VNE

eric_marsh · #1 04-30-2010, 05:04 PM

I read the material on the Van's site about horsepower and VNEs and have some questions. My understanding from reading that article is that the limiting factor is flutter. Is this flutter in the actual skin or is it movement in a control surface such as the elevators?

Why, for example, is it that the RV-7 has a higher VNE than a -6?

N941WR · #2 04-30-2010, 07:49 PM

Quote:

Originally Posted by eric_marsh

...Why, for example, is it that the RV-7 has a higher VNE than a -6?

The -7 is a different airplane than the -6 with different skins, control surfaces, longer fuselage, longer wings, longer HS and taller VS, rudder, etc. The -7 also has a higher GW and acro GW, thus it sounds like the -7 is just stronger than the -6. Since it is most likely stronger, it has a different resonance frequency, which could change the flutter characteristics.

Either that or Van?s simply threw a dart at a number on a board and used that as the Vne. Somehow I doubt that is how they picked the number.

Bill Wightman · #3 04-30-2010, 08:42 PM

Eric, this topic comes up now and then on the board. Over the years, airspeeds have crept up among these airplanes for a myriad of reasons. A few instances of flutter have been encountered, none fatal as far as I know. Another thread on this subject - which started a fire storm - is here:

http://www.vansairforce.com/communit...ad.php?t=41255

A flutter incident was once reprinted in the RVator; the same issue also contained an article talking about the Never Exceed speed for our airplanes. In that article, V_ne was technically described and related to True airspeed.

Even though that's a correct association (V_ne and TAS), general aviation aircraft are *supposed* to be designed around indicated airspeed limits. Hence, we place a red line on our airspeed indicator at V_ne. Its a fixed number on the dial, vs TAS which is altitude and temperature dependent. This is dictated by FAR, although not technically applicable to us since we're experimental aircraft.

To answer your question, MY GUESS is that flutter in the RV4/6/7, and 8 is probably restricted to the elevator surfaces. Several design factors contribute to increased flutter susceptibility:

- Relatively large, flat aluminum skins. These skins deflect easily under load, and therefore could buzz if pressed by a fast enough airflow.
- Using rod ends for hinges: We get adjustable hinge points with this scheme, but at the expense of a good stiff structure. IMHO, its not a great way to hinge a control surface.
- All mass balance placed at the tip of each elevator. With all the mass at the tip, the reaction forces are driven into the outboard elevator hinge, which then transfers those loads into the outboard horizontal stab. And, as you may guess, the most outboard portion of the structure is also where we get the most deflection.

All that adds up, in my view, to control surface flutter in the elevators. Not structural flutter (that's where the entire horizontal stabilizer gets into the act). Your supposition about the skin fluttering is probably correct, but it would be felt as a buzz at high frequency, not a shake in the stick.

At any rate, now that we know the limitation IS there, I'd keep an eye on the TAS's but still mark V_ne as a red line on the airspeed indicator, as required by Vans.

... and make sure you put those "rod-end-hinges" in tight

Snowflake · #4 04-30-2010, 10:43 PM

Flutter is highly dependent on the particular configuration of the aircraft in question. While I would not be surprised if the speed at which flutter occurs is in the same ballpark for each RV model, I would not be at all surprised if someone said the flutter speed for a -7 was lower than that for a -6. Not because I think the -6 structure is any stronger than the -7, it's just that flutter can happen in different airframes at different speeds for different reasons. Because there are so many changes between the -6 and -7 in terms of the structure, it's not possible to armchair a guess as to which is higher.

I've heard a few reports of people taking -4's over 250mph with no flutter. It's possible the same has happened in -6's, it could easily happen on the downline of a screwed-up aerobatic manoeuver.

It can be triggered by loose control surfaces, as someone mentioned... Trim tabs are particularly bad for this, when they flutter it's not unheard of for an elevator to depart an airframe shortly thereafter.

eric_marsh · #5 05-01-2010, 07:03 AM

Thanks for the replies. What prompted my question is the thought that if there is a known weak point then would there be an easy to implement way to increase the margin of safety in regard to it?

A while back I was trying to identify a skin and someone asked me to measure it's thickness. When I did so I was told that the previous owner was probably trying to fabricate a thicker skin. I made the connection between that and the possibility of flutter and thus asked about it.

If this skin is actually the source of flutter then I've got a thought that I'd like to toss out. How about filling the part in question with lightweight expanding closed cell foam. It would have to be closed cell because open cell foam can absorb and hold water. I'd think that foam, especially foam that's somewhat pliable and not rigid, would have a dampening effect on the sheet metal.

Sam Buchanan · #6 05-01-2010, 08:14 AM

Quote:

Originally Posted by eric_marsh

Thanks for the replies. What prompted my question is the thought that if there is a known weak point then would there be an easy to implement way to increase the margin of safety in regard to it?

A while back I was trying to identify a skin and someone asked me to measure it's thickness. When I did so I was told that the previous owner was probably trying to fabricate a thicker skin. I made the connection between that and the possibility of flutter and thus asked about it.

If this skin is actually the source of flutter then I've got a thought that I'd like to toss out. How about filling the part in question with lightweight expanding closed cell foam. It would have to be closed cell because open cell foam can absorb and hold water. I'd think that foam, especially foam that's somewhat pliable and not rigid, would have a dampening effect on the sheet metal.

Eric,

Congrats on your new project!

As someone who has been in the RV community for over a dozen years, and as an EAA Technical Counselor who has inspected many RV projects, the best advise I can give you is:

Build your RV per the plans or Vans' service letters with no airframe modifications.

The RV is a design that has been proven by hundreds of thousands of hours of field history. Airframe modifications that are carried out via the TLAR school of engineering (TLAR=that looks about right....) are executed at the builder's peril. An inquisitive approach is fine, but departures from the design must be made only after careful engineering studies are made.

Best wishes for an enjoyable journey through the exciting world of custom-built aircraft!

Mel · #7 05-01-2010, 08:22 AM

I've been an EAA TC since the early '80s and I've inspected more than a few myself as both a TC and DAR. I can't emphasize Sam's comments enough. Build it per the plans period!
If you MUST make structural modifications, please consult an aeronautical engineer first.

Toobuilder · #8 05-01-2010, 08:24 AM

Quote:

Originally Posted by eric_marsh

Thanks for the replies. What prompted my question is the thought that if there is a known weak point then would there be an easy to implement way to increase the margin of safety in regard to it?...

The likelihood of finding a single "weak point" that could easily be fixed in a mature design like the RV series is small. Most likely, an attempt to "beef up" one element of the structure would create a new problem somewhere else. Aircraft structures are a system, and have to be considered as such.

Also, expanding foam in an aircraft structure is generally a bad thing because even if it does not capture water (which it does), it eliminates ventilation and promotes corrosion. Also, it's heavy (a major factor when it's behind the hinge line), which will require even more weight on the counterbalance horns, which will then have a significant effect on weight and balance of the airplane due to the long moment arm of the tail. It's a slippery slope...

The question you really need to ask though, is how real is the problem you are trying to "solve"?. In general, RV's do not seem to suffer from flutter problems despite a huge sample population, great disparity in craftsmanship, and generally being flown faster than the designer ever imagined. I'd say that unless your airplane's performance is going to be a significant departure from the rest of the fleet, you are going to be OK. Just build it light, and build it well.

Alan Carroll · #9 05-01-2010, 08:33 AM

Quote:

Originally Posted by Toobuilder

The question you really need to ask though, is how real is the problem you are trying to "solve"?. In general, RV's do not seem to suffer from flutter problems despite a huge sample population, great disparity in craftsmanship, and generally being flown faster than the designer ever imagined. I'd say that unless your airplane's performance is going to be a significant departure from the rest of the fleet, you are going to be OK. Just build it light, and build it well.

I'd add to this that some of the faster racers probably exceed the TAS Vne fairly commonly (particularly in descents), and Dave Anders' Triaviathon speed in the RV-4 was 244 mph. Not something I'd advocate doing, but it seems to show that modifications are not needed for normal operations.

Bill Wightman · #10 05-01-2010, 09:12 AM

... and I'll reinforce what Sam and Mel said: build it to the plans.

The tail surfaces on ANY aircraft are safety-critical components and often are a more sensitive point in the design of the airplane, both from a flying qualities point of view as well as from a structural point of view.

Given their nature, every builder needs to pay close attention to the build on the tail: stabilizers and control surfaces. Be very careful and critical of your work.

Adding weight to the control surface will also require more counter balance weight (and its NOT a 1:1 balance ratio). In the worst case, hanging more weight well aft of the h-stab's torsional center may open the door to structural flutter, which is catastrophic usually.