Van's Air Force
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VNE questions?
- Thread starter Deven
- Start date
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
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
Russ McCutcheon
Well Known Member
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
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
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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
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
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.
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.
Russ McCutcheon
Well Known Member
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
Russ
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
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
Russ McCutcheon
Well Known Member
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
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
John Clark
Well Known Member
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
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
Russ McCutcheon
Well Known Member
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
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
mgomez
Well Known Member
TAS = CAS + 1.5% per thousand ft
Up to about 10,000' or so, I use this rule of thumb: add 1.5% to CAS for every 1,000', and that's my TAS.
So I'm indicating 120 KCAS at 4,000', my TAS is 120 + (4 * 0.015 x 120) = 127.2 KTAS. (The correct number is 127.3)
If I'm indicating 120 KCAS at 10,000', my TAS is 120 + (10 * 0.015 x 120) = 138 KTAS. (The correct number is 139.6)
Up to about 10,000' or so, I use this rule of thumb: add 1.5% to CAS for every 1,000', and that's my TAS.
So I'm indicating 120 KCAS at 4,000', my TAS is 120 + (4 * 0.015 x 120) = 127.2 KTAS. (The correct number is 127.3)
If I'm indicating 120 KCAS at 10,000', my TAS is 120 + (10 * 0.015 x 120) = 138 KTAS. (The correct number is 139.6)
Low n Slow
Well Known Member
In the certified world where I?ve done the most of my flying, Vne is based on IAS and not on TAS. As my instructor used to say, the FAA doesn?t want pilots, in a potentially stressful situation, be required to do math in order to keep the wings or control surfaces on the airplane. This is one regulation I couldn?t agree with more and I wish it applied to the experimental world too.
During a descent when you have constantly changing temperatures and altitude and if you have steam gauges, just how the h_ll are you supposed to know the TAS at any given level?!? The need to use some ballpark estimate or rule of thumb formula to seems so amateurish to me.
In the glider example earlier in this thread, it was a Certified glider.
You may want to click on that link as well. In it, Van explains why the majority of spam-can (you might even say low n slow) pilots don't need to worry about TAS effects on VNE.
I myself have no experience with fast, high ceiling certified ships. Perhaps their POH goes into this issue. Perhaps their steam airspeed gauge is one of those fancy TAS reading types, those have been around a long, long time.
John Clark
Well Known Member
Vne indication
In the case of the big turboprop that I flew for a living, the airspeed indicator was IAS, but the red line (actually a striped "barber pole") moved as the altitude increased. At sea level it was 250 kts, at 25,000 it was 210 kts. Both those numbers are .50 Mach. Also, since it was a turboprop, the limitation was at Vmo not Vne, but that's another story.
John Clark
RV8 N18U "Sunshine"
KSBA
In the case of the big turboprop that I flew for a living, the airspeed indicator was IAS, but the red line (actually a striped "barber pole") moved as the altitude increased. At sea level it was 250 kts, at 25,000 it was 210 kts. Both those numbers are .50 Mach. Also, since it was a turboprop, the limitation was at Vmo not Vne, but that's another story.
John Clark
RV8 N18U "Sunshine"
KSBA
Thanks John. I suspected that there were fancier airspeed gauges than the one in the old Skyhawk I putt around in.
It's worth noting that a 180hp RV4 can exceed VNE in level flight, and has a solo weight ceiling of over 28000ft! At that altitude, I bet pointing the nose down until the IAS reads 183kts is going to get you in a heap 'o trouble.
John Clark
Well Known Member
FL280
Ted,
The numbers get a little frightening at that end of the scale. At 183 indicated the TAS would be around 280 kts. Yikes! Besides, it is way too cold up there. I have had my RV8 up to 22,000, but it isn't much fun.
John Clark
RV8 N18U "Sunshine"
KSBA
Ted,
The numbers get a little frightening at that end of the scale. At 183 indicated the TAS would be around 280 kts. Yikes! Besides, it is way too cold up there.
I have had my RV8 up to 22,000, but it isn't much fun.John Clark
RV8 N18U "Sunshine"
KSBA
pierre smith
Well Known Member
Adjustable AS
Hi Tom,
I've seen many adjustable airspeed indicators where you line up the temp with the appropriate altitude to see your TAS. After a few flights where you calculate your TAS on the E-6B, you'll have a good feel for what your TAS is or make up a small card to stick near the A/S indicator. Personally, I always have my E-6B in the side pocket....use it often,
Regards,
Hi Tom,
I've seen many adjustable airspeed indicators where you line up the temp with the appropriate altitude to see your TAS. After a few flights where you calculate your TAS on the E-6B, you'll have a good feel for what your TAS is or make up a small card to stick near the A/S indicator. Personally, I always have my E-6B in the side pocket....use it often,
Regards,
