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Static port airspeed error update

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Tim(at)MyRV10.com
Guest

Posted: Sun Sep 24, 2006 4:12 pm Post subject: Static port airspeed error update

As I've mentioned before, my previous speeds listed when

I gave performance stats were inaccurate.  For over 100

hours I hadn't ever officially flown the plane against

the 4-way spreadsheet to see how accurate the IAS and

TAS speeds were.  Lately, I've been fighting with the airspeed

error doing many test flights with various things.

My original Cleaveland static ports were flush.  The older

ones were made that way, and they needed to be mounted on

bare, unprimed skins, and they needed to be painted with

the airframe, to ensure they had a .010" "stickout" from the

fuselage.  The error on those ports was -6.5 to -7.5kts, but

I had never been informed that I should have painted them,

so this will have an effect too.  I wanted the airspeed to

be as accurate as possible because on the Chelton system

with it's wind computer and everything else, I wanted it

to accurately depict the winds that I was seeing at altitude.



My first test was to put a penny-sized disc of .050"

over them with a hole in it, and the error went down to -4kts.



Next, I put a bridge (like what John sent to the list in the

photo) in front of them and the error was -4 to -4.5 kts.



I wanted to do anything I could to prevent installing different

ports, but after getting a set of the new style rounded head

ports from Mike (Cleaveland Aircraft Tools), I decided this

weekend I was desperate and drilled out my old ones and put

in the new rounded static ports.  If there is any installation

flaw left, it's that there is still primer on the skin inside

the airplane (and the port isn't painted either...but I'm

not sure if the new ones are supposed to be)  Talk about

an un-fun experience, my lovely wife was the lucky one who

went into the tailcone to disconnect the old ports and

buck the rivets on the new ones....and with the AHRS and

Magnetometer shelves in there it wasn't any fun for her.



On my test flight tonight, the airspeed was held very steady,

which gave me statistics on the computer spreadsheet that

rendered 0.0 standard deviation, so it was very accurate and

I knew I got good data.  As it turns out, I'm now only -1.5

to -1.8 kts lower than the displayed TAS on the Chelton, and

considering I was taking the measurements with only 1kt

resolution, I think this is now the best that I can get...and

it's finally within the 3kts that I felt would be "good enough"

for me.



Oh, I also had previously checked for static and pitot system

leaks, by the way.  The static system was perfectly tight,

but the pitot system had a leak rate of about 1kt every

20 seconds while at 170kts.  That kind of rate I would

consider to be not capable of affecting a really dynamic

airspeed indication, but who knows, maybe it makes up

a .1 or .2kt error.



The nice thing is, today my wind calculations are now

also very good looking compared to actual winds on the

speed runs.  They were accurate to within a knot or

two of speed, and the direction was within 5-10 degrees

too...and usually on the <5 degrees.



So, I guess with the new ports from Cleaveland, I think

I have this thing licked.  Thanks Mike!  I'm having

a pretty hard time finding things to be unhappy about

these days. 



-- 

Tim Olson - RV-10 N104CD - Flying

do not archive

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ron.mcgann(at)baesystems.
Guest

Posted: Sun Sep 24, 2006 4:23 pm Post subject: Static port airspeed error update

Hi Tim,   

Any idea what the altitude error may have been with the original ports?   

cheers,  

Ron (back in Oz)  

187 finishing   

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Tim(at)MyRV10.com
Guest

Posted: Sun Sep 24, 2006 5:38 pm Post subject: Static port airspeed error update

Oooooh, you gave me a tough question.    There's a lot of variables

in that, with temperatures and such, so getting an exact number

is very hard.  So, don't trust this figure, but I think I have it

roughly figured out.



First I had to look at 170kts and 160kts and find how much

pressure in inH20 difference there was in 10kts, then just

estimated how many inH20 per Kt there was at that those

speeds, and figure out how many inH2O variation I had.

Then since I couldn't find a good chart on altitude vs. inH20

I had to get a chart that showed altitudes of 6562' and 8202'

and the associated inHg, and then I just linear-ly came up

with approximately how many feet per in/Hg that was.



Then I took that in/H2O in speed and converted it to inHg

and then multiplied it out with the altitude factor.

I came up with a difference of 131.26'.   Now I know

that these pressures aren't exactly linear and there are

more factors involved, but this should give a really

rough idea of what 7kts variation would be.  131' of

altitude could be significant, but remember that both

the airspeed and altitude errors would be completely

different during an approach into and airport at 1000'msl

flying 100kts, so it may not be as bad in that situation.

Tim Olson - RV-10 N104CD - Flying

do not archive

McGANN, Ron wrote:

[quote] Hi Tim,

 

 Any idea what the altitude error may have been with the original ports?

 

 cheers,

 Ron (back in Oz)

 187 finishing

 

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DAVELEIKAM(at)wi.rr.com
Guest

Posted: Sun Sep 24, 2006 8:33 pm Post subject: Static port airspeed error update

While you are on the subject of static ports, should I be looking to 

purchase a static port system and install it in the tail cone during the 

build?  And if so, you talked of a system from Cleveland.  Better than 

Van's?  Others?



And another emp. question while I have your attention.  Any suggestions on 

the position/strobe light for the tail?

Thanks!



Dave Leikam

Muskego, WI

#40496

N89DA

About to start tailcone



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Tim(at)MyRV10.com
Guest

Posted: Mon Sep 25, 2006 3:05 am Post subject: Static port airspeed error update

Hi Dave,



I guess that question is a very tough one (the static port question).

In general, these aftermarket static ports are much nicer looking and

provide a better attachment for the static line.  The cleaveland ports

allow me to use the very flexible Tygon tubing, and the barb fittings

seal really well.  So the aftermarket ports have some benefits besides

looking nicer.  One of the big problems though is, on the RV-10,

I've never once heard anyone else post numbers that they've got

from flying a 3 or 4-way box to find their true airspeed.  Some people

have bought the SafeAir ports, which I'd assume are good, and

some have stuck with the Van's pop rivet.  But, until we get numbers

from people, how would we know how accurate they are?  Also, the

people should take the extra step as I did to have the tester run

on their pitot system when they get their IFR pitot static test

done.  Once we get a few people running the numbers we can better

evaluate the ports.  Until then, people can make claims one way

or the other, but I wouldn't believe any anecdotal "These work

great" claims without data....I thought my old ones worked

great too.



As for the position and strobe lights for the tail, I got the

Whelen combo strobe/tail light from Van's.  It seems to work

fine, so I couldn't ask for much more.  You don't need that

though until it's time to attach that lower rudder fairing, or

even until final assembly if you wish.



Tim Olson - RV-10 N104CD - Flying

do not archive

Dave Leikam wrote:

[quote] 

 

 While you are on the subject of static ports, should I be looking to 

 purchase a static port system and install it in the tail cone during the 

 build?  And if so, you talked of a system from Cleveland.  Better than 

 Van's?  Others?

 

 And another emp. question while I have your attention.  Any suggestions 

 on the position/strobe light for the tail?

 Thanks!

 

 Dave Leikam

 Muskego, WI

 #40496

 N89DA

 About to start tailcone

 

 do not archive

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