VNE dive for certification

[lee-wave]

May I ask the collective thoughts of members here please...

 

As a general rule of thumb the VNE IAS is reduced by 2% for every 1000 ft increase in altitude.  The permit test flight for an aircraft requires a dive to a VNE of 120knts...  if this dive is carried out at say, 3000ft, should not the VNE IAS be reduced to 113knts so as not to exceed the aircrafts flight envelope ...?

[Thruster88]

I think you will find for most non turbocharged piston aircraft VNE is an indicated airspeed. It certainly is for the Beechcraft Musketeer and Cessna 172. The turbo sr22 has a tas VNE, they can fly at 25,000 feet.

 

I haven't been able to find were Van's say VNE is IAS or TAS for different models.  

[djpacro]

Depending on the design basis for the detail, but taking FAR 23 as the typical one as most are consistent with this: Vne is defined as IAS, full stop, for little airplanes. Determination of flutter margins must take into account the maximum TAS that the airplane can achieve. Testing therefore, the test pilot takes the airplane to the maximum altitude then dives to the required test airspeed.

This article is useful reading: https://www.australianflying.com.au/news/vne-and-flutter-explained

 

(Earlier versions of FAR 23 required the use of CAS rather than IAS so you will encounter variations with altitude. Faster airplanes will have a scheduled Mach number limit as well).

 

You must read FAA AC 23-8C Flight Test Guide for Certification of Part 23 Airplanes for background - see Page 106 onwards. https://www.faa.gov/documentlibrary/media/advisory_circular/ac_23-8c.pdf

 

Then their guide for homebuilt airplanes at https://www.faa.gov/documentLibrary/media/Advisory_Circular/AC 90-89A.pdf

 

CASA also has their own flight test guide for home-built aircraft which is based on AC 23-8. https://www.casa.gov.au/airworthiness/standard-page/flight-test-guides as does the LAA http://www.lightaircraftassociation.co.uk/engineering/flight_testing.html

 

EAA too https://www.kitplanes.com/new-flight-test-manual-from-eaa/

[mkennard]

54 minutes ago, djpacro said:

Depending on the design basis for the detail, but taking FAR 23 as the typical one as most are consistent with this: Vne is defined as IAS, full stop, for little airplanes. Determination of flutter margins must take into account the maximum TAS that the airplane can achieve. Testing therefore, the test pilot takes the airplane to the maximum altitude then dives to the required test airspeed.

This article is useful reading: https://www.australianflying.com.au/news/vne-and-flutter-explained

 

(Earlier versions of FAR 23 required the use of CAS rather than IAS so you will encounter variations with altitude. Faster airplanes will have a scheduled Mach number limit as well).

 

You must read FAA AC 23-8C Flight Test Guide for Certification of Part 23 Airplanes for background - see Page 106 onwards. https://www.faa.gov/documentlibrary/media/advisory_circular/ac_23-8c.pdf

 

Then their guide for homebuilt airplanes at https://www.faa.gov/documentLibrary/media/Advisory_Circular/AC 90-89A.pdf

 

CASA also has their own flight test guide for home-built aircraft which is based on AC 23-8. https://www.casa.gov.au/airworthiness/standard-page/flight-test-guides as does the LAA http://www.lightaircraftassociation.co.uk/engineering/flight_testing.html

 

EAA too https://www.kitplanes.com/new-flight-test-manual-from-eaa/

Just finished reading the aricle in Australian Flying, very useful info. Amazing what you find on reading different threads.

[Yenn]

Vne is nearly always TAS, You can change that if the designer of the plane says it is not TAS. Vans aircraft which form a large part of the homebuilt fleet use TAS. You need to compute the TAS for the flight, because it is very easy to go past Vne if you are using Indicated air speed.

[facthunter]

Especially at higher altitudes. Vans have gone into this subject extensively. Nev

[rgmwa]

Link to a related article that Vans did some years ago that may be of interest.

https://vansaircraft.com/wp-content/uploads/2019/01/hp_limts.pdf

[Thruster88]

I got the original builders manual for the RV6a out and it says all the airspeeds are indicated, IAS. There is no mention of Vne being treated differently to the other V speeds.  For later Van's models the Vne is TAS. I feel such aircraft should have a placard adjacent to the airspeed indicator stating that Vne is TAS.

 

This is what the professional pilots say about Vne. https://www.pprune.org/private-flying/479052-vne-true-airspeed.html

 

What are new pilots taught about Vne?

Edited September 24, 2021 by Thruster88

[lee-wave]

Thanks for the replies and some very interesting links...

 

After some research and as discussed in the comments above VNE in the Jabiru (120knts) is max permitted IAS.   This airspeed is about 10% below VD which is the maximum design airspeed.   This provides a healthy margin of around 15knts before reaching VD.

So it is safe to dive to 120knts during the annual permit test flight at least below 5000ft.  Thereafter when flying at 120knts above 5000ft TAS gets very close to VD.   

[facthunter]

Personally I  wouldn't fly near any structural limit speed unless you had some good reason. You can always get a gust that will change things a bit. Flutter is something that can kill you. Check for any play in hinges or linkages. Jabs Cable system can wear though it's fairly reliable even if it doesn't have the best feel. Nev

[Yenn]

There is no need to fly to any speed in the annual inspection. That is done in the test flying stage. It would be possible to get a TAS higher than 15 kts above IAS in a lot of places in Australia. Older Vans designs pre RV6a use TAS for Vne. For example the RV4.

[Thruster88]

4 hours ago, Yenn said:

There is no need to fly to any speed in the annual inspection. That is done in the test flying stage. It would be possible to get a TAS higher than 15 kts above IAS in a lot of places in Australia. Older Vans designs pre RV6a use TAS for Vne. For example the RV4.

I checked my flight manual for the RV6a to be sure it was IAS for the Vne. The RV6 shares the wing and tail from the RV4 I think. They have been well tested in the form of the Harmon Rocket, a hotrod with a six cylinder engine of the Van's design with very minimal changes to the wing and nothing on the tail.

 

[rgmwa]

Just to confuse the issue, the RV-12 POH states Vne of 136 KIAS up to 16,000', but 136 KTAS (at higher altitudes - implied but not very clear in the POH).

This assumes both you and the plane can get that high. 

Don't ask me how Vans decided on those apparently contradictory limits, but I'm sticking with TAS at all altitudes.

 

[Thruster88]

A quick check would indicate the service ceiling for a RV12 is 12,000. Feet. At least we are all looking at our POH. The manufacturer writes the POH, for experimentals the manufacturer is the builder and they can put in it whatever they like. 

[rgmwa]

16 minutes ago, Thruster88 said:

A quick check would indicate the service ceiling for a RV12 is 12,000. Feet. At least we are all looking at our POH. The manufacturer writes the POH, for experimentals the manufacturer is the builder and they can put in it whatever they like. 

Vans currently say the absolute ceiling for the 912 iS and 912 ULS versions of the RV-12 is 19,200' and 14,600' respectively. Their service ceilings are 17,300' and 13,000'.  I assume these are at standard temp and pressure.

I took mine up to 12,400 once before deciding that was far enough for me, although the plane was still climbing.

[facthunter]

Where you can climb to has a lot to do with the air temp. Service and absolute are different definitions. Nev

[Yenn]

Be careful looking at RV flight manuals or POH. Who wrote them?

For my RV4 I wrote the POH and I am no expert on doing that. I used what others have done before as a guide and put in the numbers which came from my test flying.

Would you really accept a POH written by an amateur as being the last word in how to handle the plane?

[rgmwa]

Yenn, you're correct for all the RV's except the 12. Vans manufacture and sell the RV-12 as a completed fly-away LSA (SLSA), and they also sell the kit which the builder can register either as an ELSA or an experimental (EAB). If it's built as an ELSA, which most of the ones in the USA seem to be, it has to be an exact copy of the original with no deviations, and Vans then remains the registered builder - even though they didn't (crazy system). Therefore, because it can be either an SLSA (factory built) or ELSA (homebuilt copy of the factory original), Vans has to provide the POH and acceptance test procedures, so they write and publish those documents. However, if you choose to build the kit as an experimental (EAB), then you can write your own POH as per the other RV's, but why would you when it comes with the kit anyway.

 

Incidentally to add to the craziness in the US, once the plane receives its `pink slip' for airworthiness as an ELSA, the real builder is allowed to modify it in any way they like, provided they don't violate the LSA performance or design parameters.  In other words, it effectively becomes an experimental but with Vans still as the registered builder and with no control over any modifications. They must worry about that!  Over here, CASA doesn't care whether you register it as an ELSA or ABE. If you build it as an ELSA, you're the builder, not Vans. Consequently most if not all 12's over here are built as ABE. In the US, the perception is that an ELSA will hold its value better, so that's the main reason most seem to go that way. It also only needs a 5 hour Phase 1 instead of 25 or 40 hrs, so that's another reason.

 

Edited September 28, 2021 by rgmwa

[djpacro]

Often the critical flutter mode is at an intermediate speed i.e. it is not as simple as the higher the TAS the closer to flutter (although, yes TAS is the appropriate figure to consider). I know of one high wing GA airplane - fabric covered with struts where the flutter speed was around 100 kts - fixed with the appropriate aileron mass balance - Vne is 133 kts IAS.

 

This set of slides is from the FAA showing the typical  process of flutter analysis and testing https://www.kimerius.com/app/download/5784128936/Flutter+and+aeroelastic+stability.pdf

 

Page A-59 shows an example of flutter at less than Vd, design design speed, and the flutter margin improves at higher speeds such that there is no flutter at Vd and above.

 

[djpacro]

More real examples of flutter on GA aircraft where the crritical mode is way below Vne at http://acversailles.free.fr/documentation/08~Documentation_Generale_M_Suire/Aerodynamique/Flutter/Taming_the_flutter_monster.pdf