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10-25-2007, 09:09 PM
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Join Date: Jul 2007
Location: 1
Posts: 35
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VNE questions?
111111111111111
Last edited by Deven : 03-22-2009 at 10:58 PM.
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10-25-2007, 09:27 PM
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VAF Moderator / Line Boy
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Join Date: Jan 2005
Location: Dayton, NV
Posts: 12,256
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Deven,
I n general, Vne can be based on several different parameters. Sometimes, it is Dynamic Pressure, which is measured int eh cockpit as IAS. In the case of the RV's (according to Van, the designer) it is based on the speed at which we could encounter flutter. If you look at the equations and physics governing flutter, it is based on the actual velocity of the air molecules passing the surface, therefore, it is based on TAS.
You're right - this does not make it simple if you are flying along at altitude and go into a dive - you really can't tell from your airspeed indicator how close you are to the limit. One of the nice thing about the newer EFIS systems is that some of them can and do display TAS all the time. Mine does, and when I am cruising along at 8,000', a few blips of nose down trim without a power reduction takes me up to the 200 knot TAS limit fairly quickly, I usually descend that way in smooth air - nose over to 200 and hold that speed coming downhill, reducing power as required to stay below 75%, or further if I need a greater descent rate.
I'm sure others can expand on my simple explanation!
Paul
__________________
Paul F. Dye
Editor at Large - KITPLANES Magazine
RV-8 - N188PD - "Valkyrie"
RV-6 (By Marriage) - N164MS - "Mikey"
RV-3B - N13PL - "Tsamsiyu"
A&P, EAA Tech Counselor/Flight Advisor
Dayton Valley Airpark (A34)
http://Ironflight.com
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10-25-2007, 09:36 PM
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Join Date: Oct 2005
Location: Vancouver, WA USA
Posts: 908
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I have a question about this also as I have 200HP and a CS prop on my -4, I cruse at about 22” and 2400rpm, as I climb I can maintain this power level up to about 10k’ and still indicate the same or even slightly higher indicated airspeed as I did at 1k’.
My question is, if I maintain the same indicated speed from 1k’ to 10k’ how much true airspeed did I increase for each 1000’ I went up? Is there a rule of thumb??
Also if you check your ground speed in four directions is the average your true airspeed? I made a two way trip recently and my two way ground speed average cruising at 10k’ was 176 knots or like 203mph. So if my average ground speed was 203mph was I under the VNE of 210mph of true airspeed or does it even apply at all?? Of cores I no ground speed is not airspeed but is or can average ground speed be true airspeed??
I feel I need a real lesson in this area as I think my plane is operating near VNE often, I have asked CIF’s these questions several times lately and they don’t know anything more then me and that isn’t much!
Russ
Last edited by Russ McCutcheon : 10-25-2007 at 09:57 PM.
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10-25-2007, 10:29 PM
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VAF Moderator / Line Boy
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Join Date: Jan 2005
Location: Dayton, NV
Posts: 12,256
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The short answer to your question Russ is yes - you are probably operating pretty close to the flutter limit. (Now I might be very wrong in assuming that the flutter limit is the same for the -4 as it is for the -6,-7, and -8....they were designed at different times, and might very well have different design parameters - but the principles are the same.) Does that mean you are close to imminent disaster? I can't answer that, because we don't know how much margin there is int eh numbers. My personal experience in Phase 1 testing is that I have some margin....but again, I can't quantify it, because I can't ensure that I have truly tested the entire envelope - so I try and stay under Van's limit (well, I might occasionally see a few knots more, but I always bring it back into the box pretty quickly.
As for the increase in TAS at a fixed IAS with altitude, I forget the rule of thumb, but you can compute it with your trusty E-6B (or equivalent) - just assume a standard temperature lapse rate and a fixed IAS, and compute the numbers all the way up as high as you like.
In calm air, TAS and Ground speed should be the same thing, and while the formula to use to turn a four-way GPS run into true airspeed is a little more complex than just averaging the numbers, that is a decent ballpark figure for example purposes. I have no doubt that a -4 with 200 HP is going to be very close to the 200 knot limit, based on what a much heavier -8 with a 180 can do.
Once again, I know there are knowledgeable -4 drivers like Smoky who can probably give you more detailed advice, but this is one of the reasons Van is not hot on people putting much larger engines on his designs - they already operate pretty close to his defined envelope limits.
Paul
__________________
Paul F. Dye
Editor at Large - KITPLANES Magazine
RV-8 - N188PD - "Valkyrie"
RV-6 (By Marriage) - N164MS - "Mikey"
RV-3B - N13PL - "Tsamsiyu"
A&P, EAA Tech Counselor/Flight Advisor
Dayton Valley Airpark (A34)
http://Ironflight.com
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10-25-2007, 11:27 PM
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Join Date: Feb 2006
Location: Sherwood, Oregon
Posts: 236
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Doug needs a FAQ for this stuff...
http://www.vansaircraft.com/pdf/hp_limts.pdf
I remember reading in this forum that gliders often have a table posted on the panel for VNE vs altitude. The long flexy wings are easier to flutter, and gliders can see serious altitude in the right conditions. Anyone in a hurry to get down from the teens needs to understand VNE is based on TAS.
Regarding TAS increase relative to IAS as you climb, my whiz-wheel says roughly 3.2 knots per 1000 ft, at STP, at typical RV speeds (170kts). You realize of course that it is really IAS that is decreasing, due to less dense air producing less Q, or dynamic pressure at the pitot tube.
__________________
Dog is my co-pilot.
Ted Johns
RV9 emp & wings
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10-25-2007, 11:35 PM
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Join Date: Oct 2005
Location: Vancouver, WA USA
Posts: 908
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Well I?m not terribly worried about it as this -4 has proven its self in this configuration for the last 1400 hours and 18 years, so far the margin has been adequate, Also the empennage is not stock aerodynamically or in construction but in any case it would be nice to know how fast it is going and make an educated decision on how close to or how far beyond ?Vans? VNE your willing to go. Down low it will go and has been tested well beyond Vans numbers.
Russ
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10-25-2007, 11:49 PM
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Join Date: Oct 2006
Location: Melbourne, Australia
Posts: 427
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TAS varies with Density Altitude
Don't forget that TAS varies with density altitude, so your TAS might be a fair bit higher than you first thought...
My rule of thumb is ~7% more at 5k' ~17% at 10k', but these are density altitudes and assume, I guess, that you are ISA conditions at sea level.
A
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10-25-2007, 11:53 PM
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Join Date: Oct 2005
Location: Vancouver, WA USA
Posts: 908
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Thanks Ted and Paul
Ok at 1000? I am indicating 155knots and at 10,000? I am indicating 155 knots, I am doing + 3.2 times 9 = 28.8 +155 = 183.8 knots which is 210 or 211 mph which Is Vans VNE for an RV-4 I think, if that?s the case then I?m over by about 10 knots in decent from altitude in smooth air, good to know!
Russ
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10-26-2007, 12:01 AM
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Join Date: May 2005
Location: Santa Barbara, CA
Posts: 1,324
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Vne calculations
Here is a link to a "spagetti chart" to calculate TAS and Mach number. The whole Vne flog would be simple with a Mach meter since Mach numbers are corrected for temperature and altitude. 200 knots TAS, ISA standard day, is .30 mach. Not a very impressive number is it?
http://www.tscm.com/mach-as.pdf
John Clark
RV8 N18U "Sunshine"
KSBA
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10-26-2007, 12:06 AM
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Join Date: Oct 2005
Location: Vancouver, WA USA
Posts: 908
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Thanks Andy,
Your 17% came out closer to the actual numbers I thought I was doing and a bit slower then adding 3.2 per thousand feet, 17% puts 155 knots indicated at true of 181.35 knots if I did it right which is just under Vans VNE of 210mph.
Thanks also to Deven for asking the question!
Russ
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