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.