VA

[mdenney]

I am having a problem with Va in my C182RG.

The published Va speed is 112kts but recently I found a formula for Va which states VA= Vs * sqroot of n limit.

N limit for the C182 is published as 3.8gs an its sqroot is 1.949.

1.949 * Vs (42) = 81.87KTS, this is << than the published Va of 112kts.

Why such a difference? Is the limit load really 7.1gs, which would give a Va of 112? If this is so, then why is it published at 3.8gs?

Thanks,

Max

 

[bob_albertson]

I am having a problem with Va in my C182RG.

The published Va speed is 112kts but recently I found a formula for Va which states VA= Vs * sqroot of n limit.

N limit for the C182 is published as 3.8gs an its sqroot is 1.949.

1.949 * Vs (42) = 81.87KTS, this is << than the published Va of 112kts.

Why such a difference? Is the limit load really 7.1gs, which would give a Va of 112? If this is so, then why is it published at 3.8gs?

Thanks,

Max

Look at FAR 23.3

Va is a function of the square root of weight 2 divided by weight 1 times the original Va - that gives the new Va speed. Manuvering speed varies by weight, not by Vne.

A cessna 182 is in the "normal category" and has G load limits of positive 3.8 to negative 1.52 (the negative load is 40% of the positive load except for acro which is 50% of the positive load limit--- Look at FAR 23.337).

Bob

 

[mdenney]

Look at FAR 23.3

Va is a function of the square root of weight 2 divided by weight 1 times the original Va - that gives the new Va speed. Manuvering speed varies by weight, not by Vne.

A cessna 182 is in the "normal category" and has G load limits of positive 3.8 to negative 1.52 (the negative load is 40% of the positive load except for acro which is 50% of the positive load limit--- Look at FAR 23.337).

Bob

I understand that Va is effected by change in weight however the formula that I gave, negated any weight. Va= Vs *sqroot of load factor (n).


Ref Aerodynamics for Naval Aviators pg.339

This formula works great on all Vn diagrams I have seen, which is I know is only valid for a certain weight.

Thoughts?

 

[bob_albertson]

I understand that Va is effected by change in weight however the formula that I gave, negated any weight. Va= Vs *sqroot of load factor (n).


Ref Aerodynamics for Naval Aviators pg.339

This formula works great on all Vn diagrams I have seen, which is I know is only valid for a certain weight.

Thoughts?

I'm not sure, but in thinking aloud...



....Vs varies with the location of the C.G. This means we use the Calibrated airspeed to figure Va. Since stall speed increases with the square root of the load factor we can say the square root of our maximum load factor (3.8) multiplied by the stall speed (Vs): (50 x 1.95 = 97.5) will be our maximum speed.

In your example Va = (42 x 1.95 = 81). If you add a the most aft CG location (perhaps) and with that higher stall speed you may get 112kts.....

Again, still thinking out loud and I'm not sure of the true answer.

Bob

 

[mdenney]

I'm not sure, but in thinking aloud...



....Vs varies with the location of the C.G. This means we use the Calibrated airspeed to figure Va. Since stall speed increases with the square root of the load factor we can say the square root of our maximum load factor (3.8) multiplied by the stall speed (Vs): (50 x 1.95 = 97.5) will be our maximum speed.

In your example Va = (42 x 1.95 = 81). If you add a the most aft CG location (perhaps) and with that higher stall speed you may get 112kts.....

Thats the best I can come up with, and I am pretty sure it's wrong

Thanks! I finally get it

I was using the published VS which was IAS. Looking at the performance section CAS (55)* sqroot of 3.8 = 107kts....much closer to the 111kts published.

You rock,

Max

 

[zzuf108]

VA would have to be the most misunderstood aircraft operating speed.
It seems this comes about by operational people not correctly reading or understanding an airworthiness design requirement.
Nowhere in FAR's will you find a requirement for VA to be related to stall speed and load factor.
You will find that VA minimum is related to VSxsqroot ultimate load factor, but the designer is free to choose any speed from VA minimum to Vne as the design VA. This is clearly spelled out in FAR 23.335 (c)
Similarly the only structural strength requirements for full control deflection at VA are for the fin, stabilizer, and ailerons. These requirements are detailed at FAR 23.423,441and 455.
As a design speed VA does not necessarily change with weight. However some manufacturers do give a VA v's weight but give no reason for doing so. It may have been handy, in the past, if the design VA was equal to VA minimum. The introduction of VO, some years ago, has made this now unnecessary.
Further information, from AC23-19A:
48. What is the design maneuvering speed VA?
a. The design maneuvering speed is a value chosen by the applicant. It may not
be less than Vs√ n and need not be greater than Vc, but it could be greater if the applicant
chose the higher value. The loads resulting from full control surface deflections at VA are
used to design the empennage and ailerons in part 23, §§ 23.423, 23.441, and 23.455.
b. VA should not be interpreted as a speed that would permit the pilot
unrestricted flight-control movement without exceeding airplane structural limits, nor
should it be interpreted as a gust penetration speed. Only if VA = Vs √n will the airplane
stall in a nose-up pitching maneuver at, or near, limit load factor. For airplanes where
VA>VS√n, the pilot would have to check the maneuver; otherwise the airplane would
exceed the limit load factor.
c. Amendment 23-45 added the operating maneuvering speed, VO, in § 23.1507.
VO is established not greater than VS√n, and it is a speed where the airplane will stall in a
nose-up pitching maneuver before exceeding the airplane structural limits.

As a result of an Airbus structural failure further information has been published in respect of VA and FAR 25 aircraft - the philosophy is much the same as for FAR 23 aircraft.
http://rgl.faa.gov/Regulatory_and_G...8AB55A430816ADE786257627004F06C6?OpenDocument

 

[zzuf108]

Funny, some days ago I made a post explaining some misunderstandings about VA between operational people and design engineers. The moderator has not permitted this to be shown - this is a pity, when considering the number of people who died in AA587 partly because the crew did not understand the structural aspects of VA.

 

[bob_albertson]

Funny, some days ago I made a post explaining some misunderstandings about VA between operational people and design engineers. The moderator has not permitted this to be shown - this is a pity, when considering the number of people who died in AA587 partly because the crew did not understand the structural aspects of VA.

It may be under review still. With all the new threads posted each day there are a lot of busy moderators. Your post may still show up, and you can always ask any moderator about it. If your post violated the TOS or has another issue (most likely not related to the content) then I'm sure they will be more than happy to allow you to make it compliant.

Either way, I doubt the moderators are not allowing it to be posted with no general reason in mind. So..... ask

 

[tonycondon]

moderators here do not screen every post before it is posted. i suspect the problem with your post had more to do with your account not being completely activated yet, I notice that you have only 1 post. Welcome to PoA!

 

[gismo]

I am having a problem with Va in my C182RG.

The published Va speed is 112kts but recently I found a formula for Va which states VA= Vs * sqroot of n limit.

N limit for the C182 is published as 3.8gs an its sqroot is 1.949.

1.949 * Vs (42) = 81.87KTS, this is << than the published Va of 112kts.

Why such a difference? Is the limit load really 7.1gs, which would give a Va of 112? If this is so, then why is it published at 3.8gs?

Thanks,

Max

42 KCAS seems awfully low for the stalling speed in a 182 at max gross weight. Are you by any chance using the indicated stalling speed and/or the flaps extended stalling speed? IIRC there's quite a spread between CAS and IAS at the low end of the 182's speed range and of course Va in the flaps up configuration has to be based on the accurate stall speed with them up. In any case you can be certain that the load limit of a 182 isn't 7g.