Gedifroggy
5th Dec 2001, 00:46
On the diagram of the flight envelope JAR25-333 (b), VA is not coincident with the point A (Cn max, Vs)-JAR 331 explains the manoeuver at VA and starts the manoeuver at point A1 then up to A2- Why A2 is not coincident with the point A- (Checked on the FAA site same diagram)? Thanks :cool:
fantom
9th Dec 2001, 01:05
bonjour froggy type person. go to Flight Testing forum. there is enough there on Va to keep you going for the rest of your froggy life.
Genghis the Engineer
9th Dec 2001, 03:09
Mind you, I'm not sure we answered the question even there.
I have a sneaking suspicion that the diagram in JAR-25 may just be wrong.
G
Not 100% sure of what you are asking....hope this helps
Maneuvering envelope
http://www.access.gpo.gov/ecfr/graphics/ec28se91.035.gif
Gust envelope (now removed)
http://www.airweb.faa.gov/Regulatory_and_Guidance_Library/rgFAR.nsf/bf94f3f079de2117852566c70067018c/33cfeb3483910f0085256672004ea7f2/SectionRule/2.248A?OpenElement&FieldElemFormat=gif
Under FAR Section 25.331: Symmetric maneuvering conditions.
(a) Procedure. For the analysis of the maneuvering flight conditions specified in paragraphs (b) and (c) of this section, the following provisions apply:
(1) Where sudden displacement of a control is specified, the assumed rate of control surface displacement may not be less than the rate that could be applied by the pilot through the control system.
(2) In determining elevator angles and chordwise load distribution in the maneuvering conditions of paragraphs (b) and (c) of this section, the effect of corresponding pitching velocities must be taken into account. The in-trim and out-of-trim flight conditions specified in §25.255 must be considered.
(b) Maneuvering balanced conditions. Assuming the airplane to be in equilibrium with zero pitching acceleration, the maneuvering conditions A through I on the maneuvering envelope in §25.333(b) must be investigated.
(c) Pitch maneuver conditions. The conditions specified in paragraphs (c)(1) and (2) of this section must be investigated. The movement of the pitch control surfaces may be adjusted to take into account limitations imposed by the maximum pilot effort specified by §25.397(b), control system stops and any indirect effect imposed by limitations in the output side of the control system (for example, stalling torque or maximum rate obtainable by a power control system.)
(1) Maximum pitch control displacement at VA. The airplane is assumed to be flying in steady level flight (point A1, §25.333(b)) and the cockpit pitch control is suddenly moved to obtain extreme nose up pitching acceleration. In defining the tail load, the response of the airplane must be taken into account. Airplane loads that occur subsequent to the time when normal acceleration at the c.g. exceeds the positive limit maneuvering load factor (at point A2 in §25.333(b)), or the resulting tailplane normal load reaches its maximum, whichever occurs first, need not be considered.
(2) Specified control displacement. A checked maneuver, based on a rational pitching control motion vs. time profile, must be established in which the design limit load factor specified in §25.337 will not be exceeded. Unless lesser values cannot be exceeded, the airplane response must result in pitching accelerations not less than the following:
(i) A positive pitching acceleration (nose up) is assumed to be reached concurrently with the airplane load factor of 1.0 (Points A1 to D1, §25.333(b)). The positive acceleration must be equal to at least
http://www.access.gpo.gov/ecfr/graphics/ec28se91.033.gif
where --
n is the positive load factor at the speed under consideration, and V is the airplane equivalent speed in knots.
(ii) A negative pitching acceleration (nose down) is assumed to be reached concurrently with the positive maneuvering load factor (points A2 to D2, §25.333(b)). This negative pitching acceleration must be equal to at least
http://www.access.gpo.gov/ecfr/graphics/ec28se91.034.gif
where --
n is the positive load factor at the speed under consideration; and V is the airplane equivalent speed in knots
If you look at the historical changes that have been made to thei reg you find that
the 1964 amendment to FAR 25.331 (http://www.airweb.faa.gov/Regulatory_and_Guidance_Library/rgFAR.nsf/HistoryFARPartSection/8E9A6FCE98C5954085256672004EA630?OpenDocument) states
(1) Maximum elevator displacement at VA . The airplane is assumed to be flying in steady level flight (point A1, Sec. 25.333(b)) and, except as limited by pilot effort in accordance with Sec. 25.397 (b), the pitching control is suddenly moved to obtain extreme positive pitching (nose up).
Notice of Proposed Rulemaking. Notice No. 94-29; Issued on 09/08/94 (http://www.airweb.faa.gov/Regulatory_and_Guidance_Library/rgNPRM.nsf/2ed8a85bb3dd48e68525644900598dfb/cb596265d0548e8a85256923005c3c71?OpenDocument)
Amdt. 25-23, Eff. 5/8/70 (http://www.airweb.faa.gov/Regulatory_and_Guidance_Library/rgFAR.nsf/HistoryFARPartSection/9526350063C91FB685256672004EB051?OpenDocument)
(1) Maximum elevator displacement at VA . The airplane is assumed to be flying in steady level flight (point A1, Sec. 25.333(b)) and, except as limited by pilot effort in accordance with Sec. 25.397 (b), the pitching control is suddenly moved to obtain extreme positive pitching (nose up).
(2) Checked maneuver at speeds between VA and VD. The airplane is assumed to be subjected to a checked maneuver (a maneuver in which the pitching control is suddenly displaced in one direction and then suddenly moved in the opposite direction) from steady level flight (points A1 to D1, Sec. 25.33 (b)), and from the positive load factor (points A2 to D2 , Sec. 25.333(b)) as follows:
(i) Unless lesser values could not be exceeded, a positive pitching acceleration (nose up) is assumed to be reached concurrently with the airplane load factor of 1.0 (points A1 to D1 , Sec. 25.333(b)). This positive acceleration must be equal to at least
http://www.airweb.faa.gov/Regulatory_and_Guidance_Library/rgFAR.nsf/bf94f3f079de2117852566c70067018c/9526350063c91fb685256672004eb051/SectionRule/0.10CE?OpenElement&FieldElemFormat=gif
where--
(a) n is the positive load factor at the speed under consideration; and
(b) V is the airplane equivalent speed in knots.
(ii) Unless lesser values could not be exceeded, a negative pitching acceleration (nose down) is assumed to be reached concurrently with the positive maneuvering load factor (points
A2 to D2 , Sec. 25.333(b)). This negative pitching acceleration must be equal to at least
http://www.airweb.faa.gov/Regulatory_and_Guidance_Library/rgFAR.nsf/bf94f3f079de2117852566c70067018c/9526350063c91fb685256672004eb051/SectionRule/0.2160?OpenElement&FieldElemFormat=gif
where--
(a) n is the positive load factor at the speed under consideration; and
(b) V is the airplane equivalent speed in knots.
(3) Specified control displacement. Instead of the conditions in subparagraph (2) of this paragraph, a checked maneuver, based on a rational pitching control motion vs. time profile, may be established in which the design limit load factor specified in Sec. 25.337 will not be exceeded. Unless lesser values cannot be exceeded, the airplane response must result in pitching accelerations not less than those specified in subparagraph (2).
(d) Gust conditions. The gust conditions B' through J', Sec. 25.333(c), must be investigated. The following provisions apply:
(1) The air load increment due to a specified gust must be added to the initial balancing tail load corresponding to steady level flight.
(2) The alleviating effect of wing down-wash and of the airplane's motion in response to the gust may be included in computing the tail gust load increment.
(3) Instead of a rational investigation of the airplane response, the gust alleviation factor Kg may be applied to the specified gust intensity for the horizontal tail.
Hope this helps...
Z
Keith.Williams.
10th Dec 2001, 02:33
Gedifroggy,
I cannot claim to know the answer to your question, but I can suggest something that might be.
The speed marked VA on the diagram provided by Zeke does not produce the JAR limiting load factor of 2.5. But the text appears to be describing the process for flight testing a prototype aircraft to establish its manoeuvre capability and perhaps VA.
If this is the case, then point A1 might represent an initial estimate for (guess at) VA. If the test at para ( C ) (1) is conducted at this speed and achieves a load factor less than the limiting value, then a slightly higher speed might be selected and the test repeated. This process will eventually lead to the identification of the speed at which A1 is directly below A, and the limiting load factor is achieved. This speed will be the true value for VA. Given the fact that the test involves pulling the stick back as quickly as possible, it would clearly be safer to start with a fairly low estimate of VA to avoid overloading the aircraft.
If the above suggestion is correct, then the diagram just represents one of the early stages before the true VA has been identified.