Determination of Vne & How to Increase It

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batesjoe

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I have searched through the forums and haven't seen this question raised yet. Hopefully, some insight is out there that can clear a few things up for me.

I am looking to build a single seat kit aircraft. This same aircraft model is also being put through the certification process for LSA in the US. In the end, you can buy a kit and build it yourself, or buy is ready to fly as an LSA. Therein lies my curiosity; one which (for obvious reasons) the manufacturer is reluctant to talk about.

The aircraft specifications list the cruise as 118mph with a Vne of 130mph. On the surface, the 118mph cruise is just within the LSA limitations. The listing of the Vne is likely based on the margin factor of cruise, as is common practice in light and slow aircraft.

As an experimental amateur built aircraft, my kit would not be subject to the LSA cruise speed limit. Without having the engineering data available, I have no way to know if the 130mph Vne is based on structural maneuvering loads or control surface flutter, some other factor or if it is simply a lawyered-up number to keep the sibling aircraft within LSA criteria.

Within the Experimental Category, how would I go about determining a safe increase in the Vne of the aircraft? Could incremental increases, say 2mph at a time be done within the Phase 1 testing period. Ideally, I feel this aircraft would be capable of 150mph, I just want to make sure it is done responsibly.

Any ideas?
 

bmcj

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Vne is often set not by engineered values, but by demonstrated speed during test flights. In those cases, a plane may be capable of being flown safely beyond the Vne, but YOU are the test pilot each time you do so. Typically, flutter is the big concern in establishing Vne. As such, you might demonstrate flight at a higher than Vne speed, not knowing that you are on the ragged edge of entering a flutter condition.
 

batesjoe

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Hello bmcj,

Thank you for your input. I am mostly concerned with flutter, hence my tip-toe speed increase idea. I truly don't want to operate the aircraft needlessly fast or on the edge all the time. By responsibly demonstrating an increase in Vne, I afford myself a comfort margin for a higher cruise speed. That is ultimately what I hope to achieve.

Various engine installations in this aircraft range from 22hp to 50hp with no distinction in the cruise or Vne with different engines. The only perceptible differences in data are the weights and climb rates. To me, it doesn't make sense that I would get a cruise of 118mph with 22hp or with 50hp regardless. That's why I think it could go faster, but is limited on paper to meet LSA and European UL criteria.

I want the bigger engine because the local conditions in the summertime have density altitudes approaching 10,000ft and I am surrounded by mountain ranges over 11,000msl. As a result, I would like to take advantage of the extra power and cruise more effectively. In fact, I have identified an engine within the size and weight limits of the manufacturer's data that is rated at 63hp. With that, and a good prop, I expect I could achieve phenomenal performance.
 

bmcj

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I just want to caution you that the onset of flutter can happen fast, with little warning, and may be capable of breaking the plane before you can react.
 

batesjoe

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bmcj,

Thank you! I have seen the scary effects of flutter in videos and I have felt onset of flutter as a mechanic on a test flight doing dive testing. You are absolutely correct. It is very rapid and violent. I wish there was a better way to know for sure. I hate to have an aircraft with a MGTOW of 533 lbs, fitted with 50hp or more, getting over 1500fpm climb rate just to have to throttle back to 65% to keep from busting Vne.
 

Jay Kempf

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bmcj,

Thank you! I have seen the scary effects of flutter in videos and I have felt onset of flutter as a mechanic on a test flight doing dive testing. You are absolutely correct. It is very rapid and violent. I wish there was a better way to know for sure. I hate to have an aircraft with a MGTOW of 533 lbs, fitted with 50hp or more, getting over 1500fpm climb rate just to have to throttle back to 65% to keep from busting Vne.
Who made it and is the designer still around to talk to?
 

autoreply

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3 criteria usually prevail:
*Forward loading. At VNE, your fwd lift component can be large enough to fold your wing fwd (max G flight, or gust load)
*Gust loading. A quick rule of thumb (though not holy) is to check Va. If VNE >sqrt(2)*Va, gust loading at VNE is larger than at Va and frequently limiting.
*Flutter.

Remember that point 2 also loads up the tail a lot, which might very well be the limiting case.

To be honest, with 3 fully balanced controls I don't think flutter in most conventional aircraft is a big deal. A ground shaker test gives certainty, plan 2-4 grand for one...
 

batesjoe

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Hello Jay,

The aircraft design is originally from the Czech Republic. The designer is a fella named Spacek. I have written them a few times and received responses that while informative, were also a bit off topic. Perhaps it is a language thing, but I finally determined that I wasn't going to get far with them.

The aircraft is the SD-1 Minisport. An American outfit (Skycraft Airplanes) in Orem, Utah is the new and sole dealer in the US. They are the ones going thru LSA cert and prepping to sell ready-to-fly LSA. I am interested in building it myself instead of buying out of the box. I have had great communications with the folks at Skycraft. They are quite helpful, but since they are not the designer, there is a limit to what they can really get into.

There are a few dozen of these aircraft flying, but none in the US yet. Several kits have sold here in the states, but I don't know anything about their progress. There are a few videos on YouTube of the aircraft.

The American LSA version of this aircraft would feature the Dynon skyview and 50hp Hirth F23.
 

batesjoe

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According to the manufacturer's specs, Va is 98mph. Therefore, gust loading calculation would lead to 138mph which is greater than the Vne of 131mph. I suppose the window for increase in Vne is not as large as I hoped.

I appreciate the info. I am the first to admit that I often know just enough to do something wrong, so I ask.
 

autoreply

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Using those numbers it SEEMS that it's G-loading, not gust loading that's limiting. Check the regulations, several of them (including Far23 I think) have a "shortcut" to calculate G-load due to gust loading (15/7.5 m/s) Then try to pin down at which speed gust-load and VNE G-load are similar. I'd be reasonably confident that by doing so you can make a safe extrapolation to a higher VNE, where you're still safe from gust load.

Then you'd only have to deal with flutter. Get an experienced test pilot that knows what he's doing (including flutter, which means very few pilots...) and get him to systematically test for flutter, or just source it out and do a ground vibrational analyses. Trying to let someone graduate on it might be an alternative if there is any major aerospace university nearby you (Alamogordo didn't ring a name, sorry)
 

batesjoe

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Thanks for the keen insights. The aircraft is specified at +4/-2 G's. I will research the FAR shortcut you mention and see what it gives me.

Fortunately, I do know a couple DER Pilots, but I am not sure of his availability. I know one was involved with flutter and Mach Rolls on the Lear45 and the other did flutter dive testing on a B767 SATCOM antenna installation. They are both very busy throttle jockeys. Maybe I can tease one of them with flying something under 12,500lbs for a change!

Not surprising Alamogordo doesn't ring a bell. An hour north of El Paso in the midst of tumble weeds and sand dunes, only interrupted with sand storms and sonic booms from nearby Holloman AFB and their F22s.
 

batesjoe

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Hello Cavelamb,

Well, there is that approach. I had to laugh, thanks. Hopefully, I won't have to ops check the BRS chute.
 

batesjoe

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Hmmm. At quick glance at FAR23, it appears that the "shortcut" calculation method would not be applicable in this case because the aircraft (SD-1 Minisport) is a T-Tail aircraft and the methods cannot be applied to T-Tails or Tube Tails. I will have to go back to the Mfg for more info because FAR23 references flight test proving up to 1.2 X Vd. Well, this number is not in the published specs for the aircraft. The quest continues!

BTW, Cavelamb, you were closer than you may have thought!
 
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fly2kads

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Backing up all the way to the beginning of this thread...the upper speed limit for an LSA is 120 kts CAS at maximum power at sea level. It is not a cruise limitation, as stated. Per FAR 1.1:

Light-sport aircraft means an aircraft, other than a helicopter or powered-lift that, since its original certification, has continued to meet the following:
...
(2) A maximum airspeed in level flight with maximum continuous power (VH ) of not more than 120 knots CAS under standard atmospheric conditions at sea level.

I don't mean to be nitpicky, I just want to make sure you're clear about what the limitations are. If you are going to fly it as a Sport Pilot, the aircraft needs to meet the LSA definition in sec. 1.1, whether it is certificated in the Light Sport or Experimental-Amateur Built categories.
 
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SVSUSteve

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I don't mean to be nitpicky, I just want to make sure you're clear about what the limitations are. If you are going to fly it as a Sport Pilot, the aircraft needs to meet the LSA definition in sec. 1.1, whether it is certificated in the Light Sport or Experimental-Amateur Built categories.
I see no problem in designing the frame to exceed the minimum standard (within reason and weight limitations obviously) in the event that someone who is a private pilot wants to put a good engine on the aircraft and turn the "sunny day noise maker" into a decent traveling aircraft. That said, I am willing to bet a lot of the "experimental LSAs" are only LSA performance limited in terms of cruise speed on paper if you catch my drift.
 

Dana

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One other thing to consider that has nothing to do with the main subject of Vne: If the company is getting SLSA certification and is also offering a kit, if it's an ELSA kit then it likely will be too prefabricated to meet the 51% criteria for E-AB.

-Dana

99 percent of lawyers give the rest a bad name.
 

batesjoe

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Hello fly2kads,

You are correct in the specific wording of the FAR. I would be operating this under a PPL, not sport pilot, so the LSA limit would not apply. It then becomes a matter of the aircraft design and capabilities.
 

batesjoe

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Hello Steve,

What you say is really the essence of my question/dilemma. I am searching for the best & most responsible way to determine if the published Vne is based on flutter, structural load limits, or administrative considerations to meet LSA or European UL criteria.
 
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