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Lost 3 kts After Paint

db1yg

db1yg

Well Known Member
After the paint work was completed on my RV9a I have determined that I lost 3 kts tas at curise power settings. This was confirmed on numerous flights using identical power settings and density altitudes as those captured during many pre-paint flights.

I initially concluded that I must have installed some of the wheel/leg fairings out of trim upon reassembly after paint, but that was checked by a full realignment process with the aircarft on jacks--just like I did when making the initial install. Still missing 3 kts.

I then re-checked the pitot static for accuracy in flight by doing box runs (N., S., E., W.,) in smooth air with a/p engaged, capturing the gps ground speed data, and converting to actual tas using Kevins spread sheets---still no joy.

I have a theory on what happened but I would appreciate any ideas that others may have on the lost ktas.

Best Wishes and Cheers,

db
RV9a/ECI0360/James Cowl/Catto Prop--102 hrs and a permanent smile!!!
 
What type of static port do you have? If you have one of the flush types, make sure that you don't have a large buildup of paint around it. It's possible that the paint is disturbing the air around the port and lowering the pressure that the static port is seeing.

Karl
 
GPS....

RV8N said:
What type of static port do you have? If you have one of the flush types, make sure that you don't have a large buildup of paint around it. It's possible that the paint is disturbing the air around the port and lowering the pressure that the static port is seeing.

Karl
Click to expand...


But Karl... this wouldn't affect his GPS readings and the TAS calculations based on GPS....

gil in Tucson
 
Paint

Does your new paint job include stripes along the leading edges and minute ridges where it joins the base colour? Steve.
 
Weird. I would think if anything that it would go faster (smoother, less drag). Climb less because of weight, but go faster from drag. So what's your theory? Guess that's a "good" excuse for mine to have to wait even longer for paint. ;)

Scott
RV-9A - N598SD - Flying - ~90 hrs
 
az_gila said:
But Karl... this wouldn't affect his GPS readings and the TAS calculations based on GPS....

gil in Tucson
Click to expand...

I didn't interpret his question that way. His only mention of GPS was that he had "...re-checked the pitot static for accuracy in flight by doing box runs (N., S., E., W.,) in smooth air with a/p engaged, capturing the gps ground speed data, and converting to actual tas using Kevins spread sheets---still no joy."

Its not clear to me that the GPS confirmed that he had lost 3 knots. I read it to mean that the GPS didn't confirm the lost 3 knots. I guess I replied to quickly. The static port, to me, was the logical answer.

Karl
 
Lower Pressure?

I would think that if the pressure was lower at the static ports, your indicated airspeed would be higher? :rolleyes:

Just a thought.
 
I would not expect 3 kts change....

But if the paint changes you CG, you could see a shift in speed.

Kent
 
Sand it back to rough conditions! Obviously the paint is too smooth.

Or you're not getting a true representative before and after comparison.
 
Hummm

Wow kind of nightmare if true, 3 kts is kind of significant. On one hand we all probably have a 1-2 kt variation at best in measuring TAS at different times, more depending on technique.

I have to believe you since you probably are "in-tune" with your plane and it probably is a little slower, but 3 kts **represents: 14.6 lbs drag, 7.5 hp!

I like the Gents hypothesis about the span wise stripe (with a lip) on the wing in the first 20-30% of the chord (ie top / bottom surface near the leading edge).

Sure pant could affect pitot static, of course not GPS. However GPS is not 100% accurate and there is positional error with the satellites which varies at different times (depending on positions in sky), but that might account for a few tenths knot error not 3 kts.

Paint does weigh more and does MOVE CG aft (you just have more paint aft of the CG). However that should not affect top speed in a measurable amount. Usually after paint you might expect the same speed, may be a 1 kt more from a smoother surface from better finish but not 3 kts loss?

Thanks for the post. Love a good riddle. Hope we can find a definitive answer like measurement error or something miss rigged during reassembly. My wild guess is its a mesurment / recording error.


** Assume going from 193.5 mph to 190 mph.
 
Last edited:
Of course George brings up a good point too. How long ago were the pre-paint tests done? You sure the engine isn't possibly down on some power since then? Old oil, new oil, more oil, less oil, big weather differences between tests which can affect power output, etc, etc....

Scott
RV-9A - N598SD Flying - ~90 Hours
 
Different Trim (?)

kentb said:
But if the paint changes you CG, you could see a shift in speed.
Click to expand...
This is what I was thinking... or at least one or more control surface balance has changed. No expert here, but I'd be awfully curious if the airplanes trim requirements have changed resulting in more surface 'in the breeze'...
 
Race car analogy...

I remember seeing a movie about race car drivers picking a particular time of day, especially before a rain shower moves in to make qualifying runs. The reason, lower air pressure, less wind resistance.

Did you pay attention to the barometric pressure when you made the two speed runs. Different altimeter settings could be the source of the error. Try two more flights to test that theory. One flight should be on a cloudy day with lower barometer readings and more moisture content to the atmosphere. The other flight should be on a day with higher readings and severe clear air. That clear day should give you a lower speed. Prop and engine efficiency will also be affected by the atmospheric changes.

Use the GPS four-course runs when you make these flights. Pleas let us know what you discover.

Jerry K. Thorne
East Ridge, TN
RV-9A -- N2PZ
Hobbs = 212.5 hours
 
Temp. could be effecting engine power

This time of year the plane/engine preformance should be up.

When were the two speed runs made?
We need more data to continue this guessing game.

Kent
 
What about the rigging?

Is the plane rigged the same now as before the paint job? I assume most paint jobs require the removal of control surfaces. Could they have been put back on the plane a little out of rig?
 
First I want to thank all of you for your ideas/responses--this site is incredibly valuable. I will try to respond to as many of your questions as possible and provide more data to consider.

Instrumentation

The airplane has been certified for IFR flight by a reputable avionics shop. The a.s. and alt. indications ON THE GROUND and attached to the test equipment show that the Dynon D10a and the pitot/static system are right on with the calibrated instruments up to 20k ft and 200kts. This was before paint.

Static Port

The static system was originally the flush mount system (with significant error while in flight) and was modified after MUCH testing by myself and help from listers. A thread that Scott DeAngelo started on his Performance Numbers contains the steps I went through which eventually gave me good tas accuracy. The key was adding a dome to the port and raising it into active air by moving it out from the fuse by about 40/1000. This was also before paint.

Testing Technique

The gps testing that I did before and after paint have confirmed the 3 kt loss. The testing before and after were done at consistent density altitudes and power settings (mp/rpm/ff). Additionally, the tas on the Dynon after paint continues to be consistent with the box run gps grnd speed data converted to tas using Kevins spread sheet. I concluded from this that the static port was not affected by the paint.

Rigging

The flight controls were reinstalled by me after paint in the same positions as before paint. This was done by marking the rod end bearings prior to disassembly for paint and not moving the lock nuts. Additionally the plane did not show any obvious change in handling after the paint--ie it still tracked straight and true--no heavy left or right and ball still centered.

Paint Stripes

The only place that paint transitions occur along the leading edge are the last 2 ft of the wing. There could be ridges along this area. Could this cause a loss of sufficient lift to loose 3 kts?? Pic attached
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Conclusions

I believe that my instrumentation is good to within 1%. I am confident that the aircraft is rigged just as it was prior to paint due to the care taken when disassembling the plane and the additional alignment checks on the wheel and leg fairings outlined in my prior post.

Additional Observation/My Theory

What I did notice on a recent trip to Az which I had not noticed prior to painting is that the front of my elevator counter wt is up by about 1/2 inch while in cruise flight with just me on board and about 30 lbs in the baggage. I did not remember seeing this prior to paint. My theory (Kent and Mark were suggesting this) is that the wt of the paint, most of which is aft of the cg, resulted in a need for a slight down elevator to counter the added aft. wt. This of course results in some additional drag to the airframe--which at 190+ mph may be considerable.

Is it possble that this would/could make such a difference??
Any other ideas??

BTW, this airplane exceeded all my expectations regarding performance even with the 3 kt. reduction---but I would like to get it back!!

Thanks for all the help

Cheers,

Dave B.
RV9a/ECI0360/James Cowl/Catto prop--100+ hrs and a permanent smile!
 
Steve Sampson said:
Does your new paint job include stripes along the leading edges and minute ridges where it joins the base colour? Steve.
Click to expand...

Good question... Typically the leading edges that are painted different colors than the wing will make the transition lengthwise down the wing right around the point where the leading edge top skin joins with the wing top skin. This is supposed to have the potential to cause a burbling of the airflow if the tansition isn't smooth between the colors. When doing paint schemes all of my ideas with colored leading edges and stripes to set the color apart from the wings were eliminated after reading about this so that airflow would not be affected. Is this a real condition? I don't know.

Steve might be onto something though. Are you wings painted like the above description?
 
Even if the density altitude was the same, if your "after" test was done on a more humid day, wouldn't that have a negative impact on engine power since there would be more water molecules per cubic whatever of air?

Do you have a digital tach and digital MP gauge? Was the OAT the same on both days? On a cooler day, your oil temperature was probably cooler, causing more energy consumption due to pumping losses.

Was the airplane at the same weight during both sets of tests? A 50 pound difference (8 gallons of go-juice) might make a difference that would round to 1 knot...

How 'bout the vents or heater? Are you certain both were at the same setting on the initial and subsequent testing?

Anyway, there are many, many things including measurement error which could result in a measure difference of 3 knots. Those things may or may not have anything to do with the aircraft's paint or lack thereof.
 
db1yg said:
What I did notice on a recent trip to Az which I had not noticed prior to painting is that the front of my elevator counter wt is up by about 1/2 inch while in cruise flight with just me on board and about 30 lbs in the baggage. I did not remember seeing this prior to paint.

Is it possble that this would/could make such a difference??
Click to expand...
Good question, and I'm wondering too. Move the 30 lbs to the passenger seat or whatever else is necessary to get the CG to a point where the elevator is no longer deflected into the air stream as much, and run your test again. Let us know what comes of it.
 
Interesting problem. There has to be an explanation.

How many times did you test the speed before paint? If you look at the results from tests on different days, how repeatable was the result? How many times have you checked the speed after paint, and how repeatable is the result? Data from any one day can be slightly affected by weather effects such as rising or falling air masses.

How much did the temperature differ for the before and after tests? If the temperatures at altitude were significantly different, even using different pressure altitudes to keep the density altitude the same won't yield exactly the same speed. Engine power is not exactly a function of density altitude.

Is it possible that your tach, MP or OAT gauge calibration has changed? Has the location of the OAT probe remained constant? Where is it located?

Are there any other clues as to whether the engine power might have changed? How does the climb rate compare to before paint?

Has the condition of the prop leading edges changed? Was the prop painted when the aircraft was painted?

Is it possible that some fairing was not put back on properly, so its leading edge is catching in the breeze?

I'm assuming the cowling was off when painting. Is it possible that there is not a good seal to the air box now? Could the air filter be dirty?

Any other mods since the last time you checked the cruise speed?
 
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