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Bubbleboy

Prop recommendations please

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I need to buy a prop for my Rainbow Cheetah aircraft. It is the older Brother to the current SkyReach Bushcat. Im not looking to start a slanging match on here I just need peoples lived experiences of what they have used and any positives and negatives. If its too bad an experience, maybe message me direct. The manual recommends a Kiev prop. Just looking for other options. Aircraft will not be used for training. Just my toy.

 

I need a 3 blade to go on a Rotax 912ULS.

 

Regards Scotty

 

IMG_3798(1).thumb.jpeg.88260c9c5938db82d6fbe50f523988d8.jpeg

 

 

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Bolly, Aussie, well made, tested, proven, adjustable, good value. 

 

 

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Bolly for sure. They're great people to deal with, will help you select the right prop and being a local manufacturer, they're close by for repairs or advice if needed. Also, they make good gear.

Probably the Bolly BOS3.

 

 

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I've had a warp drive for 7 yrs, 600 hrs. Plenty of dirt strip landings. Holding up well.

 

Reasonable strength (with nickel) edges. (Carbon fibre blades)

 

Good range of sizes.

 

Ground adjustable.

 

Replacement blades good price. Can be matched to existing serial number, so dont need to buy all 3 if 1 is damaged.

 

Probably not the most efficient or good looking prop but a work horse rather than show pony.

 

Quite popular and plenty around.

 

 

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My  19 year old, 850 hr, Zephyr uses a two blade Fiti connected to a Rotax 912ULS (100 hp) . Three blade versions are also available.

 

I find the Fiti to be exceptionally good at translating the engine power to thrust  - sub 100m take off, up hill, on grass ,with full fuel combined with excellent cruise, sea level 4800 rpm 100 knots + @ 12.5 L/H.

 

It is a very quiet prop, as are most Europena ones,  being designed to meet very low noise emission standards. 

 

You sound decided on the 3 blade option - In my experience, unless you have clearance/ rpm issues ,all a 3 blade will do for you is add cost, weight, complexity and sex appeal. Two blade prop, on low hp engines, is the way to go for best efficiency.

 

 

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My Bolly Optima 72 inch has served well and reasonable price. I am looking at the Whirlwind 75 inch STOL prop though for my next aircraft I am going to build when it arrives. I am trying to decide just how good it is above the Bolly. The price is US$1900 so quite expensive compared to the Bolly Optima. Tim Howes just got his Storch flying yesterday after a major refurbish..he has the Whirlwind on it now and was blown away by how good it was compared to his original prop...So I suppose it all comes down to the amount of bucks your willing to spend

 

 

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...So I suppose it all comes down to the amount of bucks your willing to spend

 

Yes & No Kyle,

 

Yes  - if you go with a "often in the spotlight brand"

 

No  - if you are willing to do a bit of research and take a punt - there are quite a few very good props, that we dont hear all that much about (in Au), that do not demand sacrificing your first born child to acquire.

 

Another thought - it strikes me as odd when I hear that a newly fitted prop  produces "blown away by how good it was compared to his original prop" - how can this be?? I agree some props may be better than others in particular applications, however unless the original prop was just unsuitable for the job/aircraft (a pilot/ fitter problem) then improvements are surely minor/small? Much more info needs to be supplied eg:

 

Original prop - swept area, fixed, ground adjust, CS ??

 

Pilot observations - ground role, climb, cruise, noise  (with respect to flying condits) ??

 

New prop - as above

 

Comparison (price may also be a factor) with adjustments for differing flying conditions

 

I guess if his aircraft is exactly the same as yours and his new prop is the only diff/change and he"leaves you for dead" on all performance areas, then you should surely go with a new prop

 

 

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My Bolly Bos5 is miles ahead of the previous wooden one. Mind you it was known to have poor performance when I was given it (yes it was free) but at least it got me flying when I damaged the first one beyond repair. The wooden one was supposed to be 63x58. I got reasonable climb out of it but hopeless cruise of 90 knots at 2950 rpm. I was always fixing chips in the leading edge & tips. The new carbon fibre Bos5 is 60xwhatever as it is ground adjustable. A bit of experimentation & I get 1200fpm climb at 80 knots  & 120 knots cruise at 2800 rpm & cooler temps. 2 for go & 3 for show still applies. 2 blades are always more efficient than 3.

 

 

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My only experience with the Whirlwind was when I flew the new Rans S21 while over in Kansas last year. It is a stock 912ULS and the aircraft with 2 POB and half fuel which it was etc we would have been around 620kg take off weight...the MTOW with the Rotax is 727kg. Hays is around 2000 ft AMSL and the weather was cool maybe 10 degC. Our climb was around 700ft/min quite easily at around 75mph with 5000 rpm. I was pretty impressed with those figures considering all the numbers and knowing what my Sav does at 600kg and around 5200 rpm and a climb of 60knots

 

The storch that Tim has and he currently only has the 80hp Rotax in it of course is a very different beast but he has enough hours flying in it and in all sorts of scenarios I would think he would notice major differences when something has changed

 

Dont know if I would spend the dollars yet on a Whirlwind but the numbers I just quoted first are real numbers that are even on the video if you want to look certainly in the first minute or so  

 

 

 

 

 

 

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+ 1 Vote of thanks to JG for a great study, and thanks to M61A1 for posting the link.

 

Just the info I was requiring, too!

 

 

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JG who frequents this site on occasion has done some testing. Link below.

 

https://stolspeed.com/nid/46

 

The results were  very much as I suggested earlier.

 

Would have liked to see comparisons between 2 & 3 blade props (so popular these days) and some comment however subjective of noise  generated both inside the aircraft and as heard by those on the ground.

 

 

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 A noisy prop is because the tip speed is too high. This is also an efficiency loss. If your plane is draggy or you fly high you need wider blades. A two blade prop should be chosen generally if it has the ground clearance. ( They used to have ONE bladed props.?? )A wooden prop is easy on the engine but won't be as efficient as composite or metal. .I've seen plenty of props tried where the owner has eventually gone back to the original.  Anytime I've flown a 'Kieve " prop it's been pretty impressive..These are general comments. Nev

 

 

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Single bladed props are often used in motorgliders mainly due to engineering storage issues. With the right counterbalance they have fewer vibrations than multi blades but are not more efficient outside of the inertial losses from spinning a heavier prop. Efficiency losses as you add blades comes from drag generated by each additional blade.

 

 

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Yes. More blades more drag  A bit like biplanes. There is an interference factor as well.  The single bladed things were used on smaller conventional but lightweight planes in the 20/30's Must cause a lot of work for the engine mounts inevitably. Nev

 

 

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Yes. More blades more drag  A bit like biplanes. There is an interference factor as well.  The single bladed things were used on smaller conventional but lightweight planes in the 20/30's Must cause a lot of work for the engine mounts inevitably. Nev

 

The counter weight could be positioned so there would be no vibrations at a certain thrust and RPM, without thrust this would then cause a small vibration. 

 

 

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 There's an offset bending moment on the crankshaft due to the single blade. The thrust line is not on the crankshaft axis, but at some point say 1/3rd  at best from the hub.. The engine mounts have to cope with this pushing and pulling, once each time the motor turns. Horrendous..  There's no easy way to compensate for it. It's not large but still significant. We go to a lot of trouble to ensure blades are set at exactly the same angle and track accurately. You are correct in saying you could position the balance weight to correct for ONE particular thrust condition.  It could be at the other end of the engine but even that has problems. You are better to have all weight at ONE End. Good thinking 99.  The fact is they were used ..  Nev

 

 

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 A noisy prop is because the tip speed is too high. This is also an efficiency loss. If your plane is draggy or you fly high you need wider blades. A two blade prop should be chosen generally if it has the ground clearance. ( They used to have ONE bladed props.?? )A wooden prop is easy on the engine but won't be as efficient as composite or metal. .I've seen plenty of props tried where the owner has eventually gone back to the original.  Anytime I've flown a 'Kieve " prop it's been pretty impressive..These are general comments. Nev

 

This would imply that the swept are/length of prop (on a given engine - Rotax 912 ?) varies significantly. Without doing the research I suspect that tip speed is not the only factor but I agree is likely to be the main one.

 

 

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Having a 3 blade prop over a 2 blade will provide additional thrust  but more drag. The thrust is a lot greater in the climb than the drag induced so performance is generally better but once in the S&L cruise configuration the extra thrust is negligible and the drag remains. On recreational aircraft 3 blades are usually a bit better with lower horsepower engines. On large aircraft with lots of horses and larger diameter props the number of blades needs to increase to get that horsepower converted into thrust. Also they need to limit the RPM to stop the blades reaching supersonic speed as the bigger the diameter the faster the tip speed. A lot of turboprop aircraft have multi blade props these days e.g ATR 72 & the later model Hercules. They are almost like an unducted fan.

 

 

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 Once you get a good forward speed the blades are less likely to interfere with each other. The helix the tips describe is Coarse at high forward speeds.. Those props are geared down and usually don't do much above 1200 rpm. High altitude fliers Usually have wider (chord) blades. They make a  totally different noise on takeoff. The DC 3's in PNG had them. I think they called them Paddle props. Most of the common Radials are 2: 1 reduction. the turbo ones are over 10:1 to much higher for small ones, where the turbines do crazy revs.

 

     The Ruskies used a lot of high subsonic speed complex turbo props for big transports, mainly for the extra range over pure jet .  The later high bypass ratio axial flow jet engines with variable incidence Guide vanes may have closed the gap, on fuel efficiency now so you don't see them..  High performance prop planes need a lot of safety features on the prop[s) to prevent overspeeding. There are extremely complex auto-coarsening  and feathering systems and various pitch locks for safety.. Nev

 

 

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JG,s stolspeed blog for prop comparisons on 912 uls is pretty much unbeatabble for information, an absolute gold mine of australian info

 

 

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Completely subjective - There are quite a few  STOL type aircraft operating in my area, using the same engine as my aircraft (Rotax 912 ULS). The STOLS. All use 3 blade props (dont know the brand) and make noticeably more noise at every stage of powered flight than my aircraft with a 2 blade Fiti.  My aircraft has almost as good low speed/landing characteristics but much higher cruise than the STOLS - go figure!

 

 

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