How is Vne calculated?

In this month's Flying, Peter Garrisons article describes how Vne is determined for certified aircraft. Using his method and plugging in number for an RV, I can't get it to align to Vans numbers.

Peter states the following

• Design Cruising Speed (Vc) = 33 * Sqrt (wing loading)
• Design Dive Speed (Vd) = 1.4 * Vc
• Never Exceed Speed (Vne) = 0.9 * Vd

His article includes comments that these are "general" formulas, and also that Vne can be limited below the calculated value for other reasons - specifically he discusses flutter.

My confusion is that if I plug in an RV wing loading (say 12.9 lb / sq ft for an RV-9A from Vans website), I get the following:
Vc = 118 kts
Vd=166 kts
Vne=149 kts or 171 mph

That Vne is too low - it comes out 33 kts or 38 mph below Vans published Vne. In fact it is below the listed cruise speed (189 mph) for a 160HP RV-9A.

For comparison I plugged in the numbers for a Cessna 172R (wing loading 14.1). The computed Vne is much closer, only off 6.9 kts from the published value.

Am I doing something wrong here, or is there more to the story?

I wonder if this is just a result of the fact that certified aircraft manufacturers are required to use more conservative calcuations than the experimental kit manufacturers choose to?

The numbers you quote are what's written in FAR/JAR-23 (http://rgl.faa.gov/Regulatory_and_G...0AC58EE778317F6085256687006D0B6E?OpenDocument). Vc may not be less than 33*sqrt(W/S) for normal/utility and 36*sqrt(W/S) for aerobatic category. Vd needs to be at least 1.4 Vc (1.5 for aerobatic), etc.

So those numbers don't have anything to do with the actual airplanes. It's minimum requirements according to the FAR/JAR. The real value for Vne most likely is the result of flutter calculations/tests with some safety margin.

An instructor I know states that Vne is the speed at which 1) the test pilot chickened out or 2) the maximum speed successfully achieved in flights prior to the crash of the prototype. I'm almost positive he's joking...

Not sure about the later models. (Things have changed a LOT at Van's) but on the early models Van himself stated that he tested to 210 mph, everything passed, and that's a number he was comfortable with.

V_a; V_c; V_d; V_ne

As Hendrik stated, these are minimum design speeds dictated in FAR 23.335. Certified aircraft must have design speeds equal to or higher than these design minimum speeds. So, there's no actual correlation between the FAR design speeds and the speeds the aircraft is designed to, other than the fact that certified airplanes must have design speeds that are *at least* what the FAR minimums specify.

Checking the numbers for the RV9 myself, here's what I get for minimum design speeds: (wing area = 124 ft^2; gross weight = 1750 lb. W/S = 14.11)

V_c = 123.9 keas
V_d (normal cat) = 173.5 which is V_c * 1.4
V_d (utility cat) = 185.9 which is V_c * 1.5
V_ne = 156.1 or 167.3 knots, depending on category. This is a 10% margin under V_d.

Note all speeds are KEAS, or knots equivalent airspeed. Equivalent speed is the same as indicated airspeed, corrected for installation and instrument error (and compressibility). Its essentially IAS. In FAR 23, limits presented to the pilot are expressed as IAS. The aircraft must be tested and found to be safe to operate at any IAS / altitude. (even though, it's TAS that's the driver for aero flutter, flight test should prove the design safe at all IAS's and altitudes)

V_ne, from the point of view of the engineer, isn't really the result of a single calculation. As mentioned in other threads, V_ne can be driven by a whole slew of possible factors, but its mostly a mission requirement of the airplane that's then supported by adequate structure for the speeds needed in the design.

Applying the FAR's, the engineer must design structure that can support the aero loads imposed on the airplane. Loading case studies are done that sample all possible flight conditions the airplane might be subjected to. We take speed, weight, altitude and CG - along with a list of maneuver type as required by FAR23 as variables and work up an aero loading case study to see what combination of the variables produce the highest loads. The study will produce hundreds, and maybe even thousands of possible combinations of speed, weight, altitude, CG and maneuver type which are then sampled to find worst case combinations.

V_ne is an interesting part of all this, because the FAR's require the airplane to be designed to operate at V_d which is above V_ne. And, where does that come from? Maybe the mission of the airplane would only require meeting the minimum FAR design speeds. Maybe the mission statement for the airplane would require operating well above the minimum speed requirements, such as for a high speed transport or military mission. In extreme cases, aero heating rates can drive design speeds (X-15).

The truth is the design speed numbers - V_ne and V_d - are selected based on the mission requirements of the airplane and then structure is designed to handle the resultant loads. Flight test is the last step in determining if the design is adequate. The airplane should be tested to V_d (above V_ne) at all altitudes up to its design ceiling.

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The above dissertation may not (probably does not) bear any resemblance to how the RV's are engineered. But it does help to show some of the factors that go into determining V_ne and to meet certified design criteria.

Rather than post all of the technical reasons why thsoe generic formulas are flawed, I highley recommend picking up a copy of "Aerodynamics For Naval Aviators." I personally think it should be mandatory reading even at a Private Pilot level, and is one of the "must have" books for a serious pilot to have in his professional library.

Long story short, wing area and loading are only two of about 20 variables in airfoil design that lead to specific performance numbers.


http://www.amazon.com/Aerodynamics-Naval-Aviators-FAA-Handbooks/dp/156027140X

Thanks for the education part (after all that is 50% of the reason we are here) - I'll look for that book too!

flion said:

An instructor I know states that Vne is the speed at which 1) the test pilot chickened out or 2) the maximum speed successfully achieved in flights prior to the crash of the prototype. I'm almost positive he's joking...

Click to expand...

Any type-certificated aircraft must be flight tested at speeds well in excess of VNE. FAR 23 requires that VNE be no higher than 90% of a speed that is demonstrated in flight test - i.e. the aircraft must be tested to at least 111% of VNE. Some manufacturers chose to conduct flight testing at speeds much faster than required by the FARs, as they want to make sure their design has very large margins. Other manufacturers do the absolute minimum.

If the RV-9A were designed and tested to FAR 23 criteria, it would have been tested to at least 233 mph (assuming that VNE is 210 mph - I don't own an RV-9A, so I'm not 100% sure of VNE).

I haven't buried my nose in the FAR's in a while, but FAR 23 doesn't apply to experimentals does it?

Sig600 said:

I haven't buried my nose in the FAR's in a while, but FAR 23 doesn't apply to experimentals does it?

Click to expand...

There is no requirement that experimental amateur-built aircraft be tested to or beyond VNE. That's why Kevin started his post with "Any type-certificated aircraft"....

Personally, I tested my RV-6 to 235 mph. It is not required, but recommended.

Mel is of course correct, but I've found that there is a lot of wisdom buried in those certification rules... and when I have a question about how something should be done I often check there first.

Like Dave says, even though it is not required, it can be a very good guide.