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I replaced an O-ring and gained 10 knots!!!


SpamPilot

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No, this is not a clickbait test. :)  This is actually a public service announcement.

I bought my Rocket 305 a couple of years ago.  I've flown her all over the western US, loving the thrill of massive power and a climb rate that just goes on forever.  But cruise speed - well, I've learned to settle.  I mean, no aircraft reaches book numbers, right?  The performance chart I've perused so many times it's burned into my retinas tempts me with a sexy 200+ knots true without even needing to put on my oxygen mask.  In the flight levels, things skew pornographic.  Alas, reality has been a good 10 knots slower than that, maybe as much as 15 if I honestly correct for the aviation equivalent of beer goggles.  She's got a belly covered in antennas and towel racks on the rudder, and she's got a lot of mileage behind her.

So a few days ago, I'm flying along at my usual "need to get somewhere before nightfall" cruise setting.  There is no mountain wave, no obvious up- or downdrafts.  The air is perfectly smooth.  In the direction I'm headed, I usually have a modest tailwind.  GPS Ground Speed shows a few knots above 200.  I think, OK, that makes sense, she's doing 190 knots true with a 10-ish knot tailwind.  I check OAT and dial in the correction factor on the airspeed indicator for temperature at this altitude to get true airspeed.  Wait, hang on - that's not 190 knots, that's 200 knots!

Convinced I'm just in a localized column of rising air, I continue for another 90 minutes.  The airspeed fluctuates a bit, as it will, but yes, on average, I'm seeing 200 knots, maybe a couple knots over.  Whatever my actual airspeed indictor error is, I'm about 10 knots faster than I usually am.  I know this because I have stared at that indicator so many times, wishing it higher.  And that's with the added drag of an underwing camera that is surely a 2-3 knot penalty at that speed.

The next day I reproduce this result.

I scratch my head trying to figure out what changed.  I was flying with my usual near-gross load, including full fuel, a copilot, a dog, and luggage, so that's not new.  She (the airplane) had her annual about three months back, but nothing needed to be fixed.  She did get an IFR cert done right before the flight, but how could that possibly...

No wait, that can't be it...

During the IFR cert, the technician identified a leak and traced it positively to the static drain in the fuselage.  Pouring alcohol into the static drain resulted in the alcohol pouring straight out, even with the drain closed.  He couldn't complete the precision altimeter portion of the certification with such a leak.  He recommended replacing the valve outright, but I didn't want to wait for the part and have the rivets drilled out and all that.  So I found SBM20-167, which describes replacing the O-ring in the static drain, discussed it with my A&P, he came out and replaced the O-ring, the IFR cert passed with flying colors, and I was on my way.

Airspeed, as calculated by the airspeed indicator, is a function of both dynamic and static pressure.  The primary static source is from the static ports in sides of the fuselage, which are drained of accumulated moisture by this drain.  It is reasonable to assume that the leak at the drain changed the pressure sensed by the airspeed indicator.  In partial support of this theory, I notice that the altitudes ATC calls me out at are closer to my altimeter readout than before, by about 100 feet.

Have I really gained 10 knots by replacing an O-ring?  Obviously, no - but I have gained the same incremental happiness as any other speed mod worth 10 knots, multiplied by the joy of the miniscule cost.  Now with no camera and getting rid of as many antennas as I can, book speeds don't seem so unattainable.

So if you think your Mooney isn't quite as hot as the POH promised - well, sure, that's likely, but checking the airspeed measurement system for leaks might reveal she's underappreciated.

Edited by SpamPilot
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Glad it made you happy!

If you ever have time and care to know, do a 4 way gps groundspeed test.  N E S W write down groundspeed at each heading.  Average them.  That will be almost exactly your TAS.  This takes out all your indicator error.  Still will have a tiny bit of wind error From crosswind component.

If you want to be test pilot precise, you actually only need 3 way groundspeed test to get your actual TAS.  You need to be real good at math then or put them in an online 3 way groundspeed calc.  That will get you exact.

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12 hours ago, SpamPilot said:

... because I have stared at that indicator so many times, wishing it higher. 

I tell people that they would beg for my problems (meaning they are mostly trivial and I live a good life).  Being a lowly caretaker of a C model, I too stare longingly at the ASI begging for more but mine reads a paltry 150 MPH and I would beg to read 190 - 200 KIAS or even a scant 150 KIAS!

  Well done solving your pitot static issue and glad to see Rockets still screaming along.  A boy can dream :)

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1 hour ago, Brian E. said:

I tell people that they would beg for my problems (meaning they are mostly trivial and I live a good life).  Being a lowly caretaker of a C model, I too stare longingly at the ASI begging for more but mine reads a paltry 150 MPH and I would beg to read 190 - 200 KIAS or even a scant 150 KIAS!

  Well done solving your pitot static issue and glad to see Rockets still screaming along.  A boy can dream :)

Me, too. But 145 mphi at 9500 msl is still respectable. Especially for only 180 hp at sealevel. 145 + 19% = 173 mph true.

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Here are good resources on checking airspeed accuracy from the National Test Pilots School. Scroll down to the bottom of the page. https://www.ntps.edu/information/downloads.html

Remember that there is a pitot drain of the same design also. Most of the time, unless you specifically ask, a normal biennial 91.411 "pitot static" test will not test airspeed accuracy because it is not required by regulation. A lot of test sets do not even have a calibrated airspeed indicator. It's called a pitot static test because it is necessary to connect the test set to the pitot as well as the static system to avoid damaging the airspeed indicator during the test, but the pitot system is not necessarily leak tested.

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I put about 1000 hours on a Rocket. Usually 73% 20 GPH that gave 200 knots @12,000. Faster higher. The Rocket supplied chart was spot on at every altitude and power setting. Those numbers came from real world flight tests not the “sales department”

I owned it from 1994 until 2004 with totally stock avionics and their attendant antennas. 
 

If a Rocket does not match Rocket’s data something is wrong. 
(No data or experience with TKS)
 

She climbed like a homesick angel. On two different occasions I flight tested major work up to 26,000’. Both times at about 2900 pounds she was still climbing at over 1500’ per at 26. There is nothing like it. 

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17 hours ago, PT20J said:

Here are good resources on checking airspeed accuracy from the National Test Pilots School. Scroll down to the bottom of the page. https://www.ntps.edu/information/downloads.html

Remember that there is a pitot drain of the same design also. Most of the time, unless you specifically ask, a normal biennial 91.411 "pitot static" test will not test airspeed accuracy because it is not required by regulation. A lot of test sets do not even have a calibrated airspeed indicator. It's called a pitot static test because it is necessary to connect the test set to the pitot as well as the static system to avoid damaging the airspeed indicator during the test, but the pitot system is not necessarily leak tested.

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Interesting to me that those papers have you record groundspeed and TRACK.  Not heading.  The online calculators I’ve always used have you fly a constant heading and the record the groundspeed.  They still provide winds and tas.  I wonder if it’s two different ways to get to the same thing?

http://www.csgnetwork.com/tasgpscalc.html

 

Edited by Ragsf15e
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20 hours ago, Ragsf15e said:

Interesting to me that those papers have you record groundspeed and TRACK.  Not heading.  The online calculators I’ve always used have you fly a constant heading and the record the groundspeed.  They still provide winds and tas.  I wonder if it’s two different ways to get to the same thing?

http://www.csgnetwork.com/tasgpscalc.html

 

As I understand it, there are a couple of methods. If you use headings, they need to be at 90 degree intervals giving rise to the name “horseshoe” technique. The track method allows arbitrary headings for each leg.

Here’s a good summary:

http://www.nar-associates.com/technical-flying/horseshoe_heading/horseshoehead_screen.pdf
 

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13 minutes ago, RobertGary1 said:

I’m surprised because it’s still static air at the drain. I replaced the seals on both my drains and got bloody knuckles. But the avionics folks passed me with the leak saying it’s still a static source. 

Well, they shouldn't pass it with a leak. The issue is that the pressure varies at different points on the fuselage and so care is taken to locate the static ports in a location where the pressure matches that of the free stream static pressure.

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