jplavoie

Hi Bob,

which prop sizes are recommended for a Jett .35. The Jett website provides data for APC 9x6, but I wonder if a 8x7 or 8x8 would work well too. My engine has the red tuned muffler.

Thanks

JP

bob27s

My Feedback: (19)

Thanks for writing...

Yes, the 9x6 is the baseline prop for performance comparison. However you can select and use the best prop for your application.

Anything from an 8x7 up through a 10x4 is fair game.

My main concern with most "Stock" 8" diameter props is that the hubs and blade roots are fairly thin. It all depends on the prop, how it is constructed, material, prop thickness, hub thickness ...... you get the idea. At 19-20k rpm, you may risk shedding a blade. So mainly for that reason, I usually recommend the 9" props. If you wish a smaller diameter, start with a 9x7 or 9x8 and trim/balance it, or look toward the APC 8.5x7 D-1 series Q-500 prop. The Vess 9x6, is OUTSTANDING.

Prop the engine so it hits over 17,500 peak ground rpm. On the 35, that is usually easy to do You are generally looking for around 18,000 or more so you can launch over 17,500.

Not a big concern with letting it turn rpm - it likes rpm. One of the best props I found on my BSE-30 was a 9x5, turned up around 19,000.

Find peak rpm, back off about 600-800 rpm from peak ... fly it.

Tell me the aircraft application, and I can try to suggest a good prop to look at.

jplavoie

Bob,

thanks for the advices.

The aircraft is a Sig Kobra (.25 size). It was build without landing gear to reduce drag. It is quite fast for such a small aircraft. I had some downwind passes over 100 mph with the 9x6.

However, I lost it due to radio failure (transmitter) and the engine is lost as well. Since it was a very good flying aircraft, I will build another one and I decided to put a brand new FIRE .35 (ordered yesterday). I will probably put a Macs tuned pipe instead of the red tuned muffler and ask Dub to tune it for 9x6 prop (this will be my starting point). Let me know if you have some suggestion (tuned pipe, prop, or anything else that can improve the aircraft performance).

Thanks again for your prompt reply

JP

bob27s

My Feedback: (19)

sorry to hear you lost the first plane ..... but you know what to expect now !

I am confused ... briefly, did you order the pipe tuned by Dub along with the engine, or are you going to add the pipe later?

The FIRE rear exhaust engines typically use a full pipe. Dub's pipes are slightly modified to help with exhaust velocity and backpressure.

Dub typically pretunes his pipe systems per your prop/application request . Once Dub has the system tuned .... he should let you know what the rpm target is. From there, you can play with props a bit, with the goal of loading for the same ground peak rpm.

If you build the plane without landing gear, you will probably want to stick with the stock 9x6 for a while. You need the immediate thrust for hand launch.

The racing props are a good alternative for better top end speed, but the plane will not accelerate out of your hand quite as rapidly. Smaller 8" diameter will have the same issue.

As far as aircraft performance ..... you are doing about all you can there. It has a fairly thick wing, and a good deal of frontal area.

Compairing performance between the red muffler and a full pipe - gain is about 500-700 rpm more on the top end. Change in aircraft speed for your Kobra would not be substantial.

jplavoie

Yes, I did order the pipe along with the engine. I asked it to be tuned for 9x6. I agree that more thrust is needed for hand launch.

The aircraft will require some modification to fit the engine/pipe, but it is quite easy to customize (this is the main advantage of a kit).

Thanks again

JP