Best cruise prop ??  

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Jabiru Aircraft
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22/05/2018 7:04 am  

Best cruise prop ??1 year 3 months ago#485

·         Russ

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2200 jab........which is proven best cruise prop.

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Best cruise prop ??1 year 3 months ago#490

·         Doug Smith

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Hi Russ,
Unfortunately you've asked the classic "how long is a piece of string" question there... the answer depends on the installation, airframe, cruising altitude etc.

If you're using an airframe that cruises below about 90 - 95 KIAS then chances are that there's too much airframe drag to make a "cruise" prop really worthwhile. In that case focusing on drag reduction is your best bet to improve speed. I've attached a picture of my plane - if you look closely you'll see nose leg fairings, main leg fairings, flap hinge fairings, a non-standard cowl and extended trailing edge fairing on the struts. With those mods it gets about 108 KTAS @15 Lph using a pretty stock 2200. I know another aircraft with a similar suite of mods but with the short wings (mine are the long UL wings) that gets more like 113 KTAS for the same fuel burn. Very importantly, both of us are able to get decent RPM on take-off and climb (means the engine is not being over-loaded and in danger of pinging).

Assuming you have a reasonably sleek airframe, from my personal experience I find that props which are somewhat light on blade area and are a little higher pitched work well... the "low" blade area allows the prop to "slip" on take-off and climb so you still get good RPM, then once you're up to speed and altitude it slips less and the higher pitch pulls you along. If you're flying mostly at lower altitudes and don't need awesome short-field performance you can get this effect from a smaller diameter prop with higher pitch (I was running an ugly wooden prop of 48" pich and about 54" dia for years and it went pretty well - except at altitude!). If you want to go higher (above about 6,000 ft ASL) then you'd want a bit more diameter, but you'll probably have to cut the pitch/area back to make it work.

So, in terms of what's been proven, I can say from personal experience that "tuned" 2 blade wooden props work pretty well. If you don't have a spoke-shave and a Ouija board in the shed I'd say to look at designs with more slender / flexible blades, possibly lower diameter and/or drag reduction at the tips (though this never works as well as the marketing claims!). If you find a design that has those features plus is ground adjustable then it should go ok.

Obviously the other thing to keep in mind is the category of your aircraft - if it's experimental then you can fit whatever you like (at your own risk: I've broken my engine a few times with my prop tinkering) but for other categories you need to go with an approved type.

Regards,
Doug.

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