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1. 07:47 AM - Re: Re: VNE ((null) raykrause)
2. 07:09 PM - Re: Re: VNE (Steven Dortch)
Message 1
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Oscar,
Thanks for the math exercise! I really enjoyed it. I like it because it makes
my brain work a little harder, removes some cobwebs.
Nice to hear from you,
Ray Krause
Plugging along on the SkyScout...no hurry!
Sent from my iPad
> On Aug 24, 2017, at 9:02 PM, taildrags <taildrags@hotmail.com> wrote:
>
>
> There is indeed a lot of food for thought in that article. And since I can't
stand to read things like this without playing with numbers, I decided to run
some numbers for the Air Camper through the formulae that are presented in the
article.
>
> First, there is the theoretical "design cruising speed" Vc, which, for aircraft
with wing loadings of less than 20 pounds per square foot, is stated to be
33 times the square root of the wing loading. The "Improved" Air Camper plans
show a span of 29 ft and a chord of 5 ft, which gives 145 sq.ft., and a max gross
weight of 1,080 lbs (varies, depending on where you're looking). Resulting
wing loading is 7.45 lb/sq.ft., so 33 times the square root of that is 90 KIAS.
As the author observes, many airplanes can't actually cruise at this design
cruising speed and that's very likely the case for most Air Campers since
that translates to almost 104 MPH indicated. But let's continue.
>
> The article says that Vc provides the basis for determining the design dive speed
Vd, which is 1.4 times Vc or 126 KIAS (146 MPH indicated). Furthermore,
the never-exceed speed Vne is nine-tenths of Vd or 113 KIAS (131 MPH indicated).
The 1932 plans give a maximum speed Vne of 90 MPH (78 kt). It goes on to
say that if the airplane can't achieve its design Vd in flight, then the dive
speed actually attained in flight test, Vdf, replaces Vd, and Vne is nine-tenths
of that. I would be very curious to know if anyone has ever flight-tested
their Air Camper to maximum dive speed, or had one anywhere close to 131 MPH indicated.
I have not.
>
> Now for the interesting, if somewhat esoteric, part of the discussion. Vne is
an indicated airspeed, but the critical *flutter* speed may be a true airspeed
and as a result the margin separating Vne from the critical flutter speed
gets smaller as you gain altitude because true airspeed increases as the air density
decreases, and air density generally decreases with altitude. So for example,
if John Dilatush's turbocharged Subaru-powered Air Camper, operating out
of his home airport of Salida Colorado (7,523' MSL) is flown at 2500' AGL,
it would be at roughly 10,000 MSL. If it were summertime and the OAT were 95F
with a relative humidity of 30%, the density altitude would be 14,620'. If he
was flying at Vne of 90 MPH (78 KIAS), his TAS would be 97 kt. The article
states that the cumulative margin between Vne and critical flutter speed is 33%,
so in this (theoretical) case, that would be 26 kt. In the condition described,
this airplane could be flying within 7 kt of its!
> critical flutter speed but showing that it had not exceeded the designer's Vne
of 90 MPH (78 kt). In the mountains, in the summer, with thermals... could
the airplane gain another 7kt on a strong ridge lift or downdraft and hit critical
flutter? Does anyone really know how close to actual any of these numbers
are? Who developed the formulae and rules of thumb, and how much fluff or
slack did they put in the numbers?
>
> So numbers exercises aren't just for the fast planes. Even for us low and slow
flyers who may never see airspeeds in the triple digits, aerodynamic effects
are still in play. No, we're not all flying ticking time bombs here, but since
Steve raised the issue by posting the link to the article, I thought I would
exercise my calculator to see what the numbers might be for this what-if.
I like what the author says: "...if I were to ride a wave to 35,000 feet in a
172, I would not be in a hurry to peg the airspeed at redline on the way back
down." I think I would, though! Do you know how COLD it is up there??!!
>
> --------
> Oscar Zuniga
> Medford, OR
> Air Camper NX41CC "Scout"
> A75 power, 72x36 Culver prop
>
>
>
>
> Read this topic online here:
>
> http://forums.matronics.com/viewtopic.php?p=472162#472162
>
>
>
>
>
>
>
>
>
Message 2
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Okay, Oscar! What is your VNE!
On Aug 24, 2017 11:04 PM, "taildrags" <taildrags@hotmail.com> wrote:
>
> There is indeed a lot of food for thought in that article. And since I
> can't stand to read things like this without playing with numbers, I
> decided to run some numbers for the Air Camper through the formulae that
> are presented in the article.
>
> First, there is the theoretical "design cruising speed" Vc, which, for
> aircraft with wing loadings of less than 20 pounds per square foot, is
> stated to be 33 times the square root of the wing loading. The "Improved"
> Air Camper plans show a span of 29 ft and a chord of 5 ft, which gives 145
> sq.ft., and a max gross weight of 1,080 lbs (varies, depending on where
> you're looking). Resulting wing loading is 7.45 lb/sq.ft., so 33 times the
> square root of that is 90 KIAS. As the author observes, many airplanes
> can't actually cruise at this design cruising speed and that's very likely
> the case for most Air Campers since that translates to almost 104 MPH
> indicated. But let's continue.
>
> The article says that Vc provides the basis for determining the design
> dive speed Vd, which is 1.4 times Vc or 126 KIAS (146 MPH indicated).
> Furthermore, the never-exceed speed Vne is nine-tenths of Vd or 113 KIAS
> (131 MPH indicated). The 1932 plans give a maximum speed Vne of 90 MPH (78
> kt). It goes on to say that if the airplane can't achieve its design Vd in
> flight, then the dive speed actually attained in flight test, Vdf, replaces
> Vd, and Vne is nine-tenths of that. I would be very curious to know if
> anyone has ever flight-tested their Air Camper to maximum dive speed, or
> had one anywhere close to 131 MPH indicated. I have not.
>
> Now for the interesting, if somewhat esoteric, part of the discussion.
> Vne is an indicated airspeed, but the critical *flutter* speed may be a
> true airspeed and as a result the margin separating Vne from the critical
> flutter speed gets smaller as you gain altitude because true airspeed
> increases as the air density decreases, and air density generally decreases
> with altitude. So for example, if John Dilatush's turbocharged
> Subaru-powered Air Camper, operating out of his home airport of Salida
> Colorado (7,523' MSL) is flown at 2500' AGL, it would be at roughly 10,000
> MSL. If it were summertime and the OAT were 95F with a relative humidity
> of 30%, the density altitude would be 14,620'. If he was flying at Vne of
> 90 MPH (78 KIAS), his TAS would be 97 kt. The article states that the
> cumulative margin between Vne and critical flutter speed is 33%, so in this
> (theoretical) case, that would be 26 kt. In the condition described, this
> airplane could be flying within 7 kt of its!
> critical flutter speed but showing that it had not exceeded the
> designer's Vne of 90 MPH (78 kt). In the mountains, in the summer, with
> thermals... could the airplane gain another 7kt on a strong ridge lift or
> downdraft and hit critical flutter? Does anyone really know how close to
> actual any of these numbers are? Who developed the formulae and rules of
> thumb, and how much fluff or slack did they put in the numbers?
>
> So numbers exercises aren't just for the fast planes. Even for us low and
> slow flyers who may never see airspeeds in the triple digits, aerodynamic
> effects are still in play. No, we're not all flying ticking time bombs
> here, but since Steve raised the issue by posting the link to the article,
> I thought I would exercise my calculator to see what the numbers might be
> for this what-if. I like what the author says: "...if I were to ride a
> wave to 35,000 feet in a 172, I would not be in a hurry to peg the airspeed
> at redline on the way back down." I think I would, though! Do you know how
> COLD it is up there??!!
>
> --------
> Oscar Zuniga
> Medford, OR
> Air Camper NX41CC "Scout"
> A75 power, 72x36 Culver prop
>
>
> Read this topic online here:
>
> http://forums.matronics.com/viewtopic.php?p=472162#472162
>
>
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