Parasitic drag question

We are currently test flying an all fiber-glass experimental plane in an
unpainted state and wondered if any of you have done the same and
experienced as much as a 10% increase in air speed and improved climb rate
after painting your planes.
I know most of you are happy to wait until your pride and joy is painted but
we opted to test fly in the raw. The manufacturer who has a similar plane is
achieving around 10 knots better speed for the same revolutions, at 4,600
rpm on a Subaru conversion with a 2:1 reduction he gets 100 knots and we are
only just getting 90. The only difference in configuration is the addition
of front wheel fairing and three leg fairings plus nice shiny paint. Our
climb rate is also very low compared to predictions. It will be several
months before it is painted and back in the air so am asking the group.
Replies appreciated. Derek.

"Derek Anderson" wrote in message
...
We are currently test flying an all fiber-glass experimental plane in an
unpainted state and wondered if any of you have done the same and
experienced as much as a 10% increase in air speed and improved climb rate
after painting your planes.
I know most of you are happy to wait until your pride and joy is painted
but
we opted to test fly in the raw. The manufacturer who has a similar plane
is
achieving around 10 knots better speed for the same revolutions, at 4,600
rpm on a Subaru conversion with a 2:1 reduction he gets 100 knots and we
are
only just getting 90. The only difference in configuration is the addition
of front wheel fairing and three leg fairings plus nice shiny paint. Our
climb rate is also very low compared to predictions. It will be several
months before it is painted and back in the air so am asking the group.
Replies appreciated. Derek

Sounds like 8 knots for the farings and wheel pants, and 2 knots for the
paint, would be closer.
--
Jim in NC


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On Fri, 12 Mar 2004 08:41:08 +1000, Derek Anderson wrote:

We are currently test flying an all fiber-glass experimental plane in an
unpainted state and wondered if any of you have done the same and
experienced as much as a 10% increase in air speed and improved climb rate
after painting your planes.
I know most of you are happy to wait until your pride and joy is painted
but we opted to test fly in the raw. The manufacturer who has a similar
plane is achieving around 10 knots better speed for the same revolutions,
at 4,600 rpm on a Subaru conversion with a 2:1 reduction he gets 100 knots
and we are only just getting 90. The only difference in configuration is
the addition of front wheel fairing and three leg fairings plus nice shiny
paint. Our climb rate is also very low compared to predictions. It will be
several months before it is painted and back in the air so am asking the
group. Replies appreciated. Derek.

Well designed fairings and wheel pants can make quite a difference. RV
builders frequently report around 20 mph speed increase once they have
fitted the pants and fairings. So you could pick up several kt with the
nose gear fairing and gear leg fairings.

Rate of climb is less affected by the fairings though, as there is less
drag at the speed for best rate of climb. The fact that you report a
significant difference in rate of climb could be a bad sign. But, it all
depends on how you measure the rate of climb. If you (or the
manufacturer) just look at the VSI, then it could be in error.

Many years ago there was an article in one of the popular general aviation
magazines reporting on a Debonair engine upgrade. The author was very
impressed at how his aircraft climbed at 5,000 ft/mn after the upgrade to
300 hp. I wrote a letter to the editor saying that this was obviously
wrong, as the laws of physics simply would not allow an aircraft that
heavy to climb at 5,000 ft/mn with only 300 hp. The author wrote me a
very snotty letter, as he was very upset that I was questioning his
integrity. We wrote a couple of letters back and forth, and he finally
sent me a video he had taken. Sure enough, you could see the VSI sitting
on 5,000 ft/mn after take-off. But over in the corner of the screen you
could see the altimeter, and I timed a rate of climb of about 1,500
ft/mn. The author was very apologetic once he realized that he had been
misled by a grossly inaccurate VSI.

Do you have the exact same prop as the manufacturer? Differences in prop
efficiency could make a huge difference in performance at a given rpm
(assuming full throttle operation). If you are not at full throttle at
4,600 rpm then small differences in prop pitch would make a large
difference in the speed you get. If the prop is from a different
manufacturer, then you can't compare pitch from one to the other, as each
manufacturer has his own way to measure pitch.

How do you know your tach is accurate? How do you know his tach is
accurate? If you and he are really at different rpm that could explain
much of the difference.

--
Kevin Horton RV-8 (finishing kit)
Ottawa, Canada
http://go.phpwebhosting.com/~khorton/rv8/
e-mail: khorton02(_at_)rogers(_dot_)com

paint adds zilch, nada, nothing. Smooth means a lot, but you
can't get smooth by painting. It takes sanding of the paint (or gel coat)
until the divits and lumps do not exceed .003 +_ inches in a 2 inch arc.
For most power type planes, lumps and bumps don't mean anything
behind about 20 or 30 percent chord in any case.


In article , "Morgans"
wrote:

"Derek Anderson" wrote in message
...
We are currently test flying an all fiber-glass experimental plane in an
unpainted state and wondered if any of you have done the same and
experienced as much as a 10% increase in air speed and improved climb rate
after painting your planes.
I know most of you are happy to wait until your pride and joy is painted
but
we opted to test fly in the raw. The manufacturer who has a similar plane
is
achieving around 10 knots better speed for the same revolutions, at 4,600
rpm on a Subaru conversion with a 2:1 reduction he gets 100 knots and we
are
only just getting 90. The only difference in configuration is the addition
of front wheel fairing and three leg fairings plus nice shiny paint. Our
climb rate is also very low compared to predictions. It will be several
months before it is painted and back in the air so am asking the group.
Replies appreciated. Derek

Sounds like 8 knots for the farings and wheel pants, and 2 knots for the
paint, would be closer.

Parasitic drag isn't a big part of the equation on a 90 or 100 knot
airplane. I imagine you'll achieve some of your desired 10 knot speed
increase with the addition of the fairings, but I doubt you'll get the
entire 10 knots. Remember that most manufacturer's claims are based on
*optimized* conditions, which may only exist in the marketing guru's mind...

KB



"Derek Anderson" wrote in message
...
We are currently test flying an all fiber-glass experimental plane in an
unpainted state and wondered if any of you have done the same and
experienced as much as a 10% increase in air speed and improved climb rate
after painting your planes.
I know most of you are happy to wait until your pride and joy is painted
but
we opted to test fly in the raw. The manufacturer who has a similar plane
is
achieving around 10 knots better speed for the same revolutions, at 4,600
rpm on a Subaru conversion with a 2:1 reduction he gets 100 knots and we
are
only just getting 90. The only difference in configuration is the addition
of front wheel fairing and three leg fairings plus nice shiny paint. Our
climb rate is also very low compared to predictions. It will be several
months before it is painted and back in the air so am asking the group.
Replies appreciated. Derek.

I'd second Kevin's point of asking if you have the same prop and
setting (if adjustable) as the comparison aircraft. 3000RPM in 4th
gear is a different speed than 3000RPM in 5th gear.

Also, are you both flying airplanes of the same weight, that will have
a big effect on climb.

Kevin Horton wrote in message ...
On Fri, 12 Mar 2004 08:41:08 +1000, Derek Anderson wrote:

We are currently test flying an all fiber-glass experimental plane in an
unpainted state and wondered if any of you have done the same and
experienced as much as a 10% increase in air speed and improved climb rate
after painting your planes.
I know most of you are happy to wait until your pride and joy is painted
but we opted to test fly in the raw. The manufacturer who has a similar
plane is achieving around 10 knots better speed for the same revolutions,
at 4,600 rpm on a Subaru conversion with a 2:1 reduction he gets 100 knots
and we are only just getting 90. The only difference in configuration is
the addition of front wheel fairing and three leg fairings plus nice shiny
paint. Our climb rate is also very low compared to predictions. It will be
several months before it is painted and back in the air so am asking the
group. Replies appreciated. Derek.

Well designed fairings and wheel pants can make quite a difference. RV
builders frequently report around 20 mph speed increase once they have
fitted the pants and fairings. So you could pick up several kt with the
nose gear fairing and gear leg fairings.

Rate of climb is less affected by the fairings though, as there is less
drag at the speed for best rate of climb. The fact that you report a
significant difference in rate of climb could be a bad sign. But, it all
depends on how you measure the rate of climb. If you (or the
manufacturer) just look at the VSI, then it could be in error.

Many years ago there was an article in one of the popular general aviation
magazines reporting on a Debonair engine upgrade. The author was very
impressed at how his aircraft climbed at 5,000 ft/mn after the upgrade to
300 hp. I wrote a letter to the editor saying that this was obviously
wrong, as the laws of physics simply would not allow an aircraft that
heavy to climb at 5,000 ft/mn with only 300 hp. The author wrote me a
very snotty letter, as he was very upset that I was questioning his
integrity. We wrote a couple of letters back and forth, and he finally
sent me a video he had taken. Sure enough, you could see the VSI sitting
on 5,000 ft/mn after take-off. But over in the corner of the screen you
could see the altimeter, and I timed a rate of climb of about 1,500
ft/mn. The author was very apologetic once he realized that he had been
misled by a grossly inaccurate VSI.

Do you have the exact same prop as the manufacturer? Differences in prop
efficiency could make a huge difference in performance at a given rpm
(assuming full throttle operation). If you are not at full throttle at
4,600 rpm then small differences in prop pitch would make a large
difference in the speed you get. If the prop is from a different
manufacturer, then you can't compare pitch from one to the other, as each
manufacturer has his own way to measure pitch.

How do you know your tach is accurate? How do you know his tach is
accurate? If you and he are really at different rpm that could explain
much of the difference.

On Fri, 12 Mar 2004 08:41:08 +1000, "Derek Anderson"
wrote:

We are currently test flying an all fiber-glass experimental plane in an
unpainted state and wondered if any of you have done the same and
experienced as much as a 10% increase in air speed and improved climb rate
after painting your planes.
I know most of you are happy to wait until your pride and joy is painted but
we opted to test fly in the raw. The manufacturer who has a similar plane is
achieving around 10 knots better speed for the same revolutions, at 4,600
rpm on a Subaru conversion with a 2:1 reduction he gets 100 knots and we are
only just getting 90. The only difference in configuration is the addition
of front wheel fairing and three leg fairings plus nice shiny paint. Our
climb rate is also very low compared to predictions. It will be several
months before it is painted and back in the air so am asking the group.
Replies appreciated. Derek.

Several people have given you good replies Derek. Here's my input.

Do your airplane and the manufacturers airplane weigh the same? Is
your instrument panel loaded with goodies that add weight?

Does your engine make exactly as much power as his does? Are you both
using exactly the same prop? Does the prop have adjustable pitch? If
so, is it exactly the same as where the manufacturer has set it?

Not having fairings on the gear legs or wheels will, as others have
mentioned, make a substantial difference in cruising speed, but not so
much with climb. Weight , engine output and prop make the difference
there.

Did you build the engine yourself, or is it a firewall forward kind of
powerplant? If you built it yourself, did you do all the
modifications the manufacturer has done?

No two homebuilt airplanes are ever exactly the same.

Corky Scott

One more item that I should have mentioned in my first message:

Airspeed system accuracy - you say you are 10 kt slower than the
manufacturer's claims. It isn't clear how you determined the airspeed you
are getting. Airspeed indicators have errors, and the instrument error is
often several kt, and sometimes many kt (if you have an unserviceable
ASI). You almost certainly also have some static source position error,
and this can add another several kt error in the indicated airspeed.

If you have determined TAS using the average of GPS runs, be aware that
some of the methods that people propose are not mathematically sound.

There is lots more info on how to calibrate ASIs, determine static source
position error, and determine TAS from GPS data in the Flight Test Links
section of my web site:

http://go.phpwebhosting.com/~khorton...dex.php?&PID=1

So, depending on how you determined your speed, maybe you are 10 kt slow,
and maybe you aren't.

--
Kevin Horton RV-8 (finishing kit)
Ottawa, Canada
http://go.phpwebhosting.com/~khorton/rv8/
e-mail: khorton02(_at_)rogers(_dot_)com

Derek Anderson wrote ...

...The manufacturer ... is achieving ... better speed ... and ... climb


Manufacturers ALWAYS achieve better performance than their customers.
It's a fundamental law of aerodynamics.

Daniel

"d b" wrote in message
link.net...
paint adds zilch, nada, nothing. Smooth means a lot, but you
can't get smooth by painting. It takes sanding of the paint (or gel coat)
until the divits and lumps do not exceed .003 +_ inches in a 2 inch arc.
For most power type planes, lumps and bumps don't mean anything
behind about 20 or 30 percent chord in any case.

Agreed. Given that most people sand and fill when they paint, I was
thinking the painting process was going to make it smoother, as you suggest.

Any other nits to pick?
--
Jim in NC


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