aero13

I have a few 0.60 Jett engines that need to be refurbished, but I was wondering if the 0.65 Jetts can produce enough extra thrust to offset the extra 8.6oz of weight? Plus, I've heard that there is a long back order on the 0.65 engines, and the soon I can get either engine choose the better. What are you're thoughts? Thanks for the help!

bob27s

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my apoligies, I posted on here a few days ago, but it apparently didnt "Stick"

All Jett engines are assembled to order. So there is lead time for all of them runs about 2-3 weeks.

For a SJ-65 engine - yes, much longer wait but not considered a back order. This is not a standard product. Custom built, no parts in stock. The big block SJ-76 and SJ-90L are the standard large engine products and can be delivered in a couple of weeks. The .65 has not been available for many years and is built only on special request.

The smaller SJ-60L is a standard product. Keep in mind, this is a "46" size engine. The SJ-90L and SJ-76 are (and very original SJ-60 and SJ-65 were) big-block .61 10cc engines. Bigger case, bigger muffler yes, weight more.

The 60L is a fantastic engine. Power to weight is hard to beat anywhere. Many of the classic pattern guys are flying these in aircraft originally designed for ST 60 blue heads, Webra speed .61, Webra blackhead, and the old OS60FSR. Performance and weight combination makes for great flying aircraft.

Thrust - not a good word to use really. That assumes a prop doing work, props vary, conditions and speed vary, trust varies with forward airspeed - static thrust means very little on a forward moving fixed wing aircraft.

Tell me more about your application, and I can assist with engine choice.

aero13

Well, I working on a heavy lift aircraft for a student design team. The objective is to lift as much payload weight with as little empty aircraft weight as possible. Points are awarded for every oz of payload weight and a small addition is made for shorter takeoff and landing rolls; however, max payload is the main driver. The aircraft will never need to exceed 40mph because of the wing area used. I know Jett rates engines with a given prop at a given rpm because of the uncertainty inherant in both power ratings and static/dynamic thrust ratings.

bob27s

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Understood, and I am familiar with the heavy lift projects. Several of the teams over the years have used Jett engines.

One of the reasons you will not find more information on HP from Jett is because it is directly related to how the engine is propped and setup.

HP = torque x rpm
So...... what you will see from a some manufacturers is something like (making this up): XYS .61 engine 2.7HP at 18,700 rpm.
Is that meaningful ? Not likely because unless this in a geared system or a ducted fan it will never actually see that rpm.
(also unlikely it will actually survive at that rpm)
With a proper installation, and a useful prop load something like this would probably turn an 11x7 around 13,000 rpm.
Some manufacturers are much better at this these days, providing both HP and real-world use.

Jett uses the more useful method of expressing engine performance, with a useful load and a real/tested rpm target. From there, yes, the engines can be run higher or lower rpm bigger or smaller prop but you know where the baseline is, and where Jett considers that it performs is acceptable.

Power to weight wise , the small block engines are the best opportunity. When using a full size tuned pipe, you have the opportunity to adjust your rpm range/peak torque. Long pipe setup (properly set up, good fuel system, lots of bench time to confirm your setup) will allow you to turn a large diameter prop by trading the rpm down. Something like a 13x4 or 14x4 is possible, and it will turn as well as the big block engine.

The trick is, sometimes those static thrust calculations look tempting - but for take off performance, reality does always match what is on paper. (same thing goes for aircraft speed - it does not equate to pitch x rpm the prop does work based on its shape and airfoil, and that airfoil works differently at various forward airspeeds)

Just as an example (and not directly related to your project), read the posting the Jett forum here about the SJ-50 on a Phoenix 7. I had several people tell me that the .50 would not be enough because the plane weighed 7.5 lbs and needed at least an 11x7 prop. As you can see, the 10x6 spinning at 16,000 rpm provided performance well beyond expectations. Another good example was a SU-27 project in the extreme speed forum built with an OS55 and jett-stream muffler provided great performance and saved over a full pound on airframe weight from similar builds with .61/.91 power.

We try to only provide information and basic technial advice for the academic competitions. Ultimate it is up to the teams to select and apply the prop, fuel system and engine to the aircraft.

aero13

Thanks for the information. We've used Jett engines before, so I was just curious. I'd like to send in the two .60s we have for refurbishment. I believe they are JF60Ls, but I won't be able to check until tomorrow morning. Which way (email, phone, or jett website) is the best way to get things set up? Thanks for the help.

bob27s

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email Dub at [email protected]