[eric_marsh]

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]

...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]

A confusing subject...

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/community/showthread.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]

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]

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]

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]

Listen to Sam's words!

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]

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]

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]

... 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.

 

[rvbuilder2002]

... 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.

I agree with Sam, Mel, and Bill. You could easily make things worse without even knowing it. Adding stiffness and mass at the same time would likely be taking you backwards.
RV's don't have a known weakness as far as flutter goes. What they do have is an unknown specific flutter speed. It can vary with conditions, age of the airplane (slop from wear in the control system), builder influences (workmanship quality, amount of paint applied, etc).

Every man made machine and structure has a failure point (or many depending on the job it does) On most of them that specific point is not know, but it is known that it was designed to have it conservatively beyond the normal everyday use of that machine or structure.
The Golden Gate bridge has a point that it would fail if over loaded. There is a very good chance that the designers don't even know exactly where that is, and hopefully we will never know...because they designed it with a conservative margin to account for unknowns, age, etc. The thing that would be foolish would be to start using the bridge in such a way that we reduced those margins without verifying how close to the limit we are getting.

 

[eric_marsh]

Fair enough. Thanks for the responses. I tend to be an inquisitive type and with a long history of hot rodding I like to understand why things do what they do. I can see that I'm not going to learn aircraft design overnight.

BTW, I've heard a couple people mention that the other VNE thread was contentious. Having reviewed it I consider it to be an excellent thread. The cross pollination of ideas like I found in that thread can be very informative as it tends to focus on interesting details that one might otherwise not even consider.

I'll also mention that a greater understanding of the issues involved makes me more inclined to tread cautiously when it comes to limits than I might if I were less well informed.

And yes, I will triple check those "rod-end-hinges."

Eric

 

[chunt0]

Let me throw in a couple of comments. Strength and stiffness are not the same thing. Stiffness affects frequency; strength, per se, does not.

I'm pretty sure Van's has tested each model at higher speeds than published VNE. Nothing in aviation should be normally operated right at the ragged edge. Safety factors are important. Unfortunately, if Van's told us exactly what their maximum test speeds were, many would be tempted to bump their own VNE accordingly. Since each airplane is different, this would result in unnecessary tragedies.

 

[breister]

Guys, this topic comes up all the time here.

First, EVERY aircraft is a compromise. The Vans line of aircraft happen to be TERRIFIC compromises, giving very good speed compared to production airplanes costing 5 times as much while also allowing aerobatics and very good rough / short field landing characteristics.

If you want faster, you have two viable options that don't turn you into either an Aerospace Test Pilot or an accident statistic:

a) Build or buy a Harmon Rocket - the flight testing has been done
b) Build or buy one of those annoying fiberglass airplanes you sometimes see passing you

Some folks like me who once got paid to fly supersonic and "got over" the urge to fly serious aerobatics went with option b. Oh, an occasional Harmon will pass me but he has to burn 4gph more than me to do it. The "next level up" is another $100k, so I'll be happy with what I got. But I did it knowing I am not interested in visiting my friends on grass strips unless I am VERY confident of a smooth field.

Why am I chiming in? Because I genuinely don't like to see accidents happen, and that seems to be the likely outcome if people start strapping on boosted IO-375s to a standard -6 or -7.

Choose your airframe according to your mission. There is something out there for almost every mission - even a plane carrying an IO-540 capable of 9 g's and landing on dirt strips and running circles around Vans and Lancairs in the same price category (Glasair II stretch with an IO-540, can be had in the same general price regime as a Vans or used Lancair 320/360).

But please - make holes in the air, not holes in the ground...

 

[logansc]

Bill: Do you happen to know the numbers on the Harmon Rocket? They would be helpful to me since my F1 Rocket is similar. The F1 manual does not stipulate IAS or TAS for the redline speed. I took the limitation to be an IAS limit (as is the case with most/all certified GA aircraft), but was of course, concerned about flutter at limit speeds.

Dove the aircraft during medium altitude testing to 225 KIAS at 12,000', thinking I was giving myself plenty of room relative to the (assumed IAS) limit of 250 knots. Bad math on my part: 225 KIAS at 12,000' on a standard day = 309 mph. It was at least 20 degrees warmer than standard, so I'm guessing I was a bit faster than that. Later learned that the 250 limit is TAS, so I was out in professional-test-pilot-land, unintentionally.

On the other hand, the airplane is routinely flown at very high speeds by airshow performers, racers, and others. Would be nice to know what the "community" thinks is a realistic "one-size-fits-all" speed limit for Harmons and F1's, if you happen to know.

Regards,


Lee...

 

[Bill Wightman]

Hello Lee,

No, I don't know the numbers on the F1. 309 is pretty freaking fast for this type of airplane!

Flutter has been related to TAS, but its been shown that the real flutter limit probably lies somewhere between the CAS and TAS values. So, there's a dynamic pressure component as well as a true speed component to the fluid dynamics, and of course, the structural frequencies and stiffness of the structure play a complimentary role. Your indicated of 225 matters in that regard.

I've looked only a little bit at the F1, and think its designed pretty much along the same lines as the Vans products, maybe with stiffer skins (?). I was surprised to see the same rod-end hinge hardware in place, as its not very stiff.

210 indicated at 12500 will equate to 250 true, so even an average RV could do that if headed downhill aggressively.

Wish I could help you with better advise or definitive information. One thing you can do is compute a few TAS values for altitude/airspeed and keep a little cheat sheet handy in the cockpit for reference. Or display it on your EFIS.

Unfortunately, I really have no idea what an acceptable flutter limit might be for the F1 fleet. 250? Even that might be just a guess....

 

[DanH]

If you're truly curious about flutter, your first stop is FAA AC23.629-1B:

http://rgl.faa.gov/Regulatory_and_Guidance_Library/rgAdvisoryCircular.nsf/0/371d5ea900ee1c1786256f2d0048ed41/$FILE/AC23-629-1b.pdf

(BTW, you may be familiar with 1A....1B is the 2004 rewrite.)

 

[Bill Wightman]

FAR 23.629 requirement

Yeah, Dan good point out. And that reg requires flutter testing out to 20% above max dive speed, V_d.

That means, Lee, if the F1 was tested as required by FAR, at least one airplane was subjected to 300 KEAS which would most likely be well over the 309 you did.

 

[Ron Lee]

Why am I chiming in? Because I genuinely don't like to see accidents happen, and that seems to be the likely outcome if people start strapping on boosted IO-375s to a standard -6 or -7.

Breister, why does this bother you? Weight? Speed? CG?

 

[David-aviator]

Fair enough. Thanks for the responses. I tend to be an inquisitive type and with a long history of hot rodding I like to understand why things do what they do. I can see that I'm not going to learn aircraft design overnight.

BTW, I've heard a couple people mention that the other VNE thread was contentious. Having reviewed it I consider it to be an excellent thread. The cross pollination of ideas like I found in that thread can be very informative as it tends to focus on interesting details that one might otherwise not even consider.

I'll also mention that a greater understanding of the issues involved makes me more inclined to tread cautiously when it comes to limits than I might if I were less well informed.

And yes, I will triple check those "rod-end-hinges."

Eric

Eric,
Your inquisitiveness is not to be put down but you should expand your view of the situation just a bit to gain a better perspective of what it is you want to know.

All of this ground has been plowed before. Airplanes are designed to perform a mission and it is no secret to get what you want, just don't take one design and try to use it for something for which it was not intended. You can not create super sonic performance out of a J3 cub.

I just spent this past week end at the USAF museum at Wright Patterson. It is incredible what man has created in flying machines in the past 100 years. The spectrum covers early rag wings to super sonic jets, there is no end to machines designed to perform one task or another, and most were very successful efforts.

That being said, the RV is a recreation airplane and a very good one if one recognizes its design intent and limitations. To mess with that design, from an amateur point of view, is asking for trouble. Leave it be.

If you want to go really really fast, find the resources and rescue a former military machine from bone yard in Arizona and lit it rip. It is guaranteed it will go fast enough.

 

[dedgemon]

How is certification Vne determined?

Bill,
This is timely since I was thinking about this subject this weekend. We've discussed this before wrt sailplanes.

We know the aeroelastic issues are driven by equivalent airspeed (more or less TAS) since there is an aerodynamic damping term. So this starts becoming an issue at higher altitudes for airplanes where flutter is the limiting issue (not all are).

My "suspicion" is that with the certified airplanes, that the manufacturer establishes a ceiling for the airplane and the establishes Vne knowing that max altitude value. The RV's don't have any ceiling limit and with all this power they can get really high.

I think that what you typically find in the sailplane community would work well here. The sailplane POH's (from the manufacturers) generally specify a single Vne in IAS up to a certain altitude (like say 10000' or something) and then they derate the Vne speed with altitudes above that level. This pretty much tells me that someone did some high tows and tested it around that altitude.

It would be interesting to have Ken Krueger write about this. Vans current guidance is a bit ... vague!

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.

 

[eric_marsh]

Eric,
Your inquisitiveness is not to be put down but you should expand your view of the situation just a bit to gain a better perspective of what it is you want to know.

All of this ground has been plowed before. Airplanes are designed to perform a mission and it is no secret to get what you want, just don't take one design and try to use it for something for which it was not intended. You can not create super sonic performance out of a J3 cub.

I just spent this past week end at the USAF museum at Wright Patterson. It is incredible what man has created in flying machines in the past 100 years. The spectrum covers early rag wings to super sonic jets, there is no end to machines designed to perform one task or another, and most were very successful efforts.

That being said, the RV is a recreation airplane and a very good one if one recognizes its design intent and limitations. To mess with that design, from an amateur point of view, is asking for trouble. Leave it be.

If you want to go really really fast, find the resources and rescue a former military machine from bone yard in Arizona and lit it rip. It is guaranteed it will go fast enough.

I gotcha. It seems that there is a somewhat different mindset in aviation from that which I'm used to. Take my bike, for example. If you want to double up the horsepower, just drop the compression then hang a turbo on it and if you're up to 300 hp on two wheels then go for it. I'm quickly learning that there's a different set of rules in aviation. As they say, you can't break break the laws of physics. But when a question comes to mind that I can't figure out for myself I've got to at least ask it, being of the belief that the only stupid question is the one that is unasked.

Quite honestly, I'm enjoying my exposure to a new realm of knowledge.

 

[dedgemon]

Same thing really

You can hang as much horsepower in the airplane as you want. No problem.
Of course it'll kill you just a quick as it will on the bike too!

 

[eric_marsh]

Yes but bikes don't just fall apart if you push them too hard. Not by themselves anyway. With the assistance of other immobile objects in the vicinity, of course, it's a different story.

 

[Bill Wightman]

It would be interesting to have Ken Krueger write about this. Vans current guidance is a bit ... vague!

Yup, I agree. We can guess and theorize all day long but the best and first source for technical information is Vans Aircraft. Not withstanding the lack of conformity in the build on E-AB airplanes, I think - given the safety concerns and product responsibility aspects - more specific information from Vans would be very helpful.

I think I'll work up some kind of sensible V_ne limit table for my RV8, since its gonna fly later this year and I'll share that on the board here.

 

[Ironflight]

Yup, I agree. We can guess and theorize all day long but the best and first source for technical information is Vans Aircraft. Not withstanding the lack of conformity in the build on E-AB airplanes, I think - given the safety concerns and product responsibility aspects - more specific information from Vans would be very helpful.

I think I'll work up some kind of sensible V_ne limit table for my RV8, since its gonna fly later this year and I'll share that on the board here.

Bill - I agree that specific, engineering data is what we want - but I am not sure that Van hasn't already given it. There are numerous articles in the RVator saying that the flutter limit is a TAS - what SPECIFICALLY do you want Van's to do that they have not already done? Send a letter to each registered owner (can't be done - don't know where all the kits are)? Take out a full-page ad in a magazine (Won't reach everyone - some folks don't subscribe)? Provide a table of TAS vs. IAS at altitudes and temperatures (that's pretty easy for anyone who has passed a PP test to do)?

I am with you that we want and need to know the information, but I believe it is out there. If you want an entity to do something SPECIFIC, you need to state SPECIFICALLY what it is you want....and then you can choose how you respond to their response. Fair?

Paul

 

[dedgemon]

out there?

perhaps. But I'm constantly amazed at how many pilots (both RV and non-RV) have never even heard of the issue. They simply KNOW that Vne is indicated.

Personally I would like a little more info on how Vne was determined by Vans for the various models. Call it, information for the anal retentive engineer in me.

 

[Flying Scotsman]

Yes but bikes don't just fall apart if you push them too hard. Not by themselves anyway. With the assistance of other immobile objects in the vicinity, of course, it's a different story.

Ever seen some of the crashes of the bikes at Bonneville, trying to set land speed records?

http://www.youtube.com/watch?v=_I4Yr2N6zss

http://www.youtube.com/watch?v=YkZjrmX3fzQ

Lots of these to enjoy out on the net...

 

[rv7boy]

Flutter Video

For the anal retentive engineers (as well as the inquiring minds that just want to know) among us...

There are several videos on the internet that illustrate the destructive nature of flutter. Here's one showing an RC model developing wing flutter and then suffering structural failure all in a matter of moments.

The brevity of time from initial indication of flutter to total destruction of the wing is worthy of respect. Choosing a Vne well below the first lowest critical airspeed seems to be a prudent way of avoiding flutter. I don't know how many people have experienced flutter and then lived to tell about it, but I don't want to be one of them.

Note to DR...this video was made near Ft. Worth. This wasn't one of your models was it?

Here's some more video links demonstrating flutter

B747

Piper Twin Commanche

Glider Wing

Glider V-Tail empennage there's some discussion on this one about the digital camera's effects upon the visual image

 

[Bill Wightman]

I am with you that we want and need to know the information, but I believe it is out there. If you want an entity to do something SPECIFIC, you need to state SPECIFICALLY what it is you want....and then you can choose how you respond to their response. Fair?
Paul

Paul, I actually did call Vans and ask for specific information - I spoke with Ken Krueger. I wanted to know if we needed to be restricted to TAS values for the never exceed speed, in light of the RVator article and since I own a nearly completed RV8. Maybe I need to re-read the article, but I don't recall it giving specific guidance on the need to change ops limits for the airplane. I thought this was a big deal since application of concept explained could easily force a reduction in V_ne of 30mph at altitude - or more.

The answer I got on the phone was very much the same as the article: there's no specific guidance in regard to V_ne as it relates to IAS, or TAS. The only information I got was to be "aware" of the true speeds.

Others here have asserted this entire discussion has turned into a tail chase, and I'm beginning to agree. We start with RV'ers like Eric who ask the airspeed question; then somebody like me makes an assertion that the ops limits haven't been changed; then someone else sees the information presented by Vans and says we have all we need to know and that should settle it.

Doesn't that sum it up? IMHO, the question should be settled by officially revising the ops limits - then there would be no motive to ask the question any more.

 

[Dave Cole]

FAR 91.319(b)

Bill, as I understand it, the aircraft kit manufacturer does not set the operating limitations for your aircraft.

FAR 91.319 Aircraft having experimental certificates: Operating Limitations is specific on this point:

FAR 91.319(b) No person may operate an aircraft that has an experimental certificate outside of an area assigned by the Administrator until it is shown that--
(1) The aircraft is controllable throughout its normal range of speeds and throughout all the maneuvers to be executed; and
(2) The aircraft has no hazardous operating charactgeristics or design features.

This regulation clearly puts the responsibility of defining the Operating Limitations for each individual aircraft on the person that performs the Phase 1 flight testing. It requires that, throughout the normal range of speeds and all maneuvers, it must be shown that the aircraft is controllable.

Van's can, and has, provided information that can guide you in developing a flight plan to establish your airplanes particular Op Limits. But they cannot define the limits for you.

 

[Bill Wightman]

Dave, Vans is only a kit manufacturer, whereas the builders of each individual aircraft are the "manufacturer" for their own airplane. That distinction is found elsewhere in the regs too, but most clearly stated in 8130.2F. So I agree with your observation there.

But lets back up one step, and look at the practicality of answering the question: where do the limits come from? Is every builder going to hire a consulting engineering team to figure out what its limits should be? Are the limits self determined by flight test? Can we sensibly go out and flight test the RV to see what the airframe load limits really are, and at what speed we will encounter flutter? Of course not, but the good news is that work has already been done and the data provided to us.

So, although I do agree with you about the builder needing to supply ops limits, the source we turn to are the numbers given by the kit manufacturer. There are cases that come up now and then that require ops limits to be adjusted but they are by far the exception and not the rule.

I agree the kit manufacturer doesn't "set" the ops limits for us, but they DO provide them for our use. And, absent a more detailed analysis/testing, those limits are what we use. Since Vans has done the engineering, done the testing, and substantiated the results more thoroughly than any of us will, they have determined the limits that nearly all RV's use today.

 

[eric_marsh]

For the anal retentive engineers (as well as the inquiring minds that just want to know) among us...

There are several videos on the internet that illustrate the destructive nature of flutter. Here's one showing an RC model developing wing flutter and then suffering structural failure all in a matter of moments.

The brevity of time from initial indication of flutter to total destruction of the wing is worthy of respect.

If I understand this correctly, whether flutter occurs in a skin or a control surface such as an elevator it is essentially caused by a harmonic resonance, yes? IFF this is true, then wouldn't it be prudent to utilize a mechanism to dampen the resonance? I understand that in doing so one weak link has been eliminated which will in turn expose the next weakest link in the chain, but isn't it a good thing to eliminate or minimize that first weak link and hopefully gain an additional margin of safety?

 

[rvbuilder2002]

If I understand this correctly, whether flutter occurs in a skin or a control surface such as an elevator it is essentially caused by a harmonic resonance, yes? IFF this is true, then wouldn't it be prudent to utilize a mechanism to dampen the resonance? I understand that in doing so one weak link has been eliminated which will in turn expose the next weakest link in the chain, but isn't it a good thing to eliminate or minimize that first weak link and hopefully gain an additional margin of safety?

Flutter is normally a portion of the aircraft , by some means, becoming excited at its natural resonant frequency. If that freq. is low enough the structure may be able to withstand it (as seen in some of the utube videos). I think flutter at higher freq. usually doesn't have a good ending.

The resonant frequency of a structure or object is related to a lot of
different things.

In the context of aircraft control surface flutter it is related to the mass of the control surface.
The balance of the control surface relative to its hinge point.
The stiffness or rigidity of the control surface.
The aspect ratio of the control surface.
The level of slop in the control system that activates the control surface. The elasticity of the control system that activates the control surface.
The stiffness of the airframe component (wing, stabilizer, etc) that the control surface is attached too.

And probably many others that I can't even think of.

My point is...It is much more complex than just deciding to make a skin stiffer. It would be very foolish to then assume that you had a higher VNE speed in the context of flutter resistance. Because of the interaction that all of these factors have with each other you could have actually made it worse.

 

[breister]

Breister, why does this bother you? Weight? Speed? CG?

My concern is based on comments from multiple RV flyers saying that these combinations (boosted bigger motor on an RV) will easily exceed the Vne for Vans aircraft at higher altitudes. Face it, if you strap a turbo on one of these someone will be tempted to fly much higher, and thus easily be able to achieve or exceed spec sea level IAS numbers which means a much higher TAS.

So, my point is that you can experiment with boosting speeds over Vne on an airframe knowing that ultimately you may become a statistic, which I hate to see, or you can choose a platform built for the speeds you really want to fly at and already tested and approved for those speeds.

 

[frazitl]

I think we need some real world data...

Given the range of participants here, and the breadth of experience, I suggest we create a data base of Vne tests actually completed by model. I wouldn't want to have this degenerate into a "mine are bigger than yours" forum, but many of us have tested our planes for Vne during the testing phase. I have a feeling this data is languishing out there and could collectively be much more valuable than the testing any kit builder could afford to fund. We have a range of builder skill sets involved (all above average I'm sure), and a community that needs this information to maintain the safety record of our beloved magic carpets.

We need someone with the skills to set this up. I'm not up to that task for a web based application. DR?

I'll start:

RV7A N457RV 6/15/09 IAS:220 DA: 7444 TAs: 246 Slap stick: pitch only no flutter symptoms

RV7A N457RV 6/15/09 IAS: 222 DA: 7444 TAS: 248 Slap stick: none
no flutter symptoms

 

[breister]

Given the range of participants here, and the breadth of experience, I suggest we create a data base of Vne tests actually completed by model. I wouldn't want to have this degenerate into a "mine are bigger than yours" forum, but many of us have tested our planes for Vne during the testing phase. I have a feeling this data is languishing out there and could collectively be much more valuable than the testing any kit builder could afford to fund. We have a range of builder skill sets involved (all above average I'm sure), and a community that needs this information to maintain the safety record of our beloved magic carpets.

There have also been concerns voiced that such testing might not be anything more than an over-confidence creator, because of the lack of consistency in the build process. Success one day for one person may not translate into success every day for every person.

Now, as much as I will try to discourage these experiments I will also say that one of the joys of freedom is the freedom to take risks. However, regrettably negative outcomes in these experiments can impact the entire community.

Finally, I for one try to avoid discussions of "which is better," because all are a compromise. I gave up sod and regular gas and the option to leave my bird out on the ramp year round for a few mph and a few mpg. Others will find a different "perfect balance."

Which brings me back to my original point: You should choose your airframe based on your intended mission.

Cheers all, and fly safe.