Mike;
Taking your engine off and putting it on
a dyno would be informative and fun; but complicated, time consuming, and expensive.
It would be a much simpler first step to make a change of propeller; especially
if you could find one to borrow for a test run. I don’t recall the
specifics of your prop diameter and pitch, or what static rpm you can get, but
a relatively small change of pitch, for example, can make a fairly significant
difference.
I first ran my 20B with 85” pitch
on a 66” dia 3-blade Catto (2.17 re-drive). The max engine rpm I could
get was less than I expected. Reducing the pitch to 80” (a bit less than
6%) increased the rpm by 200-300 over pretty much all regimes, and seems about
right to me. I now get about 5500 static, maybe 5600+ takeoff and climb, and 6300
WOT level flight. It gave some improvement in takeoff off and climb performance,
but hard to tell any difference in top speed, which isn’t too surprising.
It could be that someone on this list;
or your RV crowd, might just have an appropriate prop for a quick test??? You
might have a chat with Craig Catto about your plane/engine, and see if he might give a recommended dia/pitch.
Al
-----Original Message-----
From: Rotary motors in aircraft
[mailto:flyrotary@lancaironline.net] On
Behalf Of Mike Wills
Sent: Friday, March
05, 2010 9:30 PM
To: Rotary motors in aircraft
Subject: [FlyRotary] Re: TB size,
Travel and Power
I hear what your saying (and
I get it). Here's the thing, and I admit I may have a misunderstanding
here. Your explanation applies if you are measuring manifold pressure
essentially at the rotor face. If you measure MP closer to the TB as I do unfortunately,
it is possible to see ambient at full throttle but have restrictions in the
intake tract that would result in less than ambient at the rotor face.
Conversely (and again theory - I'm open to being shown wrong here), a properly
designed DIE manifold would show ambient MP measured at the TB and greater than
ambient measured at the rotor face.
I accept that by trimming the prop
I can lighten the load and gain some HP (though some here seem to think that HP
will increase linearly with RPM to infinity and I don’t buy that either).
But as you alluded to in your previous, there's some potential for mistake
in trimming any prop until you are quite sure that you are not HP limited by
something other than load. And I'm not sure yet. Just a feeling based on the
fact that I'm using a cut down RX-7 TB that’s maxed out about 1/3 short
of fully open. And honestly I still havent dug any deeper because the airplane
is flying well and has good performance. But sooner or later I'm going to
want to get more than the 5700RPM I'm currently getting - I want all the
performance that’s there.
Sent: Friday, March 05, 2010 6:36 PM
Subject: [FlyRotary] TB size, Travel and
Power
I am apparently not doing a very
good job making the point about the relationship between throttle body size,
throttle travel and engine power clear. So here is another try at it -
using extreme examples and no math. Let’s assume your volumetric
efficiency is 100% (no losses).
IF you put a ½ ” dia TB on you
engine. You could have it wide open and your engine (under normal prop
load) probably wouldn’t turn 4000 rpm. That is because even wide
open, the ½” dia TB restricts airflow sufficiently - that the manifold
air density never approaches the ambient air density. Since we know that
the engine power is directly proportional to the density of air in the
combustion chamber – and this density is limited in this case to less
than ambient, you engine is not going to produce much power, certainly not full
power. So this bit of information tells us “Bigger Lithium
Crystals, Scotty!!” – i.e try a larger throttle body.
Now if you keep enlarging the
diameter of the TB you would find that at fully open -your engine would be
producing more power than it was, but perhaps still not the maximum power it is
capable of. This is because the air density in the manifold has increase
due to the less restrictive flow, but is still below ambient. This is due
to the better, but still restrictive effect of the TB size on the air
flow. Now if you continued enlarging the TB size, you would reach a point
where with the TB just reaching fully open - your manifold air density is
exactly ambient and your engine is producing all the power it is going to.
Now if you enlarge the TB even
further, you will simply find that you can cause the manifold pressure (air
density) to reach ambient without opening the TB fully. Its simply large
enough that all the air the engine can use (ambient air density point) is
met at partial throttle opening. In fact , you can certainly continue to
advance the throttle thereby opening the throttle plate even more – but,
you are not going to increase the air density in the manifold and therefore you
will not produce any additional power for that additional throttle travel.
Bigger Throttle body’s result
in more power only up to the point the airflow they permit causes manifold air
density to reach ambient. Beyond that point, the only thing they do is
provide frustration – by having all that throttle travel remaining which
does nothing to produce more power {:>).
Now if you can somehow lighten
the load on your engine, then engine can turn faster providing more
“suction” on the manifold volume reducing the air density below
ambient, now opening your “oversize” TB a bit more will produce
more power because you are increasing the airflow again to the point where the
equilibrium point between ambient air density in the manifold and rpm is again
reached. Lighten the load further and you can again increase engine power
by opening your TB more. Etc, etc.
Ah, ain’t this hobby wonderful
{:>)
Ed
Ed Anderson
Rv-6A N494BW Rotary Powered
Matthews, NC
eanderson@carolina.rr.com
http://www.andersonee.com
http://www.dmack.net/mazda/index.html
http://www.flyrotary.com/
http://members.cox.net/rogersda/rotary/configs.htm#N494BW
http://www.rotaryaviation.com/Rotorhead%20Truth.htm