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I want to increase the low speed (60 - 120 mph) acceleration of my S-1S so
I can give Unlimited one more run. I
think thatâs the weakest aspect of my Pitts, and Iâm looking for advice on what
to upgrade.
Iâve heard a theory that the drag of the stock bungee gear
not all that bad at low angles of attack. Where it really hurts performance is at high angles of attack. If you look at the plane from the front as
it sits on the ground, it appears that the trailing edges of the gear legs are
angled inwards relative to the airflow.
If this theory is correct, then the legs are much draggier near a stall (because the legs line up properly with the airflow at low angles of attack). Consequently,
installing Wolf rod gear might give appreciably better acceleration at low
speeds. There would be a bonus of less
drag while pulling a lot of g. (Any
improvement in top speed is relatively unimportant to me.) Does anyone believe / disbelieve this
theory? Why?
Acceleration from low speeds is hampered by the 2300 rpm I
get out of my engine / prop at 60 mph.
Iâm reluctant to change the prop (explanation below), so Iâm looking for
suggestions to improve horsepower in the range of 2300 â 2500 rpm. Itâs my impression that changes to improve
breathing - cold air induction, 4 into 1 exhaust systems and port work â are
optimized for 2700+ rpm and in some cases might even reduce low rpm power. Iâm wondering if anyone makes systems that
maximize power increases below 2500 rpm. I assume that 10 : 1 pistons would help at all rpms, but are they a good idea in an engine that would spend a lot of time at full throttle below 2400 rpm? Feedback on changes people have made that seemed to improve (or
diminish) low rpm power would be welcome.
It's not necessary to read the rest of this post to address my questions. In
case someone is interested, Iâve added some background to explain why I'm asking them.
I competed in Unlimited in my Pitts from â89 to â01. I never really won anything but I found it very satisfying.
Then came the Free rule changes that penalized low performance planes
and increasingly challenging Unknowns, and I decided to just fly the Knowns for
fun without competing. (I tried
Advanced and found it too boring to get motivated to practice. The fun of competing wasnât enough to
compensate for the lack of fun during practice. People with limited talent like me spend most of their flying time practicing.) But a
medical issue (vitreous detachment â fortunately no complications) made me face
the reality that at my age (63) Iâd better get back into competition now if Iâm
ever going to do it.
My
S-1S is up to Unlimited in most performance areas except low speed
acceleration. It has Falcon extended ailerons so itâs got enough roll, and an
enlarged rudder so I need less speed at the top for a good hammer. The engine is a parallel valve IO360 with
9.2 : 1 pistons and a LyCon elbow that lets me pull bit of ram air from just
below the spinner. It has only the
basic 2 into 1 exhausts. I recently
installed RV3 wheel pants with fully faired brakes and careful shaping of the
gear leg â pant junction, which might have reduced drag a bit.
My
old 60â Sensenich has been replaced.
Through three owners and 1200 hours it had been twisted from 56 to 52 to
60 inches, so it probably wasnât airworthy.
More importantly, at 3500 rpm and 195 mph IAS - the most Iâve needed for
any figure Iâve had to fly - the plane slowed down quickly enough to throw me
forward into the belts. Iâd lose 20 mph
flying across a quarter of the box. I
read good things from the British about the MT fixed pitch, so I bought one
with 63â pitch. I immediately sent it
back to MT to be repitched down as far as possible. They say itâs now 61â, but in Sensenich numbers itâs probably 64â
or a bit more. I get 3100 rpm at 195
IAS, which is good as thatâs the redline for the prop (which MT will say
privately despite the official limit of 2700).
It also pulls hard at this airspeed, whereas the Sensenich made the
engine feel like it was running out of air above 180 mph. This prop has transformed the high speed performance
of the plane. I can get 190 easily out
of a diving figure and maintain it across most of the box, which is all I need
to fly Unlimited Unknowns confidently.
However,
I really need the low speed acceleration of the old Sensenich to be able to cope
with some of the figure combinations Iâve seen in Unknowns. (Iâve had no problem getting through the
Knowns of the last four years since I installed the MT. Some of the Known proposals to CIVA would
have been impossible, but there seems to be some commitment to making it
possible to fly the Known in lower performance planes as none of the impossible
sequences was chosen.)
One
approach would be to install a constant speed prop. A Whirlwind would give me 2600 rpm all the time (or an MT two
blade would give me 2700 rpm but no more thrust than the Whirlwind). Iâd get 15
â 20% more thrust at 60 mph, but it would add 27 pounds to the nose. To keep the C of G where I want it Iâd need
to take most of that off with very expensive components. (A SkyTec starter would save 3 pounds, a
lightweight flywheel another 2.5, a Sky Dynamics cold air induction would
remove the elbow and save 3 pounds, a Sky Dynamics fuel injector would save 4
pounds but needs a different fuel pump and accessory case, a Wolf cowl would save a few pounds, etc,
etc.) And then thereâs the cost of
changing the cases and crank.
Financially it would be make a lot more sense to get a different
airplane. (Unfortunately itâs
impractical to import Experimental Exhibition airplanes into Canada, and
anything certified is out of my price range.
That leaves no alternatives except the G200, and thereâs very few of them around.)
Another
approach might be to get a different composite prop with 60" pitch, and
try to get back high speed performance with drag reduction and modifications to
the engine that would let it make power above 3200 rpm without hurting power at lower rpm. (Moving the C of G back improved the flying qualities so much that I'm not interested in a new 60" Sensenich.)
The
approach Iâm the most interested in right now is to keep the current prop and
try get back low speed acceleration by minimizing low speed drag with the rod
gear and getting more power out of the engine below 2500 rpm. However, Iâd welcome comments on any of
these ideas. Iâm sure Iâm not the first
to try to solve this puzzle.
Edited by: allanf
