Old 09-19-2011, 03:14 PM
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Default RE: UMS Evolution Radial Care and Operation

Thanks for your post Lars. You might show pictures. I need to figure out how to post pictures myself!

I would like to discuss correct propellors for these Engines. When the 7-35 was released last year the top recommended rpm was a ridiculously slow 5200. The engine would be loaded down severely at that rpm and less than 1 hp would be developed! An APC 17x8 was turned at about 7000 and a Xoar 17x8 wood was about 6500. The Xoar has a larger hub size which matches the prop driver perfectly. The engine ran well at these speeds. The early carbs had to have a rich idle setting or else the mid to full range would be too lean regardless of high speed needle settings. The new carbs will address this. Notice these engines have an under square stroke to bore ratio along with minimal valve overlap. This translates into ideal loads at lower rpms. The 7-70 on my 91 inch corsair turns a 22x10 3 blade warbird Xoar prop at 4500 rpm. There is no detonation and the engine runs quite cool! As mentioned it flies this plane with authority. I take off at half throttle. A two blade prop should be around a 22x12. Some of you might say why the high pitch? This is not high pitch for this diameter. A diameter to pitch ratio should be around 2:1 or more for improved aerodynamic efficiency. Many people say I have a slow WW1 plane, well throttle back and it will fly well and the engine will sound more realistic. 3D is where low pitch props with great diameter work well due to the high thrust at near zero speed. Increase speed with these props and the aerodynamic efficiency drops dramatically. My 9-90 swings a 24x12 at around 5800 rpm and hauls the 24 pound Robin Hood strongly. The plane will fly at 1/4 throttle with added up trim very slowly and sound outrageously realistic! Cannot do that with low pitch. I do experiment with many props. My Saito 90 r3 and 325r5 have a very over square stroke to bore ratio. This translates into developing power at much higher rpms. The 90r3 can only turn a 13x7 with out overloading and loosing performance. The 325r5 is suffering with anything above a 20x10. That is the advantage of these engines from UMS. They can turn the larger props and be in their ideal operating range, look more scale and enjoy the efficiency gained from larger props spinning more slowly. Think about the P51 and corsair etc. (full size) They have gearboxes in front of the engine that drops rpm, increasing torque so they could turn those huge props with high pitch for increased performance.

Recall the newer engines have greater displacements so the props might have to be bigger and/or have even courser pitch than before.

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