Ed,

 

 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.

 

Mike Wills

From: Ed Anderson

Sent: Friday, March 05, 2010 6:36 PM

To: Rotary motors in aircraft

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