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  #11  
Old 07-14-2010, 05:17 PM
jerryab jerryab is offline
 
Join Date: Dec 2008
Location: Northwest Arkansas
Posts: 40
Default Indicated or TAS

Just curious if you are talking indicated or True airspeed. I can get 160 kts true at 6500' 24/2500, indicating 138 kts.

I believe you guys, why shouldn't I? Just trying to evaluate my plane. Heading out this evening to Texas, will use new IPhone apt and get a better indication of what I'm really doing.

Thanks, Jerry
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  #12  
Old 07-14-2010, 05:25 PM
n5lp's Avatar
n5lp n5lp is offline
fugio ergo sum
 
Join Date: Apr 2006
Location: Carlsbad, NM
Posts: 1,912
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Quote:
Originally Posted by jerryab View Post
Just curious if you are talking indicated or True airspeed. I can get 160 kts true at 6500' 24/2500, indicating 138 kts.

I believe you guys, why shouldn't I? Just trying to evaluate my plane. Heading out this evening to Texas, will use new IPhone apt and get a better indication of what I'm really doing.

Thanks, Jerry
I would hope everyone is using TAS or whatever they explicitly state as IAS is pretty much meaningless for any performance comparison both due to the variation of IAS with altitude and other factors and with likely airspeed system errors.

By the way, there is a quick and dirty method of finding TAS that doesn't involve flying the triangle and higher math. Simply do a slow turn while watching the GPS ground-speed. By doing this you can closely approximate the wind direction (slowest GS is directly into the wind). Fly directly into the wind until everything is stable. Look at and record the GS. Do a 180, stabilize and record the GS. The number half way between those two numbers is the TAS.

This isn't perfect but it is way way better than looking at an airspeed indicator and it doesn't take much time or preparation. It does have all the same possibilities of errors as the more sophisticated techniques for performance evaluation, for example generally rising or subsiding sir, which can have a huge impact.
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Larry Pardue
Carlsbad, NM

RV-6 N441LP Flying

Last edited by n5lp : 07-14-2010 at 05:31 PM. Reason: Added stuff
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  #13  
Old 07-14-2010, 05:31 PM
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twsurveyor twsurveyor is offline
 
Join Date: Jan 2005
Location: Ridgetop, Tennessee
Posts: 134
Default RV Speeds

Quote:
Originally Posted by n5lp View Post
Well "comfortable cruise speed" is highly subjective of course.
Larry, I have no doubt your an honest man and I sure hope you didn't take offense to my post.......... I just wish my 6A would post close to the numbers your getting. Maybe it's just the curse of being a "lowlander" east of the Mississippi, don't know. I do know my palne never has been able meet Van's numbers. Oh it's "in the ball park" but as the 0-320 guy says, it's always 8-10 kts off of Van's numbers. Maybe I have too many steps and antennas hanging out in the wind?

Strange thing is, I never have trouble keeping up with most of the RV Guys (mostly 180 HP 6' & 7's) I fly with. I also watch FlightAware regularly and most RV-6(A)'s & 7(A)'s gennerally post cruise numbers between 150 & 160 kts?? Go Figure!

I'd sure like to know what the secret is cause I'm always open to suggestions. If anyone has a "magic bullet" please let me know where I can get one!
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Tommy Walker
Ridgetop, TN (1M5)
RV-6A, N 350 TW
830 Hrs & "Climbing?!
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  #14  
Old 07-14-2010, 05:45 PM
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apkp777 apkp777 is offline
 
Join Date: Oct 2008
Location: Schaumburg, IL
Posts: 2,053
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Quote:
Originally Posted by n5lp View Post
By the way, there is a quick and dirty method of finding TAS that doesn't involve flying the triangle and higher math. Simply do a slow turn while watching the GPS ground-speed. By doing this you can closely approximate the wind direction (slowest GS is directly into the wind). Fly directly into the wind until everything is stable. Look at and record the GS. Do a 180, stabilize and record the GS. The number half way between those two numbers is the TAS
Or you can just look at your SkyView! Gotta love love EFIS!

You're right though. IAS is meaningless.

I think it's an interesting comparison between the -6 and the -9. No doubt the -6 is faster, more responsive and aerobatic capable. The -9 is slower, less responsive, non-aerobatic. Still a real joy to fly. I have flown lots of airplanes, gotta say mine -9 is my favorite!
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N524AP, RV 9 (tail wheel)
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  #15  
Old 07-14-2010, 05:48 PM
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n5lp n5lp is offline
fugio ergo sum
 
Join Date: Apr 2006
Location: Carlsbad, NM
Posts: 1,912
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Originally Posted by twsurveyor View Post
Larry, I have no doubt your an honest man and I sure hope you didn't take offense to my post.......... I just wish my 6A would post close to the numbers your getting. Maybe it's just the curse of being a "lowlander" east of the Mississippi, don't know. I do know my palne never has been able meet Van's numbers. Oh it's "in the ball park" but as the 0-320 guy says, it's always 8-10 kts off of Van's numbers. Maybe I have too many steps and antennas hanging out in the wind?

Strange thing is, I never have trouble keeping up with most of the RV Guys (mostly 180 HP 6' & 7's) I fly with. I also watch FlightAware regularly and most RV-6(A)'s & 7(A)'s gennerally post cruise numbers between 150 & 160 kts?? Go Figure!

I'd sure like to know what the secret is cause I'm always open to suggestions. If anyone has a "magic bullet" please let me know where I can get one!
Tommy, I can't answer why your airplane may be slow, but as for flying with the other guys, I find that it is rare indeed that people cruise at 75% power and at optimum altitude. There really is a comfortable cruise speed and it is usually around 65% power (or less), where the speed isn't all that much slower and fuel burn is a lot less. When I first started flying my plane I tended to cruise at 165 to 170 knots but now that I know I can do that I usually fly a safe-and-sane (and cheaper) 145 to 160 knots.

To be sure your airplane really is slow, you need to test at the conditions specified by Vans. Be sure you are producing the specified power at the specified altitude and that you go to the trouble to find your actual TAS, including flying early in the morning on different days and/or weeks and that you use one of the methods of finding an accurate TAS from GPS data.
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Larry Pardue
Carlsbad, NM

RV-6 N441LP Flying

Last edited by n5lp : 07-14-2010 at 05:53 PM.
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  #16  
Old 07-14-2010, 06:18 PM
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twsurveyor twsurveyor is offline
 
Join Date: Jan 2005
Location: Ridgetop, Tennessee
Posts: 134
Default RV Speeds

Larry, The 145-160 KT cruise your quoting meets my definition of "comfortable cruise". What kind of fuel burn to you typically get at a "comfortable" setting of 145-160 kts (LOP or ROP)?

I think the difference in your numbers and mine might be in the determination of % of power. At full throtle I have no problem reaching Van's max speed numbers for a 180HP 6A ( 205 MPH). Only problem is, I'm turning 2740 RPM @ 8500'!

I had a guy tell me that you loose 1.5-2 kts for every antenna and step etc. you have hanging out in the wind. I find that hard to believe but, I have 5 such things hanging out in the wind. When you add in the 2-3 kts for the nose wheel you come up with 10-12 kts more or less, and that would put me "in the ball park" with your numbers. Like I said, "don't know" and "open to suggestions"!
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Tommy Walker
Ridgetop, TN (1M5)
RV-6A, N 350 TW
830 Hrs & "Climbing?!
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  #17  
Old 07-14-2010, 06:28 PM
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n5lp n5lp is offline
fugio ergo sum
 
Join Date: Apr 2006
Location: Carlsbad, NM
Posts: 1,912
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Originally Posted by twsurveyor View Post
Larry, The 145-160 KT cruise your quoting meets my definition of "comfortable cruise". What kind of fuel burn to you typically get at a "comfortable" setting of 145-160 kts (LOP or ROP)?

I think the difference in your numbers and mine might be in the determination of % of power. At full throtle I have no problem reaching Van's max speed numbers for a 180HP 6A ( 205 MPH). Only problem is, I'm turning 2740 RPM @ 8500'!...
It sounds to me like your RV is fast. Those Van's numbers are at approximately that RPM and altitude. That is one reason most people don't like to fly at maximum cruise speed. 2700 RPM just seems kinda busy.

75% power at lower altitudes is going to be slower and at higher altitudes you can't reach 75% power. It is the theoretical magic place those max cruise speed figures refer to.

The capability of the airplane and how it is normally operated are two different things.
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Larry Pardue
Carlsbad, NM

RV-6 N441LP Flying
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