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VNE, IAS or TAS?

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  • VNE, IAS or TAS?

    A couple more questions on the Bearhawks. I'll ask them in separate post.
    I've noticed that the 4 place Bearhawk VNE of 175 mph is specified as IAS rather than TAS. I am not an aero engineer, but my understanding is that VNE is TAS. While I doubt there are many Bearhawks approaching 175 mph in level flight, in a steep decent after cruising in the mid teens TAS could potentially reach 175 mph TAS although the IAS would be much lower. Mostly because of where I live, I regularly cruise in the mid teens with a steep decent at the destination. My most frequent cross country is KPNA-KSHR. Plug that into Weathermeister.com and take a look at the terrain profile.
    So, TAS or IAS? This is probably a question for Bob.
    Also, the Bearhawk website specifies VNE for the Patrol as 165 mph. But doesn't say if that is TAS or IAS. Is this specified somewhere other than the website?

  • #2
    I wondered about this as well, so I checked a few sources. It appears that on a certificated airplane, all airspeed limits are IAS, not TAS. Vne is set to 90% of the maximum demonstrated (tested) airspeed (Vds), which is usually in a dive, to provide a small safety margin. Here's a good explanation from a DAR, who is also a certification test pilot. Several other sources who appear to be knowledgeable confirm that information.

    But there are two possible reasons for the Vds testing to be suspended where it was... The most common reason is that the aircraft had reached the maximum design speed, the stress limits of the structure itself. This scenario is 100% related to IAS, as it's basically the measure of the number of molecules of air hitting the wing/fuselage/tail/etc. per second during flight. No altitude adjustment is needed for that scenario.

    The other (less common) reason for Vds testing to stop is the onset of aerodynamic flutter at an airspeed below the maximum design speed. For various reasons, higher altitudes (reduced air density) does have a role in exacerbating flutter.

    So for aircraft where aerodynamic flutter is the reason Vds was set where it was, it would theoretically be possible for Vne to be reduced at high density altitudes (effectively making Vne TAS based). But here's an article published in Flying Magazine ("Vne and Flutter Explained") that explains how the FAA dealt with that, so it becomes a non-issue:

    [Vne varying with altitude] "is too complicated for us ordinary folks to work with on a daily basis, so the FAA made a law which basically says that aeroplanes must be able to operate safely up to the red line as long as they operate within their design parameters. These parameters include such things as gross weight and C of G. But they also include a maximum density altitude.

    This means that as long as you operate within these limits you don't need to worry about TAS and flutter, you will be on the safe side of that TAS provided you stay below the red.

    Basically, they have given you a buffer so you don't have to convert your ASI reasings to TAS to make sure you are safe."

    So, assuming Bob tested the airplane to Vds, then set the Vne accordingly (which would be the normal approach), the Vne for his designs should be IAS based. If that's not the case, all bets are off. But in any case, we – the builder / operator of our EAB aircraft – have the responsibility to establish the limitations for the aircraft we built. And that should be done via flight testing to confirm what Bob published for the airplane he built. The limits we set should not be "greater" than what the designer established, but I may find that I'm less comfortable flying with an Aft CG than Bob was, so I might only test to the aft CG limits with which I'm comfortable, and establish that number as the limit for my airplane. Ditto for VNE.

    With all that said... If you want to be really conservative, and have a way to indicate TAS in flight, you should be OK by limiting the maximum TAS you fly match the IAS Vne limit. So, for the 4-place and it's 175 mph Vne (IAS) limit, limit the TAS to 175 mph and you should be pretty safe. That's actually a pretty conservative measure.
    Last edited by JimParker256; 06-11-2019, 09:31 AM.
    Jim Parker
    Farmersville, TX (NE of Dallas)
    RANS S-6ES (E-LSA) with Rotax 912ULS (100 HP)

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    • #3




      Last edited by jaredyates; 06-11-2019, 02:06 PM.

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      • #4
        Hey Jared

        Your second link points to the Bearhawk VNE pdf as well. Not the Van's document.

        Cheers
        -------------------
        Mark

        Maule M5-235C C-GJFK
        Bearhawk 4A #1078 (Scratch building - C-GPFG reserved)
        RV-8 C-GURV (Sold)

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        • jaredyates
          jaredyates commented
          Editing a comment
          Thanks Mark, the editor has been doing some weird link parsing. Somebody should fix that.

      • #5
        I am not the bible on the subject, and don't think I have ever read "the truth". TAS based Vne never made sense to me. Vans is the only place I have read it. Everything I have flown in the last 30 years had a IAS limit (usually CAS actually), and a mach limit on the high side. The CAS limits were for various reasons. Sometimes structural (too much pressure). Most of them I don't know what the reason for the limit. At higher atlititudes your IAS can be well under half your TAS.

        I would assume light aircraft it is dynamic pressure, or flutter causing the limit. To me, both are caused by IAS, not TAS. I would love to know truth. Flutter is caused by the speed induced dynamic pressure interacting with the natural frequency of the flying surfaces. I don't think it knows or cares how fast you are going, just how much pressure there is.

        But, I think Vans knows what he is talking about. So I am not sure.

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        • #6
          Thanks for the responses! I think limiting myself to 175 mph TAS in a decent should be easy enough and not be encroaching on any safety margins. I’m not interested in finding out where flutter begins! Or ends!

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          • svyolo
            svyolo commented
            Editing a comment
            Using TAS is definitely the conservative choice. Less of a factor for a BH than for something slicker.
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