Hi Paul,

I haven't updated the group on my progress in quite a while, so
today is a good time to do that. As you know, I've been building a p-
port 20B using your design of slide throttle. The engine is together
and in the airframe. In fact, I've actually flown it with the old
intake (p-ports blocked off). The slide throttle is also technically
complete and installed in place of the old intake. Today I ran the
engine with the slide throttle, with mixed results. The slide
throttle was very smooth open-to-close, until the engine started.
With three rotors sucking against the slide, it immediately got very
difficult to actuate. On top of that, after the engine warmed up, it
became extremely difficult to slide. I figure that I can cure the
temp related problem by milling the teflon slide down .010 or so.
However, I was hoping you would have some ideas on how to reduce the
slide forces during low/mid throttle settings.

Mark

Too bad I did not think of this earlier. Your engine is one third larger
than the two rotor so the force on your slide during the over run
at 15 psi is 124 pounds as opposed to only 82 pounds on the 2 rotor.

The coefficient of friction of Teflon is reported at .06
so in theory the load on the throttle cable, worse case scenario,
is 7.44 pounds. For a two rotor it is .666 of that or about 5 pounds.
From your description it sounds like three or four times that
number or .18 to .24. Mark Supinski says throttle force is no
problem on his two rotor p-port.

The brand or exact type of Teflon makes a difference. The friction
coefficient can be as high as .2 for some brands and types.

My suggestion is uses a fish scale and a sample chunk of the Teflon
you have and place a brick on the Teflon and try to slide
it along on aluminum sheet pulling it with the fish scale.
Weigh the brick and tell us what the brick weighs and the
fish scale reads. I'll do the same tomorrow with the
Teflon sheet I have.

Try to record the manifold pressure down stream of the slide
(on the engine side). 30 inches of Hg is 15 psi.

Teflon has a coefficient of thermal expansion .000075 inches per
inch per degree
F. So a temp rise of 130 F would be .00997 inches per inch. In 3
inches it would grow about .030 so that is the total clearance you
will need at top and bottom. Thickness wise it would be only .004.
The gasket in there should cover that. It is more likely the height
of the Teflon is the high temp sticking problem.

The required slide travel is 2 inches so a throttle lever that moved
four inches would reduce the force on the slide to half of what it
is now. I think as much as six inches of throttle movement would be
acceptable. That would give you a 3:1 advantage.

The other thing to minimize is bends in the throttle cable.
The friction laws apply to that too. If you put a lever in there
you might design a pure push/pull rod system.

Paul Lamar

--

I'm surprised to hear how difficult the slide throttle is to move on
the 20b. As Paul said above, on our 13b the slide is silky smooth --
in fact it feels "just right", neither too difficult nor too easy to
move. A bit more friction on ours would not be a problem; it is a
long way from being difficult to move.

Having said that, we did at one point have a very balky slide
throttle -- we found the problem to be misalignment / binding of the
throttle cable where it entered slide body. The binding was variably
bad -- which ended up being related to engine vibration either
making the binding better or worse. I would suggest checking for similar
friction in your installation.

Our solution was simple -- we enlarged the hole where the throttle
cable enters, and then beefed up the cable mounting structure to
minimize it's ability to allow the cable assembly to impart a
sideways force against the hole.

Mark Supinski

** Btw - we have test pilot now. We're making some final mods to the
muffler & then we'll be buttoning up the airplane & seeing if she
stays aloft.

Paul,

Thanks for the suggestions. It just dawned on me that some of the
heat-related binding may be at the top and bottom edges of the
teflon. I initially was only considering the two large sides,
although there may be some there too. I'll look for signs of
binding when I disassemble it.

I replaced the throttle cable for exactly the reason Mark stated above.
Now
it is only 36" long and a straight shot through the firewall into the
slide
throttle as can be seen in the second picture. I may end up adding a
lever for a mechanical advantage as there is only 2" of throttle
travel with the current arrangement.

I will do the drag test and let you know the results.

Mark S.
Austin

You did a nice job on that 3 rotor slide throttle Mark.
Don will be in the market for one soon :)
Do you need bell mouths? I have them for $5 each :)

--------------------------------------------
I just noticed you are appearing to be using the apex
seal oil metering pump. Perhaps the 2 cycle oil in
Mark Supinski's fuel is making the slide move easier.

Paul Lamar

Paul,

Thanks, but I don't think I'll be going into the 3-rotor slide throttle
business anytime soon... I'd rather be flying. ;-)

If you check your records you'll see that I bought 3 bell mouths
about 6 months back. I'm still working on the air intake plenum
which I welded up last night after getting home from the airport.
I'll fit-check it today.

Mark

P.S. I'm using the oil metering fittings, no injection pump... premix
rules! The wiring and vac lines are temporary so I could do a test run.

If you are using them for vac lines there is a one way valve in there
so you will have to plug the atmospheric ports. In any event using
those ports with a small orifice as a source of vacuum will result in
much more consistent and easier tuning. We at first tried vac
taps in the sides of the runners between slide and engine with
less than successful results.

Paul Lamar

Paul,

Yes, I remember that scenario. Not sure it was necessary, but I poked
holes
through the diaphragms in the stock taps. Notice the aquarium
tubing? For the record, aquarium tubing doesn't work for vacuum/heat
applications. ;-) I'll be replacing the aquarium tubing with high
temp silicone tubing before the next run.

Mark S.
Austin

I can hardly wait :)
The performance of this airplane should be impressive.

BTW Don't let it get away from you on the first flight.
Hold it on the ground for a few extra few knots
to be sure you have good aileron control. Tracy nearly
crashed his RV4 the first time he had 200 HP. You are
looking at close to 850 foot pounds of prop torque.
The first thing you will notice is a much higher
climb rate.

Mark Supinski's first test pilot panicked when
the 250 HP P-port Mustang lept off the runway.

These p-port rotary powered airplanes have an unprecedented
high power to weight ratio for light planes.

What were your impression from the first ground engine run?

I am also impressed with how quickly you implemented a 3 rotor
p-port.

Paul Lamar

Paul,

My first impressions are very favorable. Keep in mind that it still needs
tuning. But it definitely isn't your daddy's Oldsmobile, if you know
whatI mean. Where the side port motor would rev without a problem
to more than 7000rpm, I can tell that this motor really "wants" to
rev. It sounds like the typical p-port at idle, rump, rump, rump,
etc. I've already flown it, but took it very easy. Bobby Hughes
thinks it is louder than my old motor, but I can tell that it is
quieter because I can do ground runs with the door open without my
ANR headset which I couldn't do with the old motor. Iguess we'll
have to take some measurements to see who's right. Unlike my old
motor which I changed out the stock exhaust inserts for turbo 13B
inserts, I left the stock 20B exhaust inserts in these housings.
That alone should be worth 5db, or so. The rest of the exhaust is
unchanged. It may be due to tuning, but the p-port motor takes more
cranking to get started, and it cranks faster. Of course the seals
aren't broken in yet either.

Mark S.
Austin

Thanks Mark for the report. The lower pumping losses are
showing up already.

Paul Lamar

Paul,

Slide-throttle update: I received the 1/32" teflon from McMaster
and made apiece to fit between the teflon slide and the inner
aluminum plate. While Ihad it apart, I milled about .040" off the
height of the slide to allow forexpansion. I'm happy to report
that the actuation forces are now veryacceptable, even after a
good heat soak. This is one powerful engine. During tuning at about
3/4 throttle, the brakes could no longer hold it. Iguess I need to
find a big tree. I've been told that the exhaust tone isn'tlouder,
just throatier. Cooling system appears to be up to the task. Stay
tuned...

Mark S.

If the brakes do not hold it you probably have static thrust about
equalto the gross weight.

Some where between 1500 and 2500 pounds. What is the weight?

Paul Lamar

2080 lbs empty.

Mark in Texas

Wow! Here is a physics question for everybody. How much
net out of the prop HP is Mark's engine generating? Assume
the coeficient of friction between tire and pavement is one.

Paul Lamar

350HP
Dale Davies

We will wait and see what others say.


Paul Lamar



Paul,

I can't say for sure if the tires were skidding or the brakes were slipping
as I was inside the cockpit.However, I can easily skid the tires if I
brake too hard on landings.So, I would conclude that the braking is
adequate.

Mark

Sounds like it. Right after landing with the flaps down
it is easy to skid the tires as the wings support the weight.
If the take off speed is 65 MPH then the load on
the tires at 32 MPH is 3/4 of the gross weight. It is a good
sign if you can skid the tires then.

Assume Mark can lock the wheels at zero speed so I will
ask the question again. How much net out of the prop HP is
Mark's engine generating? Assume the coefficient of friction
between tire and pavement is one.

No guessing allowed. Show  me how you calculated it.

Paul Lamar