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VNE

 
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steven.d.dortch(at)gmail.
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PostPosted: Wed Aug 23, 2017 7:32 pm    Post subject: VNE Reply with quote

Check out this article.
http://www.flyingmag.com/technique/proficiency/technicalities-are-you-feeling-lucky


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Lorenzo



Joined: 13 Aug 2014
Posts: 27
Location: Tellico Plains, TN

PostPosted: Thu Aug 24, 2017 4:27 am    Post subject: VNE Reply with quote

Thanks for posting. A lot of good speed info. here.


Lorenzo


From: owner-pietenpol-list-server(at)matronics.com <owner-pietenpol-list-server(at)matronics.com> on behalf of Steven Dortch <steven.d.dortch(at)gmail.com>
Sent: Wednesday, August 23, 2017 11:31 PM
To: pietenpol-list(at)matronics.com
Subject: VNE

Check out this article.


http://www.flyingmag.com/technique/proficiency/technicalities-are-you-feeling-lucky
[img]http://www.flyingmag.com/sites/flyingmag.com/files/styles/small_1x_/public/import/2010/sites/all/files/_images/201009/FLY0910_tech_674x674.jpg?itok=NSLgN5d6[/img]

Technicalities: Are You Feeling Lucky? | Flying Magazine
www.flyingmag.com
IT HAPPENED LAST MAY, DURING an air race in South Africa. An airplane was descending toward a turn point in a valley when the pilot of a following airplane saw what ...


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taildrags



Joined: 29 Dec 2009
Posts: 1532
Location: Medford, OR

PostPosted: Thu Aug 24, 2017 8:02 pm    Post subject: Re: VNE Reply with quote

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??!!


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Medford, OR
Air Camper NX41CC "Scout"
A75 power, 72x36 Culver prop
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Ray Krause



Joined: 09 Oct 2013
Posts: 436

PostPosted: Fri Aug 25, 2017 6:46 am    Post subject: VNE Reply with quote

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

Quote:
On Aug 24, 2017, at 9:02 PM, taildrags <taildrags(at)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 &quot;Scout&quot;
A75 power, 72x36 Culver prop




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http://forums.matronics.com/viewtopic.php?p=472162#472162











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PostPosted: Fri Aug 25, 2017 6:09 pm    Post subject: VNE Reply with quote

Okay, Oscar!  What is your VNE!

On Aug 24, 2017 11:04 PM, "taildrags" <taildrags(at)hotmail.com (taildrags(at)hotmail.com)> wrote:
Quote:
--> Pietenpol-List message posted by: "taildrags" <taildrags(at)hotmail.com (taildrags(at)hotmail.com)>

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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taildrags



Joined: 29 Dec 2009
Posts: 1532
Location: Medford, OR

PostPosted: Sun Aug 27, 2017 12:52 pm    Post subject: Re: VNE Reply with quote

Steve; see pic of my airspeed indicator, below. Bear in mind that I did not build the airplane.

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A75 power, 72x36 Culver prop
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PostPosted: Sun Aug 27, 2017 6:19 pm    Post subject: VNE Reply with quote

So did you pick 95 based on precise science and engineering, or is 95 fast enough?

On Aug 27, 2017 3:55 PM, "taildrags" <taildrags(at)hotmail.com (taildrags(at)hotmail.com)> wrote:
Quote:
--> Pietenpol-List message posted by: "taildrags" <taildrags(at)hotmail.com (taildrags(at)hotmail.com)>

Steve; see pic of my airspeed indicator, below.  Bear in mind that I did not build the airplane.

--------
Oscar Zuniga
Medford, OR
Air Camper NX41CC "Scout"
A75 power, 72x36 Culver prop




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taildrags



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PostPosted: Mon Aug 28, 2017 8:23 pm    Post subject: Re: VNE Reply with quote

Steve; the Vne came with the airplane ;o) No, it wasn't a mathematical operation like "top of the green is at 80, Vne is 1.2 times that... 96 MPH". No math involved. I can tell you that I've had the airplane up to a little over 90 in a dive and it did NOT feel like a happy airplane. I hope never to have to intentionally see 41CC at redline on the ASI. I now confidently cruise it at around 70 with 2350-2400 RPM on the tach, but it still likes to think it has an A65 on the nose so sometimes I throttle it back to 21-2200 and ease the stick back to hold it at 60-65.

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