Vne Testing

[LotusSeven]

Originally Posted by flyinga

Are we sure that flutter is the primary concern for Vne? Could it be the windscreen? Some other structure limit? Granted, flutter is the scariest but a colapsed windscreen could also ruin your day.

Bingo, that is why I said "[If Vne] happens to be dictated by flutter and not something else". At least none of us (that I know of) has gotten their RV up to sufficient speed to experience localized shocks on the control surfaces ala P-38

[RV8R999]

[quote=LettersFromFlyoverCountry;678661]Does exceeding it once mean the next time you exceed it you'll have the same outcome? [quote]

As has been stated previously - NO

From an engineering point of view and simplifying a bit, flutter can be modeled as a spring, mass, damper system which needs a forcing function to excite the the oscillatory behavior (2nd order approximation).

the spring (springs generate an opposing force due to displacement) can be accounted for due to material flexibility, actual mechanical spings in the system (aileron trim), hand-on-stick, etc..

The mass is the mass of the surface in question plus all connecting linkages (aileron, elevator, rudder, primer, paint, balance weights, etc..)


the damper (dampers generate an opposing force as a result of velocity) can be accounted for in system friction, hand-on stick, AND the fluid characterisitcs in contact with the surface in question.

The forcing function, in this case a moment about the control surface axis of rotation, is accounted for as the vector sum of all lift, drag and Nz components acting upon the surface.

Each of the necessary elements (spring, mass, damper, forcing function) have components associated with atmospheric condtions - primarily airspeed and density. Also, because the forces acting upon the surface are vector sums (the direction in which the force is acting is important) the machanical alignment of the surface in relation to the airflow is also a factor, therefore Gross Weight, CG (both lateral and longitudinal) are factors as manifested in changes in AOA and mechanical alignment for a given airspeed. Mass and friction in the system can vary from one plane to another because there is no way to ensure conformity between aircraft built in garages.

Therefore when any of the above variables changes the entire system dynamics are affected and results cannot be assumed to remain unchanged - in fact they will be different! You may not be able to perceive the difference with minor changes in any parameter but the system has changed.

[Bob Axsom]

Saying that it is based on TAS and not IAS puts on another layer of safety. The heavily instrumented and monitored Flutter test flight by flight test engineering is for design and requirement satisfaction verification and is not repeated on every production airplane. The design is not in question and you do not have to reverify it on every copy. I can guarantee you that the windshield does not shatter and the structure of an RV-6A with no counterbalance on the rudder of the short tail version does not start shaking at its VNE IAS even though I try to convince my race competition otherwise. If I ever start closing in on 300 kts flutter will be only one of the failure modes I will worry about.

Bob Axsom

[Jetj01]

Most pilots associated with spam can type flying, C-172's, Cherokee's, etc, never really worry much about VNE. In most cases it takes an extreme flight angle for these type aircraft to approach their designed VNE. In my experience some pilots also have a sketchy concept on TAS vs IAS and its meaning to their aircraft. However we who have ventured into the higher performance, especially homebuilt, market need to think about it and maybe 'get real' about it. My 180HP RV-4 cruises at 209 mph true air speed at 8,000' PA. That is just 2 knots under what Van's designed the RV-4 VNE max. Four days ago flying into San Antonio I was descending out of 10,500' PA, which was close to 14,000' density altitude and noted IAS below 180 mph but true airspeed, as measured on my Dynon and verified on my G496 at 227 mph TAS. Yes, well above designed VNE. The air was smooth and I monitored it as I descended and watched where TAS and IAS started to marry up, as it does when we descend...both being equal at sea level and on a standard day. I am admitting this is normal ops for me. Back when I had my Rocket that I bought I would routinely cruise at 190-200 KIAS, with airframe components that are exactly as I have on my RV-4 I built. The Rocket has an accepted higher VNE. When I did my Phase 1 testing with my newly completed and painted and W&B'd RV-4, I did incremental airpseed testing that exceeded published RV-4 VNE to determine the characteristics of my over powered aircraft and finished them knowing my RV-4 did not exhibit negative flying traits at an acceptable limit above published VNE. A 'safe' way to accomplish this testing is another discussion.

In short, if you cruise your RV type aircraft at cross country altitiudes- 10,500 Westbound and 9,500 Eastbound is gnereally where I am, and fly 'full throttle' at those altitudes, generally around 20 in MP, if you lower the nose, you WILL exceed VNE TAS, unless you significantly decrease power setting. It is just a fact. If you are one of those that NEVER exceed Van's posted limit, that is your choice and I'm assuming you understand TAS vs IAS. However, if you consider the 1000's of Vans aircraft flying out there, I am quite comfortable that the aircraft are designed, and if properly built, and verified, can exceed and do exceed published VNE routinely. I chose to test my RV-4 to a limit that I knew I would routinely fly and am comfortable with MY limit.

Not trying to convince anyone out of their opinion, just showing my point of view on it. Flutter, striking other flying objects, unbalanced aircraft etc are all serious business, as is flying in general, but an open mind to the reality of our aircraft and aerodynamics can be liberating.

"A man's gotta know his limitations"

[RBD]

Originally Posted by JonJay

Used when you hit a really bad shot that somehow works out.

It's called the "sister in law"
"Your up there laddie, but you know you shouldn't be."

Now THAT, is funny.

[Bob Axsom]

Originally Posted by Jetj01

Most pilots associated with spam can type flying, C-172's, Cherokee's, etc, never really worry much about VNE. In most cases it takes an extreme flight angle for these type aircraft to approach their designed VNE. In my experience some pilots also have a sketchy concept on TAS vs IAS and its meaning to their aircraft. However we who have ventured into the higher performance, especially homebuilt, market need to think about it and maybe 'get real' about it. My 180HP RV-4 cruises at 209 mph true air speed at 8,000' PA. That is just 2 knots under what Van's designed the RV-4 VNE max. Four days ago flying into San Antonio I was descending out of 10,500' PA, which was close to 14,000' density altitude and noted IAS below 180 mph but true airspeed, as measured on my Dynon and verified on my G496 at 227 mph TAS. Yes, well above designed VNE. The air was smooth and I monitored it as I descended and watched where TAS and IAS started to marry up, as it does when we descend...both being equal at sea level and on a standard day. I am admitting this is normal ops for me. Back when I had my Rocket that I bought I would routinely cruise at 190-200 KIAS, with airframe components that are exactly as I have on my RV-4 I built. The Rocket has an accepted higher VNE. When I did my Phase 1 testing with my newly completed and painted and W&B'd RV-4, I did incremental airpseed testing that exceeded published RV-4 VNE to determine the characteristics of my over powered aircraft and finished them knowing my RV-4 did not exhibit negative flying traits at an acceptable limit above published VNE. A 'safe' way to accomplish this testing is another discussion.

In short, if you cruise your RV type aircraft at cross country altitiudes- 10,500 Westbound and 9,500 Eastbound is gnereally where I am, and fly 'full throttle' at those altitudes, generally around 20 in MP, if you lower the nose, you WILL exceed VNE TAS, unless you significantly decrease power setting. It is just a fact. If you are one of those that NEVER exceed Van's posted limit, that is your choice and I'm assuming you understand TAS vs IAS. However, if you consider the 1000's of Vans aircraft flying out there, I am quite comfortable that the aircraft are designed, and if properly built, and verified, can exceed and do exceed published VNE routinely. I chose to test my RV-4 to a limit that I knew I would routinely fly and am comfortable with MY limit.

Not trying to convince anyone out of their opinion, just showing my point of view on it. Flutter, striking other flying objects, unbalanced aircraft etc are all serious business, as is flying in general, but an open mind to the reality of our aircraft and aerodynamics can be liberating.

"A man's gotta know his limitations"

Check out www.airventurecuprace.com. Even though today is the last day you can enter and get in the formal printed program, if you use the "Contact Us" button I'll bet Eric Whyte can even get you in there. He said in an email earlier that he is waiting on the mail today before he goes to print - so you two could exchange info today and satisfy that requirement as well.

Bob Axsom

[Jetj01]

Bob,

I have wanted to try this fun type racing ever since I first heard of it but my j-o-b does seem to get in the way of such endeavors! Such as next week I'll be international! But I'm thankful I get to fly for a living so can't complain about said J-O-B and its timing to much! First chance that it all lines up tho, I'm there! Doing the condition inspection on my plane today!

[dmaib]

Originally Posted by JonJay

Used when you hit a really bad shot that somehow works out.

It's called the "sister in law"
"Your up there laddie, but you know you shouldn't be."

I am going to remember that one!

[Tandem46]

Originally Posted by flyinga

Are we sure that flutter is the primary concern for Vne? Could it be the windscreen? Some other structure limit? Granted, flutter is the scariest but a colapsed windscreen could also ruin your day.

Pretty sure it's flutter of the tail. From Van's:

Because of the higher Vne, the RV-7/7A MUST HAVE the thicker 0.020" thick elevator and rudder skins AND a counterbalanced rudder. The thinner 0.016" thick skins that are standard on the RV-6/6A, and the older RV-6/6A rudder without the counterbalance, are NOT FOR USE ON THE RV-7/7A.

[ChiefPilot]

Originally Posted by mike in phx

Can you please tell me what you have going on here mr chiefpilot
I would like to hear your explanation
Why all the "reference"
What is your obsession with vne and my airplane ????

I started this thread back when I started putting together the phase one test plan for my project. It has nothing to do with you except that some of the folks have contradictory posts between our two threads.

Since you've had your 6 go faster, what was your plan for expanding the envelope and do you have any data you'd be willing to share?