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Don't forget that with a true constant speed prop, the governor is managing prop pitch to keep rpm at the preset number. It will likely be 'maximum fine' pitch for peak HP before brake release, and pitch will be increasing with airspeed as the takeoff roll continues, which keeps rpm constant. Fixed or manual control at one setting will see the symptom described. A c/s Lyc will be at 2700 rpm (full power rpm at sea level) from static until a control is changed, while most fixed pitch props on Lycs are set up for ~2700 rpm when the a/c has accelerated to max speed at wide open throttle at sea level. This results in static rpm of somewhere between 2100 & 2300 rpm, depending on individual prop design. RPM typically increases to 2200-2500 rpm during climbout. Try running the rpm ratios against the rotary's rpm spread to see if they *roughly* correspond to what you're seeing with the rotary. If you have in-flight manual control, you can imitate a c/s controller's behavior setting static full fine to closer to 7400, and dialing in more pitch as you accelerate to maintain rpm. But it would be a pretty high workload for one person.

BTW, that flat torque curve thing may need a bit of qualification. It's flat in the car due to the massive assistance of Rube Goldberg, which all aviators have deleted from the engine. It may very well be flat in the area that matters to us, above 6000 rpm, but I wouldn't lay money on the line without actual testing of that particular intake/exhaust.

Charlie

On Thu, Aug 6, 2020 at 10:36 AM argoldman@aol.com <flyrotary@lancaironline.net> wrote:

Hy Guy,

It is possible that your initial blade angle is set too coarse.

With a CS or Variable pitch prop, one of the purposes is to enable max TO RPM at takeoff. One of the reasons that the RPM builds up after a few seconds is that after you achieve some forward motion the angle of attack to the blades changes enabling full RPM.

There is another possibility, again relating to the prop and that is that because of the pitch, the prop is actually stalling producing little lift (thrust) until forward motion changes the angle of attack and it unstalls. This is accompanied by a rather strange sound coming from the prop until it unstalls.

I had that problem with my dragonfly. (Midwest AE110R/ CS prop (airmaster) warp drive blades) Initially when I tried it on an early x-plane I thought that it was the program, however in real life it happened exactly as simulated. I solved that problem by slowly advancing the throttle.

-----Original Message-----
From: Stephen Izett stephen.izett@gmail.com <flyrotary@lancaironline.net>
To: Rotary motors in aircraft <flyrotary@lancaironline.net>
Sent: Thu, Aug 6, 2020 9:58 am
Subject: [FlyRotary] Testing best rpm power setting

Hi guys

I’m wanting to inflight test for the best RPM for peak power of our 4 port Renesis with variable pitch prop (No constant speed control at present).

On Takeoff at Full Fine we hit 6900 after a few seconds and then rpm builds to 7400 by 500 feet and 110 KIAS

I’m wondering if we are well past the peak torque and would be better off at a lower rpm.

Am I right in thinking the following would use the aircraft as a Dyno to establish rpm for best power?

1. Set up level flight (auto pilot)

2. Maintain WOT and constant mixture (best power - say 0.9 Lambda - 13.2 AFR) via Wideband O2

3. Vary RPM in 200rpm increments for 2 minutes each setting between 5600 and 7200 by adjusting prop pitch

4. Note Fuel Flow and IAS (Logged on Dynon Skyview)

I’m thinking this would give me an indication of best RPM for highest power.

Our Inlet is just under 19” long, measured from the side face of the engine.

So I’m thinking peak torque may be around 6400 rpm.

Your insight would be appreciated.

Thanks

Steve Izett

Renesis 4 port RD1C (2.85:1) Inflight Adjustable Prop

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