Paul,
How much power can a 2-rotor make and still be considered reliable
enough for aircraft use? Is 300 hp the upper limit for a 2-rotor
supercharged, geared engine? How much would the engine weigh, roughly,
assuming a Crook gearbox and a blower weighing about 15 pounds?
Regards,
Gordon.
I really don't know. My guess is take off at 300 HP and
back off to 200 HP for cruise. Then you have the cooling
problem. 300 HP takes a heap of cooling.
--
Paul Lamar ...No rotor no motor.
My 2-cents worth on this point:
- Getting 300hp on the prop shaft out of a mechanically supercharged
engine
means developing 320-330 hp (we never managed to get more than 300hp out
of
a 2-rotor... but we also never tried!... blowing up engines gets
expensive
very quickly!)
- While it is possible to boost a 2-rotor to 330 hp, it is much
harder to
keep it out of detonation with "regular" gas (i.e. non-racing fuel). It
takes a lot of boost to get that much power (typically >60 inches
MAP), so
I
would insist on very efficient inter cooling to keep intake air
temperature
below 50°C [122 F] if possible, certainly below 60°C [140
F].
- Next, water cooling problems: you must avoid localized boiling spots.
It's generally OK if it's only a short duration power surge (like in
drag
racing), but longer than that, things can get out of control real
quickly.
- Metal apex seals won't last long. 3mm ceramic seals are probably a
must.
- Last thing: be prepared to open up the block and change parts quite
often
during tuning! Also, fuel consumption will be unacceptable for cruise
above
200-220hp.
My opinion: unpractical and unwise for any other purpose than racing...
cars or aircraft. If you need that much power, go with a 3-rotor, you
will
have none of the above-mentioned headaches!
Francois
Hi Francois,
Thanks for those intake air temperature numbers. Been looking for that
data
for a long time. Back in the 1940's 50's & 60's large radial aircraft
engines, both
supercharged and turbo charged, routinely used anti detonation fluid for
take off.
It is also common practice at Reno air racing. I suggest anybody looking
for
above
300 HP for take off out of a turbo charged two rotor think about
using it.
Also read this C&D article on how PTP obtained 360 HP out of a turbo
charged RX8 engine with ten to one compression ratio rotors using intake
air triggered anti detonation fluid.
http://www.rotaryeng.net/Turbo-RX8a.jpg
http://www.rotaryeng.net/Turbo-RX8b.jpg
Here is a power to weight ratio chart I have been working on. Still a
work
in progress. For normally aspirated only and no psru.
Paul Lamar ...No rotor no motor.
Francois,
Thanks for your input.
The intended use for this hypothetical engine is in a fully aerobatic
single-seat monoplane that would be designed with the high
power-to-weight
ratio of the rotary in mind. If we can get a better power-to-weight
ratio
than with a piston aircraft engine, we can have a lighter airframe, a
superior thrust to weight ratio and a more capable airplane.
Fuel efficiency is secondary to power-to-weight ratio, hence the
choice of
supercharger. Also a turbocharger is useless in an aerobatic plane where
you
need instant power response to punch out of maneuvers. For this
application
the positive displacement twin-screw supercharger would be optimal as it
makes even boost throughout the range.
300 horsepower at the prop is the figure I'm shooting for. If we can
have
an
engine weight of 300 pounds, I believe it would be possible to build a
+10g/-10g airframe of 500 to 600 pounds, for an empty weight of 800
to 900
pounds, and a full-up weight of maybe 1200 pounds. This would give a
power
loading of 4 pounds per hp, which is outstanding -- and a thrust to
weight
ratio of better than 1:1.
The supercharger is going to eat up about 20 to 30 horsepower as you
pointed
out, so to make 300 at the prop, we need 320 to 330. However, I'm
surprised
that would take 60 inches of manifold pressure. I had figured 48 to
50, or
8.5 to 10 psi boost. My thinking was that if a normally aspirated Rx8
makes
200 hp at 7000 rpm and a P-port makes 240 hp, it would seem to require
modest boost to reach 325 hp.
The cooling is the issue that I'm concerned about most, namely the point
you
made about localized hot spots. Assuming you have a radiator and
properly
designed ducting that can reject the amount of heat necessary, is the
engine
case up to it? Can it flow enough coolant and is there enough material
cross
section in critical areas to conduct heat away quickly enough?
I'm thinking probably yes, considering that the Le Mans engine was
making
much more power per rotor and was able to reject the heat. But I don't
know
if that was with stock or custom engine cases?
Regards,
Gordon.
PS: Paul, thanks for that chart, it really shows the notary's
advantage --
getting close to that small Garrett turbine.
-------------------------------------------------
Here is a improved version with more engines added.
PSRU weights are included but no turbo charged engines.
The power to weight ratio of a turbo rotary takes a big jump
while the aircraft engines do not due to the necessary flat rated
nature of a turbo charged aircraft engine. The air craft engine's
structure is not up to any boost at sea level.
BTW you cannot extrapolate a NA RX8 engine to 300 HP boost
wise due to the 10:1 compression ratio. Now if you are
willing to inject detonation fluid as need you might get
300 HP with less boost than Francois mentioned.
You should have said from the get go you wanted instant
throttle response. That changes everything :)
The cooling limitations Francois mentioned should be checked
with a "cylinder head" temperature probe near the lower spark
plug. Modification can be made to the rotor housing cooling passages
in this area to improve the situation. See the attached jpg's.
Also read http://www.rotaryeng.net/how-to-cool12.html
BTW the answer to vertical penetration is a larger prop and
a higher gear ratio. The geared Russian and East European
GR-360 engines are prime examples.
Paul Lamar ...No rotor no motor.
Thanks, Paul.
Can you elaborate on the housing mods? What exactly has been done? I
guess
the idea is to increase coolant flow? I assume there is enough metal in
the
hot spots for adequate conduction from the case to the coolant? (It's
hard
to add metal, if there's not enough.)
I was going to mention that I would try to find a gear ratio of over 3.
Yes
a big prop will not only give more thrust and increase vertical
capability
but also increase prop gyro effects which is useful in many maneuvers.
Btw, some versions of the Russian radials make up to 450 hp. They are
all
supercharged. The main difference is boost.
Also, I am thinking that a carburetor would have certain advantages,
probably an Ellison so-called "throttle body injector." This has the
benefit
of simplicity, lower pressures, less weight, etc. The Ellison gives very
fine fuel atomization and by mounting it just ahead of the
compressor, the
temperature of the air going in would be quite low, probably around 0 C,
due
to cooling from fuel vaporization.
This would let me go to about 10 psi boost without an intercooler. (I
haven't done the math yet, that's just a quick guess.) Besides not
wanting
the weight of an intercooler, you can't use one anyway in a suck-through
application.
This setup would be considerably lighter than fuel injection, high
pressure
pumps, intercooler, etc.
The only question is how much detonation margin and what octane fuel. I
would prefer to be able to go on 100 LL or even high-octane pump, but I
definitely do not want water-injection, for reasons of weight and its
impracticality in an aerobatic airplane.
On the plus side, you would need maximum power for relatively short
bursts.
Regards,
Gordon.
First you need to put the thermal couples where shown.
Second you are going to need a dyno to develop this 300 HP
super charged engine. Once you have a base line "cylinder head"
temperature reading super charged to 300 net HP then you can
start making water jacket modifications. If that proves successful
you are home free. If not you can try upping the water flow with
a larger water pump.
You could also call Jim Mederer at Racing Beat as he has a lot
of experience turbo and super charging rotary engines as well
as making cooling jacket and other cooling modifications. I'll
send him a copy of this but there is no guarantee he will
answer by email.
Paul Lamar ...No rotor no motor.
Hello Gordon,
While you are contacting people you might contact the people at Baylor
Univ.
in Waco Texas. They have an ethanol powered (E-95) Pitts that has
helicopter high compression pistons in it. I believe the pistons were
close
to 11:1 compression ratio. Along with with the 12% to 15% increase in
power
you also get a 75 degree decrease in intake temperature. The trade off
for
this power would be high fuel consumption which is not an issue for short
air show routines.
For shuttle flights you could switch to gasoline and keep the power
loadings
down.
What is really going to help the power to weight ratio are the
aluminum end
housings with the steel inserts faces. Any news on those??
DougFir
11:1 to one is out of the question for the rotary. The combustion
chamber shape becomes so bad it negates the advantages of the 11:1.
Nothing on the two piece end housings.
BTW Gordon a tuned p-port will improve the efficiency of
the intake plumbing. Less work for the blower and
less intake heat. I still think you are going to need
an intercooler. Don't leave home without it.
Paul Lamar ...No rotor no motor.
Thanks, Doug.
I've heard mention of the guy at Baylor with that Pitts. I think I
read an
article somewhere.
That's a good point about switching fuel for cross-country hops. Maybe
even
have a simple clutch on the blower pulley so it could be turned off --
and
then no worries about knock while running gas.
Paul,
Yeah I agree the P-port does have some advantages I overlooked. Will
have to
reevaluate that option.
Regards,
Gordon.
Hi Gordon;
Another alternative for your requirements might come right out of the
Everett Hatch/Powersport cookbook.
I understand he used Nitrous for the time to climb records. You might
talk to Ken Welter regarding long term use
and reliability limits.
A "race" prepped engine should easily produce the required HP for short
periods with either carb or EFI.
Cheers
Cary
-----------------------------------------------------------
Thanks, Cary.
Although I wasn't considering nitrous, it does add lots of boost and even
cools the charge somewhat I understand.
The only thing is that triggering the boost would mean another control to
operate, although if you mounted it as a thumb switch on the stick, it
wouldn't bee too bad.
Regards,
Gordon.
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