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Permalink Reply by Quinton Oliviero on July 27, 2015 at 5:12pm

Hello Ron,

It's hard to get first-hand information even here in Canada! I just bought a CH250 myself, with the larger Lycoming O-320 and Sensenich 74DM6 pitched to 58", and I feel like the prop is a little underpitched for the kind of cruise speed I would like to see. I had read that with the 150 HP engine it would be capable of cruising at 125-130 kts. I'm starting to question the veracity of that.

Here's some quick propeller math that you should know:

Speed (in kts) = ((RPM x Propeller Pitch) / 1215) x Drag

The Drag number is the loss from 100% efficiency caused by propeller inefficiency and aircraft drag. The factor for most aircraft is between .75 and .85, but our little CH250s are surprisingly sleek and .88 or .9 tends to make the numbers work for me.

So for example my 75% Power cruise at or around sea level is 2450 x 58 /1215 x 0.9 = 105 kts which is experimentally correct.

I'm wiling to bet that your 75% power cruise is 95 kts or so? 2450 x 53 / 1215 x 0.9 = 96 kts

In order to get the "advertised" 130 kt cruise my prop would need to be pitched to 70+ inches! Frankly I just don't think my engine is going to be able to handle that pitch. I've heard that for each inch of pitch you add, you'll lose about 40 RPM, and my static RPM is already about 2300. Such an aggressive pitch would bog my engine down too much to allow me to climb.

So long story short, the builder may have had the prop repitched to 53" because he wanted a better climb, but more than likely it's also because it was necessary to avoid bogging down the O-235.

What is your max static RPM like? (full power, brakes applied) If it's high, like say 2600, then you could definitely afford to step out the pitch. You probably don't want to let it get below 2200, so if it's closer to that then your pitch is probably as coarse as it can get without really crippling your climb rate.

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