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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