Proof That Dirt is Drag!!

USCANAM · #1 04-25-2009, 07:16 PM

Got back to Cape Cod from Sun n Fun yesterday (Friday) afternoon.
For those readers that were there on the Display field, either with their plane, or just observing, your white socks will attest to the dirt that was loose to invade every surface that was exposed to daylight, or moonlight for that matter. Not too much wind, but everytime a plane or golfcart went by it was enough to get this fine, fine abrasive dust-like dirt airborne.
I arrived Sunday afternoon, to beat the oncoming bad weather that was going to lock in the East coast, with plans to stay at least until Saturday, but the heat and the dirt was too much. Fortunately, I had a chance Tuesday morning to get to the plane while there was a lot of dew on at, and was able to clean it up a little before the judges arrived. Once they left, that was all the cleaning it got. The morning dew on Wednesday, Thursday, and Friday mornings, fixed the dirt firmly to the skin when it dried, and in some places created nice designs. Even the tears that dripped down from my eyes couldn't restore the metallic in the Claret Red finish.
However, some good did come out of this, and if you've got this far through my ramblings, you will find out why.
My plane is a -9A, IO-360 180 hp., with a 3 blade MT constant speed prop.
On the way down to Lakeland, a distance of 1041 n.m, the plane was in show condition, and we were trueing out according to the EFIS at 191 mph at 6500 feet, with a power setting of 23.5 mp, and 2350 rpm.
On the way back, at the same power setting, and at 7500 feet we were trueing out at 188 mph. In order to hit 191 mph, I had to increase power to 24 square. And to boot, being 1000 feet higher and still being able to produce the same power as the initial flight, we should have been able to go even faster at the higher altitude.
I'm sure there are no RVs flying as dirty as I was on the return trip, but I found this interesting in the speed between a clean and real dirty plane.
It was also great seeing the paint come back to life as I gave the plane it's first wash ever this morning.
Jack
close to 100 hours

bsacks05 · #2 04-25-2009, 07:57 PM

Just got back from SNF and my -9 needs a bath real bad. You are absolutely right, the dirt was the worst I have seen at SNF. I couldn't wait to get home and take a shower and turn on the AC in the house. Good time, anyhow. Pictures coming soon.

PCHunt · #3 04-26-2009, 03:11 AM

Any possibility that the dust got into the filter and slightly restricted the airflow to the engine?

Just thinking out loud.

hevansrv7a · #4 04-26-2009, 04:46 AM

Quote:

Originally Posted by PCHunt

Any possibility that the dust got into the filter and slightly restricted the airflow to the engine?

Just thinking out loud.

How would that affect MAP? It could in theory affect pumping losses, but that's a stretch.

pierre smith · #5 04-26-2009, 05:17 AM

.....already proved that by washing and waxing a Mooney and gaining 4 MPH years ago.

Keep 'em clean,

Geico266 · #6 04-26-2009, 05:36 AM

Look at the bottom of your plane if you went to SnF. If you have any oil on the bottom it is black now! Oh well, it will wash off.

At SnF in the morning when the dew is heavy, a California wiper (any auto store) works great to lift the mud and water and float it off without damaging the finish.

When flying home I try to find a rain shower to fly in!

USCANAM · #7 04-26-2009, 06:17 AM

Quote:

Originally Posted by PCHunt

Any possibility that the dust got into the filter and slightly restricted the airflow to the engine?

Just thinking out loud.

Not really. I was still able to achieve original manifold pressures. After a day, I did stuff a rag into the air inlet. Took me another day to discover I had left the cabin vents open, and since the plane was facing into the wind, we have a very dusty interior.
It's condition inspection time, so all will be clean soon
Jack

the_other_dougreeves · #8 04-26-2009, 11:38 AM

Glider pilots know this one well. It's easy to measure the improvement in minimum sink rate and L/D when the wings are clean.

TODR

Jamie · #9 04-26-2009, 12:14 PM

Just got back a few hours ago.

The dirt was absolutely atrocious. I heard that they burn the grass off down occasionally and the black stuff you see flying around is ash. Don't know if that is true or not.

On the plus side, I made it back to Atlanta (KLZU) in 2.0 hours flight time and burned 17 gallons. My route was 378.5nm which means I averaged 189kts ground speed, including climb. I came back at 11,500 and 10,500 and saw about 205kts GS during level flight at one point.

I too was watching nexrad on my 496 like a hawk looking for any little rain showers but there were none to be found.

Gotta love these planes! Gotta really love 'em when you have a tailwind.