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Discover
Joined: 26 Feb 2007
Posts: 429
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 Posted: Fri Jan 04, 2008 11:42 am Post subject: Vne at altitude TAS or IAS? |
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Gil and others,
My last email citing Fred Kokaska was from the middle of a discussion on Vne at altitude, the subject Gil brought up. I would be interested in learning more. An article I recieved from Van's started me worrying about the potential for exceeding Vne at altitude in my Sabre Toothed Tiger http://www.vansaircraft.com/pdf/hp_limts.pdf
Being that the Tiger certified service ceiling is 13,800 and the potential for the 260hp Tiger to go much higher, the article caused me some concern. Especially thinking of making an emergency fast descent from say 20,000' and whether the Indicated Air Speed on the Air Speed Indicator would keep me from breaking the Vne....or actually causing a structural failiure.
I forwarded the article to Ron Levy asking for his input and he had a good regulations oriented answer and suggested I contact Kokaska to see how the IO540 STC was stipulated as far as Vne was concerned when he achieved his STC...you saw part of Fred's answer to me. Between what Fred and Ron said I concluded not to worry. Here is what Ron said :
Ned:
The article is correct in theory. However, per 14 CFR 23.1545(c), “If VNE or VNO vary with altitude, there must be means to indicate to the pilot the appropriate limitations throughout the operating altitude range.” Therefore, for an aircraft to be certified in the Standard airworthiness category, either the published VNE is valid up to the book service ceiling, or this issue would have to be addressed. Further, to obtain an STC with a bigger engine, the expanded envelope would have to be addressed or a limitation noted. You might ask Fred Kokaska if this was covered when they developed the STC.
Ron
In addition to what I quoted earlier Fred also said: Hello Ron and Ned
Well I read Mr. Krueger's article. I began having problems with his logic early on when he started by describing the markings on the airspeed indicator. He refers to Va , the design maneuvering speed as the "blue line" on the air speed indicator. Wow, that's news to me as Va is not required and never shown as a "limit"on the airspeed indicator. Multi-engine aircraft have an airspeed indicator blue line which represents Vyse (best single engine rate-of-climb speed).
The writer lost a few points right there.........
| Quote: | In CFR Part 23 certification the term TAS is never defined, recognized or addressed. They have V speed numbers for everything but all testing, structural flight testing, climb/cooling flight testing are all related to IAS.
They will require a correction chart be developed for installation error of the pitot tube so that test result data is really based on CAS after observed temp and installation errors are accounted for.
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A manufacturer of a new design must define and demonstrate (Vd) (Dive speed as part of the overall envelope) After demonstrating a dive at Vd the FAA established Vne (AA5B 174 KCAS) as 90% of demonstrated Vd (FAA takes away 10% as a safety factor) . I went through the Vne part of certification at 100% of Vne for an AA5B (174 KIAS), There were two Part 23 objectives in this flight test (flown by FAA test pilot) of verifying the prop does not overspeed (above 2700 RPM) at Vne. This was easy as the back stop in the CS prop would probably allow 220 KIAS before it ran out of pitch. On a fixed pitch prop this can be a show stopper and require reducing Vne to prevent the overspeed. The other was pitch and roll control and airframe vibration. The test pilot started at 10 K feet and pushed over at full power to establish 174 KIAS, did some 30 degree left and right rolling maneuvers. He commented (and I looked out the right side) on the fact the ailerons were both about 2 inches above the normal even with the flap position in level flight from airloads. While this looks spooky, he noticed no binding or control input problems that he did not like.
So, to answer your question .... I have never seen any reference to the term "TAS" in any certification requirement.
Sombody ought to follow up with the FAA to get the right A/S marking in the 2400 Lb Cheetahs that have the AA5B spar installed. They approved AA5B airspeed markings for the spar upgrade in Peter Otten's IO-360 STC so you have an approved example.
Cheers Fred Kokaska
What do you think,?
Ned
[quote][b]
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az_gila
Joined: 17 Jun 2006
Posts: 99
Location: Tucson, AZ
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| Posted: Fri Jan 04, 2008 12:05 pm Post subject: Vne at altitude TAS or IAS? |
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| Quote: | .......
Ron
In addition to what I quoted earlier Fred also said:
Hello Ron and Ned
Well I read Mr. Krueger's article. I began having problems with his logic early on when he started by describing the markings on the airspeed indicator. He refers to Va , the design maneuvering speed as the "blue line" on the air speed indicator. Wow, that's news to me as Va is not required and never shown as a "limit"on the airspeed indicator. Multi-engine aircraft have an airspeed indicator blue line which represents Vyse (best single engine rate-of-climb speed).
The writer lost a few points right there......... |
Don't knock any points off here...
The article was written by one Van for his own customers (RV builders and owners), and Van does make a big deal of the Va speed since it is relatively easy to damage a RV with the stick at normal cruise speeds...
All of the airspeed indicators Van sells DO have a blue line on them...
Just consider who the article was written for, and the fact that RVs do not have to meet "standard" FAA stuff...
Ken Krueger is part of Vans Technical Support staff...
http://www.vansaircraft.com/public/pers-gen.htm
gil A [quote][b]
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