Which prop? Hanger 9 Mustang

[Dean Hunter]

OK people. I have a Hanger 9 Mustang with an OS90 FS, swinging a 15x6 APC, giving me 8300-8400 rpm. Seems a little slow on the uptake

Suggestions from various sources (club mates, art work on a Trueflite Spit) include :13x9,13x10-11,12x13,14x10

Thoughts please as which is the best ......

[Bob Cotsford]

My standard prop for a 91 4 stroke in warbird/aerobatic models is a 14*8 preferably APC. On an RCV90CD in a Harvard I had to drop to a 14*7 but that is an exception. If it's an old style pre-Surpass 90 then my suggestions might be too much (like your 15*6 for a warbird)

Google suggests a 14*6 as the top end for the OS FS90 if it is the old style engine.

Edited By Bob Cotsford on 31/05/2016 20:57:14

[..]

d I run my four os 91 surpass engines on a 13/8 prop. Generating around 9800 rpm richen'd off to about 9500 for use

[Jon H]

I would use a 14x7 and would expect to see 8500 or so on the 14x7 apc. If you have the engine well tuned for max power and have proper cooling there is no need to run the engine rich, all you do is waste fuel and make a mess.

[Dean Hunter]

Consensus so seems smaller diameter larger pitch? Not what I thought originally. I have ordered some from Slough RC this afternoon in anticipation (other model shops stock props too) - I will see how they go..........

Many thanks

[Former Member]

[This posting has been removed]

[Jon H]

Thanks Percy you beat me to it

Our 100 is quite long stroke so has quite alot of grunt for big props. Our 180 is the same.

For your mustang when selecting a prop you need to ask yourself 'what is missing' from the performance you currently have. if you open the taps on takeoff to find the model leaps forwards with great gusto but then clambers into the air revving away happily but makes little progress, although storms up any vertical maneuver you ask then quite likely your pitch is too fine as the model is reaching its maximum speed too early and then just sits there.

So in this case go down one on dia and up one on pitch before trying again.

If however the model staggers into the air before getting into its stride after half a circuit, and is a bit naff in vertical up lines but like a missile on the way down, and lands at speed not unlike the space shuttle, we can assume that perhaps our pitch is too high and the model needs a significant forward speed to get the prop into the range it needs to not be stalled out and useless. In this case, go up one on dia and down one on pitch.

 

In my experience many ic modelers are very shy when it comes to changing props and testing different combinations. If its a 46 2 stroke it gets an 11x6 on it because 'thats what the shop told me i needed'. Often model shops may not ask what type of model is being powered and so cannot correctly advise the type of prop needed. I usually get my students to fit a 12x5 to their 46 powered trainers to get the speed down and make the model more controllable. 

As an example, our Laser 80 is not far off the OS91 in terms of performance and i recommend 14x6 props for running in and sport/aerobatic models like an acrowot or cap, 14x7 for WWII models and 15x6 for something like a tiger moth.

The 14x6 gives the maximum power due to the high rpm of the engine, the 14x7 gives a little more pace to the model and the additional bite makes the model more speed stable. The 15x6 is all about thrust to overcome the drag of the biplane. Under certain circumstances a 15x8 can even be used on our 80 and i am considering this for my hurricane. Large thrust from the big diameter and plenty of speed available from the 8'' pitch at 7500rpm. I just need to be careful i dont overload the engine in climbs as it would overheat very quickly.

Another example would be my new sea fury. 80'' span and 23lbs so its a big model. The laser 360v i have in the nose is being strangled a bit by my exhaust system so was down about 200rpm on standard which was taking the edge off its performance. The 22x8 prop was not cutting it so i changed to a 21x8. I knew i needed 8 inches of pitch so had to drop dia to get the rpm i needed for the power output i was after. The drop to 21x8 did the job and the model is totally transformed in terms of its performance. It still lacks a tiny bit in the vertical, but there is nothing more i can do with the prop so its a change of the exhaust to get my rpm back. But, that is far too much hassle so i will be leaving it well alone.

The final issue to consider is propeller efficiency. Some props are better than others at turning engine power into actual thrust. Master airscrew are really naff especially at the bigger sizes and should be used only as a last resort. a 20x8 classic master on our 300v would do about 7800rpm but in flight has no bite at all and just turns rpm into noise. The menz 20x8 on the other hand runs 400rpm slower on the same engine but in flight gives almost double the performance.

This is why we do not quote hp figures as they mean nothing. If calculated using the Master prop the 300v kicks out just over 4.1hp but if calculated on the menz that drops to 3.6hp. The HP calculators do not factor in how well props pull in flight so hp on its own is meaningless when it comes to working out the performance a model will have.

 

Edited By Jon Harper - Laser Engines on 01/06/2016 08:51:16

[Bob Cotsford]

It really depends on whether it's an FS90 or a 91 FS - there's a world of difference!