoeuvring speed. Full control movement is not permitted above this speed because the aircraft can be overstressed. Furthermore, at such airspeeds, it might be possible to overstress a component using less than maximum control movement, depending on the strength of the particular structure and the aircraft motion. Therefore, the possibility exists that the vertical stabilizer and rudder separated as a result of overstress."

and ...

"Findings as to Causes and Contributing Factors

1. After painting, the rudder was not likely balanced, nor the aircraft reweighed. As a result, the rudder was susceptible to flutter at a lower speed than designed and the aircraft was over the maximum aerobatic gross weight during the manoeuvres.

2. During the manoeuvring sequence, the speed of the aircraft reached 234 knots, exceeding the 124 knot manoeuvring speed and the 200 knot never exceed speed (Vne).

3. The aircraft encountered either flutter or overstress of some rudder components. Subsequently, the vertical stabilizer and parts of the rudder separated from the empennage during flight. Consequently, the aircraft became uncontrollable resulting in the impact with terrain."

Given the number of RV-7s that have come apart in-flight, if I had an RV-7, it would have an RV-8 rudder on it.

[RV8JD]

Originally Posted by David Paule

But that's only the flutter test airplane that goes to Vd. Every other airplane, even the flight test planes, only go to Vne, which is 10% lower.

Dave

Once the flight flutter test program is successfully completed, most manufacturers will take other test airplanes to Vd. That's because other disciplines have test objectives that require flight testing out to Vd to establish their margins above Vne, and all the required instrumentation packages for all the different disciplines can't be packed into one test airplane. Those disciplines include Stability & Control, Flight Controls, Loads, etc. Those airplanes do need to have calibrated airspeed systems or trailing static cones to ensure that Vd is not exceeded.

[DeeCee 57]

I own and fly a -6 with a -8 tail, so what is my Vne?

On a serious note, here an excerpt of my builder's manual, SECT15R3.DOC 08/13/01, page 15-21 of the late series -6:

Flutter testing of factory prototypes has resulted in establishing a NEVER EXCEED SPEED (Vne) of 210 statute mph for the RV-4 and RV-6/6A, 230 statute mph for the RV-7/7A/8/8A and 190 statute mph for the RV-9A. This speed was determined through flutter testing at a speed of 20 mph above Vne. (FAA certification criteria require flutter testing up to Vne plus 10% or about 20 mph). The flutter testing performed consisted of exciting the controls by sharply slapping the control stick at various speed increaments up to this level. Under all conditions, the controls immediately returned to equilibrium with no indication of divergent oscillations indicative of flutter. This testing was performed on factory prototype aircraft, and the flutter free flight operation of subsequent amateur built RVs has substantiated published Vne.

The ?slap-the-stick? method of exciting the controls for flutter testing is potentially dangerous and requires a very skilled pilot trained to recognize the subtle control responses which indicate the onset of flutter. For this reason, it is suggested that amateur builders do not perform flutter testing of their RVs.

[Thermos]

Originally Posted by RV8JD

Given the number of RV-7s that have come apart in-flight, if I had an RV-7, it would have an RV-8 rudder on it.

Been thinking about this too since I'm preparing to fly my -7 with a -9 rudder. The -14 rudder structure is based on the -9 but appears to be stiffer.

(Warning - possible thread creep!)

In an airplane that isn't certified to FAR 23, margins past Vne belong to the Van's engineers and I don't blame them for not saying exactly how far past Vne they tested the large-tail 7 lest someone decide to go exploring. But it would be nice to have some confidence that there's test data to substantiate a reasonable margin past published Vne.

The EAA Flight Test Manual recommends setting Vne 10 percent below the manufacturer's number. Granted, the manual isn't specific to RVs and some other homebuilts may be not as well tested but in the absence of hard information on -7 flutter/loads margin I plan to restrict Vne on my airplane. Maybe not by 10 percent, but in the absence of some authoritative word from Van's, my Vne won't be 200 KTAS.

Dave

[sailvi767]

The Canadian aircraft probably fluttered at 269 MPH. The recent poster claims 20% over VNE or 276 MPH.
G

[Marc Bourget]

I'm not familiar with other threads on this subject, but no mention was made of the suggested flight profile while conducting flutter tests.

John Thorp (no stranger to flutter) recommended (of course) that you start slow,

As test speeds increase beyond the aircraft's straight and level speed a dive is employed to reach about 5 mph IAS above the "test" speed.

The technique is to enter a slight climb until the speed drops to the "test" speed, when the excitation pulse is initiated.

The aircraft has already flown at the next "test speed" and is slowing through the test speed, providing some limitation to the frequency the aircraft is exposed to.

One T-18 flutter test left the pilot with a disturbing sensation, until he realized the frequency back through the stick caused a blister to his hand.

The second point, this test protocol only tests one control surface at a time. A thorough test program would witness combined tests of two or three surfaces at a time (combination of simultaneous aileron-elevator pulsing for example)

Third point, in the above example, the undetectable flutter (until the blister) was at a higher speed, the pilot, feeling he was being careful, experienced another flutter event at about 180 MPH because of structural damage caused in the first incident.

Fourth point, reflected in the FARs is if you have a trim tab on a control surface, the Vne series should be repeated with the control arm (actuator) to the tab disconnected to determine if a broken connection will not contribute to flutter.

Be careful!

[Tom Martin]

There are continuing debates regarding IAS and TAS when determining Vne.
At ground level the two speeds are very close. As you climb indicated airspeed drops off. This was never really a factor with Cessna 172, for example, as they seldom went high. With the RV and Rocket type aircraft many of us regularly fly over 10,000 feet and there can be a huge difference between indicated and true airspeed. The actual number is somewhere between but as we can not easily determine what that real Vne number is, then take the safe route and use TAS. Most of the modern screens offer that on the EFFIS screen. If not it is easy to use one of the aviation apps and make yourself a little chart for the dash.
For example IAS for 5000, 10,000, and 15,000 feet vs. TAS at using standard lapse rate charts for temperature.
At the very least work it out for yourself and you can see the huge difference in IAS vs. TAS at higher altitudes.
Watch out during descents with power the TAS number will go increase at very quickly.
Use TAS, it may save your life.

[rocketman1988]

"...Fourth point, reflected in the FARs is if you have a trim tab on a control surface, the Vne series should be repeated with the control arm (actuator) to the tab disconnected to determine if a broken connection will not contribute to flutter..."

I would HIGHLY discourage this kind of test...from personal experience.

I had a C-421 that had a "failure" of the bolt connecting the right elevator trim tab to it's linkage. (The failure was due to negligence of the company that did the annul inspection.) The failure occurred at 16,000 feet climbing to a cruise altitude, at a speed SIGNIFICANTLY less than Vne.

When the bolt came out of the linkage, the right elevator began to flutter in a divergent mode, resulting in an airplane that was nearly uncontrollable. Luckily, the two elevator torque tubes are joined in the middle with bolts through a flange. These bolts sheared, separating the elevators. It also stopped the flutter.

This happened in the space of about 5 seconds.

The aircraft was flown to a successful landing with one elevator in control and the other in trail...lucky to say the least.

Flutter is nothing to mess around with, and it doesn't matter if you THINK you are the ace of the base...

[jask]

Originally Posted by sailvi767

The Canadian aircraft probably fluttered at 269 MPH. The recent poster claims 20% over VNE or 276 MPH.
G

The 6 above has the short tail. If you have a 6 or 7 with the tall tail, I would be very respectful of the stated Vne.

[skylor]

Originally Posted by sailvi767

The Canadian aircraft probably fluttered at 269 MPH. The recent poster claims 20% over VNE or 276 MPH.
G

The -6/-6A has a VNE of 210 MPH. 20% over 210 MPH is 252 MPH.