No easy
answer – to all the nuances of producing power, Chris.
200 HP is a
possibility depending on lots of factors (like your total induction
system set up, however, I believe that 180 HP is more in line with the
likely maximum on the older 13B. The Renesis a bit more/
Fuel flow is
as much an indication of power as anything short of a dyno and/or
extensive aircraft performance numbers.
RPM (in my
opinion) has too many variables – like I have a 74x88 prop with a
2.85:1 gearbox and can turn 6000-6200 rpm static – does that mean I am
making more or less HP than you at 6200 rpm at 35” Hg. It all depends
on what prop load each of our engines are seeing at that rpm.
Generally as a rough
rule of thumb you can figure 10 hp per gallon/hour of fuel flow. So it
you have a fuel flow of 16 gallon/hour then your engine could be
producing around 160 HP. 18 GPH = 180 HP, etc. Now your power won’t
be more than that, but it could be less. The rotary can flow
considerably more fuel without making useful power than a piston
engine, but it is a useful rule of thumb.
I street
ported my 91 Turbo block myself using a Mazdatrix street port
template. I went through 5 intake designs and several muffler
experiments before finding one that appear to give me the power I
wanted. On a cold morning with OAT < 50F, I can get up to 6200 rpm
and 18-20 GPH fuel flow with the 74x88 prop. So I feel I have my set
up just about as good as its going to get – short of a forced induction
system.
With your
prop set at max fine pitch, you have reduced the prop load on your
engine to a minimum – that permits your rpm to be at a maximum – but,
that does NOT mean you
are producing the power and more importantly - the thrust needed for safe flight.
Don’t be
mislead by rpm. As an extreme example to make my point, Without a prop (almost no load)
my engine will turn 5500 rpm at idle throttle setting – but I am only
flowing something like 1.5 – 2 GPH which means producing something like
15-20 HP even though the rpm without prop is higher than my static was
when I had my old 68x72 with a 2.17:1 gear box. So even though the
rpm is the same or slightly greater, the engine not making any where
near the 150-160 HP I made with the prop on at the same rpm. Fuel
flow is not a perfect indicator but much more useful than RPM alone in
estimating your power particularly with a variable pitch prop.
This is
important, Chris. Years ago, there was a long EZ builder who had a
non-rotary auto engine who unfortunately ended his first flight in a
fatal crash into a cactus plant. He understood the electronics just
fine, but did not understand the relationship between rpm and thrust.
He set his adjustable pitch prop for maximum RPM – which mean minimum prop load, which in his
case mean minimum thrust.
He managed to get airborne with this limited thrust, but could not
apparently climb out of ground effect with the flight ending on impact
with a cactus.
For
example, you would get your highest rpm with a prop that absolutely no
pitch which would produce minimum load on the engine – however, I think
you would agree there would not be much “push/Pull” by such a prop set
up. With a variable pitch prop, you need to find the optimum balance
between rpm and thrust. Too much rpm could indicate too little pitch
(and too little thrust), too low rpm could indicate too much pitch (and
too little thrust) – neither condition gives you maximum thrust.
The only way
I can think of to find that optimum balance between rpm and thrust is
to attach your airframe through a scale to an anchor. Then measure the
pull on the scale at various rpm and prop pitch settings. This should
help you find where combination of engine rpm and prop pitch provides
maximum thrust (at least maximum static thrust – which is a good
start). Perhaps someone else can offer a better and easier method.
Yes, you should be able to get a feel during taxi test – although some
folks frown on the idea of high-speed taxi test. Theoretically you
could use an accelerometer, some accurate speed measurements the weight
of your aircraft and calculate the effective HP – that has been done
with automobiles.
In cases
were folks are using fixed pitch props with parameters similar to
those used by others, you can make a comparison and get a rough feel
for engine performance based on RPM. Unfortunately, with a variable
pitch prop making such comparisons is more difficult and questionable.
Now if you can find someone using the same variable pitch prop you are
using and compare your rpm and prop settings, that is certainly
something worth checking into.
Just
continue to ask these kinds of questions and to think about the issues
in producing power and thrust – you’ll be ready.
Ed
.
From: Rotary motors in
aircraft [mailto:flyrotary@lancaironline.net]
On Behalf Of Chris
Barber
Sent: Wednesday, May
26, 2010 9:51 PM
To: Rotary motors in
aircraft
Subject: [FlyRotary]
Rotary HP reviisted....now that I know more about it all...kinda.
I know
this must have been discussed, but perhaps more in passing or in some
of Tracy 's literature (now kinda dated, especially with some of the
strides he continues to make), but what is the conventional wisdom as
to rotary horsepower? I know when I first started looking into it, oh
many years ago, it seemed the impression I got was that you could
pretty easily achieve 200 hp, however, that is now a bit lower. Some
of the tricks to get the higher HP was mild and medium porting, bridge,
"J" and "P" porting. Then there was the option of turbo...the one, in
combination with a medium street port, I chose.
When
started today, mine was turning about 6200 rpm at 35 MP (with a 3 lb
spring in the wastegate).....it surged forward against the chocks and
breaks. IVO prop full fine. Nice feel of power even if it likes to
heat up fast in 90 degree weather like this.
Also, I
think this was discussed before too, when I go WOT my engine develops
up to about 6250 rpm, but then drops a couple/few hundred rpm to
usually just under 6000....sometimes just above. Thoughts? Ed,
(perhaps Al)it seems you may have chimed in before. Regrettably, when
I search the archives I tend to get frustrated due to thread drift.
I heard a
lot about porting when I was initially investigating all this and I
chose to use a medium street port. I let Mazdatrix do the work. That
being said, I have read all but nothing on others porting their engines
(other than a LOT from PL on P ports). Since standard porting does not
reportedly effect reliability, only, potentially low low idle (I can
idle as low as about 1300 rpm when warm smoothly) why is it not
discussed and/or utilized more?
Just
curious guys/gals. Discuss <g>
Always loved
the lines of a Tailwind – in fact, I have the plans and license to
build a W-10 sitting on my book shelf. Unfortunately, I doubt I’ll
ever get around to it.
Ok with
thermostat in the system holding it at 190F – we really won’t know how
much reserve capacity you have in your cooling system until the power
goes up.
I agree with
George, 200HP out of an N/A 13B is really pushing it. I think 180HP is
a more realistic expectation – but, hey you never know. It amazing how
power goes up when things come together.
Enjoy and
fly safe.
From: Rotary
motors in aircraft [mailto:flyrotary@lancaironline.net]
On Behalf Of John
Sent: Wednesday, May
26, 2010 2:36 PM
To: Rotary motors in
aircraft
Subject: [FlyRotary]
Emailing: 3 view w-10 003, first flight
My most
humble thank you to flyrotary and all you fine folks that have helped
bring my project to flight. Ed and many others whom I won't try to
name here because I would miss many. The plane is a Wittman Tailwind
W-10, empty wt. is 981 and it is a tri-gear, Wt. is within 6 #'s of
similar trikes w/ 0-320 engines. I have the thermostat in the engine
because most of the year it is not very hot, if we get 50 days with the
temperature over 70, we have had a hot summer, not a lot of beach bunny
activity. 7-1 I'll be 74 and have realized my dream, now it's all
gravy. Again, thanks guys!! JohnD
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