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  #21  
Old 05-01-2007, 04:09 PM
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bumblebee bumblebee is offline
 
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Quote:
Originally Posted by Bob Axsom
Ah yes but we continue to try. Last year in the AirVenture Cup if I had averaged 1 more knot I would have beaten the slowest RV-8 in my RV-6A with a 1.5 feet longer than standard wingspan (tip tanks). It's hard but not impossible to squeeze out improvements and even harder to measure them but accepting defeat is an option that has its own drawbacks.

Bob Axsom
(Do I dare mention that merely adding length to a wing might create additional drag? )....

I race, therefore I empathize. I could sneeze and lose a knot over the course of a 100 mile race. If you REALLY want to lose sleep, start racing sailplanes.....you'll be up at 3am cooking up secret waxes and coatings all in the name of finding that elusive knot while your (soon to be former) wife calls in the guys with the white jackets.

Not to give you more ulcers, but prop efficiency is another m-a-j-o-r contributor to speed variability. Due to the immense drag rise at the tips, running a prop at 2700 RPM can create MORE drag than running at 2500 RPM. This large multiplier means it is critical to keep RPM exactly on the money for every test run. You then must correct for density altitude (temp and humidity) variability. Prop management is easily worth a knot or two all by itself over the length of a race.....

Just discussing it, I can already feel the obsession creeping back....that's why most of us don't have hair (or wives).

Keep trying! But make a consistent test program and stick with it. Consistency counts for more than exotic techniques.
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  #22  
Old 05-01-2007, 05:45 PM
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It is true there are many variables...living in the rockies, I find the vertical movement of the atmosphere the biggest challange, bumblebee has named others.

Still, by use of a little vector arithmetic, and 3 equations in 3 unknowns, we can eliminate winds aloft from the list of variables. And, better yet, by simply downloading the spreadsheet provided by the National Test Pilot School at
NTPS Spreadsheet
we can just fly the squares, and go home and type the numbers in, and we have the basis of a consistant test method.

For each square, we get four sets of three legs. The spreadsheet calculates wind velocity, wind direction, and our true airspeed (the 3 unknowns) for 4 sets of three in each square. It also calculates the standard deviation of the resulting TAS, which provides a measure of the quality of the square that was flown. If there was vertical movement of the air mass, the pilot did a less than perfect job holding altitude, the winds aloft varied over the distance of the square, etc., the std. dev. will be large, and we know to ignore the test. If the std. dev. is small, we can have some confidence in the data.

I have found that a std. dev. of less than 1.0 means a fairly good test, you can set your own criteria.

So, why not?
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  #23  
Old 05-01-2007, 06:15 PM
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rv6ejguy rv6ejguy is offline
 
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Lots of great theories here. For solo work, I forget about TAS/ GPS. Find some smooth air, same altitude and close to same OAT. Nail the VSI and altimeter for 3-4 minutes and read the ASI. Can get +/- 2 knot accuracy if you are good.
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Turbo Subaru EJ22, SDS EFI, Marcotte M-300, IVO, Shorai- RV6A C-GVZX flying from CYBW since 2003- 441.0 hrs. on the Hobbs,
RV10 95% built- Sold 2016
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  #24  
Old 05-01-2007, 06:52 PM
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Kevin Horton Kevin Horton is offline
 
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Quote:
Originally Posted by rv6ejguy
Lots of great theories here. For solo work, I forget about TAS/ GPS. Find some smooth air, same altitude and close to same OAT. Nail the VSI and altimeter for 3-4 minutes and read the ASI. Can get +/- 2 knot accuracy if you are good.
Yes, I was planning on posting a message suggesting just this. The GPS based methods are great if you really need to know TAS. But, if all you care about is how the speed with the latest mod compares to some other configuration, then you only need to know the change in speed. You don't need to know the absolute speed. So, just get a stabilized ASI reading, pressure altitude and OAT. Convert the ASI reading to TAS. This isn't the real TAS, as we haven't corrected for ASI instrument error, static source position error or errors in the OAT. But, these three errors will be about the same on all flights (assuming you don't make any changes that would affect them), so they cancel out if we are just looking for a difference in speed.

Yes, OAT and humidity will also affect the results. But, if we get a bunch of data from multiple flights at different OATs in any one configuration at the same pressure altitude, weight, CG, rpm, MP, leaning technique, etc, we can plot all the TASs vs OAT. If we get enough data we should be able to see the trend of TAS vs OAT. Once we know how much the TAS changes per degree OAT change, we have a way to correct all those TAS values to what would be achieved on a standard day. That correction should be relative unaffected by typical mods, so once we have found it, we can use it in the future.

I need to find some historical upper air data showing dew point and temperature. I have an old NACA report that shows the effect of humidity on engine power. I hope to figure out roughly how much the power would be affected by the normally seen humidity values at altitude.
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  #25  
Old 05-01-2007, 08:24 PM
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You guys are giving me ulcers. Ya'll need to go log about a dozen hours in a flight school rental 172, and then go back to your own plane. You'll never complain about that "elusive knot" again....

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Built an off-plan RV9A with too much fuel and too much HP. Should drop dead any minute now.
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  #26  
Old 05-02-2007, 12:53 PM
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Quote:
Originally Posted by rv6ejguy
Lots of great theories here. For solo work, I forget about TAS/ GPS. Find some smooth air, same altitude and close to same OAT. Nail the VSI and altimeter for 3-4 minutes and read the ASI. Can get +/- 2 knot accuracy if you are good.
It must be nice to have air that smooth, without up and down drafts.
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  #27  
Old 05-02-2007, 02:29 PM
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I'm based right beside the Rockies and it often is not smooth except in the winter months- so I test fly a lot during these times. Occasionally, I can find a smooth morning or evening. Test flying for speed measurements is essentially a waste of fuel in gusty, windy or turbulent conditions anyway. Just gotta be patient for a good day or like I said before, get your RV buddy and fly formation.
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Turbo Subaru EJ22, SDS EFI, Marcotte M-300, IVO, Shorai- RV6A C-GVZX flying from CYBW since 2003- 441.0 hrs. on the Hobbs,
RV10 95% built- Sold 2016
http://www.sdsefi.com/aircraft.html
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  #28  
Old 05-02-2007, 04:48 PM
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rv969wf rv969wf is offline
 
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Default Oklahoma winds

[quote]
Quote:
Originally Posted by rv6ejguy
I'm based right beside the Rockies and it often is not smooth except in the winter months- so I test fly a lot during these times. Occasionally, I can find a smooth morning or evening. Test flying for speed measurements is essentially a waste of fuel in gusty, windy or turbulent conditions anyway. Just gotta be patient for a good day or like I said before, get your RV buddy and fly formation.
I understand your thoughts. Trying to do any flight testing where I live in the panhandle of Oklahoma is not good. I think we have maybe 10 days out of the year that are nice. Well OK maybe 20 days. The summers here are hot, windy, rough etc. A normal day to fly where I'm at is 20-25mph gusting to 35mph +. Ugh.... We do have some nice days but not very many. I still have not been able to get apples and apples testing with any of the mods that I've done. Some days its so rough even at alltitude I can't even get my RV out of the yellow zone on the airspeed indicator to do any airspeed tests. I'm a little jealous of the RV fliers that live in nice areas without all the bumps and rough air. Ohwell, I'll quit complaining as I chose to live where I'm at for now.
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  #29  
Old 05-02-2007, 05:25 PM
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[quote=rv969wf]
Quote:

I understand your thoughts. Trying to do any flight testing where I live in the panhandle of Oklahoma is not good. I think we have maybe 10 days out of the year that are nice. Well OK maybe 20 days. The summers here are hot, windy, rough etc. A normal day to fly where I'm at is 20-25mph gusting to 35mph +. Ugh.... We do have some nice days but not very many. I still have not been able to get apples and apples testing with any of the mods that I've done. Some days its so rough even at alltitude I can't even get my RV out of the yellow zone on the airspeed indicator to do any airspeed tests. I'm a little jealous of the RV fliers that live in nice areas without all the bumps and rough air. Ohwell, I'll quit complaining as I chose to live where I'm at for now.
All that, and no women to be found! Alan, maybe you should consider moving!
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  #30  
Old 05-10-2007, 12:25 AM
Bob Axsom Bob Axsom is offline
 
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Default Well Mr. Horton & Mr. Huft I have seen the light

I had gotten back on the stay off the website wagon but I had to log back on and thank you two guys for not giving up on me. I knew very well that the U. S. Air Race, Inc. Handicap Procedure was underestimating the TAS in the presence of wind even if it is steady. I had determined that flying directly into the wind and reverse was the only way to directly eliminate a steady state wind from observation averages in determining TAS. I played around with trig and resolved better a estimate for one of my past tests but it was a lot of work and I had not gone through the work necessary to establish a procedure. Well, your persistent nudging over the past year or so, bothered me a lot. I knew you were telling me the right thing but I just wanted to do it my way even when I knew my way was underestimating the actual TAS. A few minutes ago I finally went to the reference spread sheet and plugged in the numbers from a particularly disappointing test flown two days ago. I had pulled out the lower forward baffle and the extensions that seal to the bottom of the cowl outlet and decreased the spark plug gap to the minumum 0.016". I expected to see a speed of around 172kts - I got 168.6kts. The excel spread sheet with the imbedded functions immediately told me the wind was 190.4 degrees at 36.6kts and the TAS was 170.6kts. Since all of my historic data are derived from 3-way testing I will use that page for entering the past test data and get more realistic TAS numbers for my past experiments. It may give people a head ache to read this stuff but try to experimentally increase the performance of your airplane and measure the results with a procedure that provides inconsistent and inaccurate results if you want to feel real pain. I'm sure the buddy system works for a lot of folks but when you are working alone as I am, this is a wonderful tool. I have opened the spark plug gap to the maximum 0.021" and want to fly a test again tomorrow before I make anymore mods. Thanks guys, this looks great.

Bob Axsom

Last edited by Bob Axsom : 05-10-2007 at 12:32 AM.
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