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5 Answers

What is the practical meaning/use to the pilot of Vno?

Asked by: 3945 views , , ,
Aerodynamics, Private Pilot, Student Pilot

 


Having trouble finding out the practical meaning of Vno.

I know it stands for V-normal.
I also found out that "VNo is a calculated speed based on the wing loading of the aircraft. For utility aircraft it is 33 times the square root of the wing loading. For aerobatic aircraft it is 36 times the square root of the wing loading."

I also at a Cessna site I see it described as
"Structural cruise speed "
and notice that Vno is HIGHER than the Va.
(for that Cessna 150 the Va is 97 KIAS @ 1600lb 88 KIAS @ 1300 lb and Vno is 107 KIAS.)

I find it defined also as--
"Vno is shown where the orange and green lines of the airspeed indicator meets.
It is called the structural cruise speed at which speeds must be below to avoid damage in turbulence."

That last definition sounds similar to (although not identical to) Va --- Manuevering speed.

To add to my confusion my POH for my Skyranger does list a Va but lists no Vno.
And on MY aircraft's airspeed indicator the bottom of the yellow band (top of the green band) is exactly at the list Va.
  Yet one of the above definitions suggests those colors meet a Vno --- not consistant with my research indicating Vno is HIGHER than Va.

Can you
(a) unconfuse me and others (everyone else I've asked is equally unsure of the practical meaning of Vno)/
(b) Can you speak to the practical meaning use of Vno?
(c) Is a flight examiner likely to ask me if I know my Vno (given that my POH doesn't list one, likely to be critical of me no knowing the Vno?)

Thanks for your time, knowledge, and help.

Alex

 

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5 Answers



  1. Ryan Miller on Apr 14, 2012

    Alex,
    The good thing about aviation is you will get 10 different answers from 10 different pilots and 10 different examiners.
    A simple definition to Vno is Maximum structural cruising speed (memory aid NO GO THERE speed) Its at the begining of the yellow arc also known as the caution range.
    If you are cruising along and experience severe turbulance or you make violent, abrut control input, it may overstress the aricraft and causing structural damage.
    Like I mentioned before you the speed will be at thebegining are of the yelow arc on most airspeed indicators. So not beign listed in the POH this is one way to know the speed.
    Hope this helps. Safe Flying!

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  2. Nathan Parker on Apr 14, 2012

    Vno is set so that a vertical gust of a particular velocity (33 fps) will not exceed the load factor limit of the aircraft.
     
    While this might bear a superficial resemblance to Va, the latter speed only ensures that full control deflection will not rip the flight control surfaces from the airplane.  Most manufacturers. however, also set Va to a speed that will prevent load factors that exceed the load factor limit, but this isn’t required by the regulation.  Va, can in fact, be set higher than this, which may be why it’s equal to Vno in your airplane.
     
    Vno isn’t an absolute protection against dangerous vertical gusts, but it does make them very unlikely.  Va may improve the protection, assuming that it’s set to a speed that will stall before exceeding the load factor limit, but stalling isn’t good, either.

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  3. Ryan Miller on Apr 14, 2012

    Sorry I typed the answer via my phone and my fat fingers got in the way. Hopefully it makes sense.

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  4. Best Answer


    Steve Pomroy on Apr 18, 2012

    Hi Alex.
    Very interesting question.  And you’re right, there is lot’s of confusing and conflicting information out there.  If you want a straight answer, you really have to dig into the certification standards.  Luckily, some people have already done that!
    To start with, Nathan’s answer is correct.  Although the 33 fps gust requirement is an old standard that was based on instantaneous application of the gust.  The more current version of the standard requires a 50 fps gust, but allows for non-instantaneous application because it takes some time for the moving aircraft to penetrate the gust surface.  At the end of the day, the two standards are roughly equal in terms of structural strength.
    If you’d like some more detail, check out my blog post on Turbulence Penetration.  If you’d like further clarification on Va, you can check out my blog posts on Va Here and Here.
    Cheers,
    Steve Pomroy
    http://www.skywriters.aero

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  5. Alex on Nov 20, 2012

    Thanks Steve,
    Most informative and well, if at times mind-challenging, article you provided the link to.

    Just happened to drop in on this old topic, and have some updated information,
    and a further question.

    The update is this:
    I found out why our Skyranger had no Vno either in the UK POH that unofficially came with it nor on the ASI:
    Although ours (like most Skyrangers in the USA) was certified as an ELSA, In the UK , from which the kits were modeled at the time this one was sold as a kit in the USA, their otherwise very strict certification standards don’t call out a Vno (although of course their engineers know of it) and don’t call for one on the ASI color bands. The green band thus ends at Va. Which on our copy of the UK POH and ASI is at 83 mph (Yes, our ASI is in MPH.)
    I calculated our Vno and it came out at 109MPH. Pausible because our Vne is124mph.
    Then I found another virtually identical Skyranger which DID have the greenband up above its stated Va …. right at 105mph. So I took the more conservative number of course, and, while leaving Va marked clearly on our ASI, exended our green bad up to 105mph.
    Realistically it’s a relatively moot point because in level flight as a practical matter we can at best cruise at about 93mph and rarely cruise more than 80 indicated air speed. But it’s good to know that we’ve got a bit more turbulance margin at than the original greenband (ending previously).

    Here, though, are some question (of the “I’m curious” type):
    You wrote in your article “An aircraft’s sensitivity to turbulence is based almost entirely on it’s wing loading (almost because dynamic stability will also play a role). Higher wing loading results in a smoother ride. Lower wing loading results in a wilder ride.”

    Seems to me that although lower wing loading (such as flying loaded well below MTOW) does indeed get you bounced around more in turbulance, on the other hand it means when you hit that turbulance, or do full deflection of the controls, there is less stress on the aircraft, and that you would have MORE tolerance and as a practical matter could fly in turbulance at higher speed s than the Vno or Va might indicate (as they’re set IIRR based on flying at MTOW) with less risk of structural failures than fully loaded… albeit as you say with a wilder ride.

    Did I get that right?

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