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A glance at the Bristell 915is

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10 minutes ago, Doug G. said:

Carbon Cub does it with 185 hp engine for takeoff only - "5 min. max".

 

I though you meant a placard saying "don't exceed speed X" and not an rpm limit.  The rpm limit is definitely doable, but it would probably have to be very low in this case, maybe even below where Rotax recommends running the engine.  "RPM greater than 4500rpm 5min max" would be a hilarious placard to see!  

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3 hours ago, Doug G. said:

Carbon Cub does it with 185 hp engine for takeoff only - "5 min. max".

The Carbon Cub is draggy enough that they don't have to worry about speed, and that is not the reason for the RPM limit.

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15 hours ago, Doug G. said:

Nope, I didn't say anything about RPM, and neither do the Light Sport parameters.

Yeah, but RPM limits are a great way to limit speed.  That is how the Sonex with the Jabiru 3300 stays LSA...the engine rpm is limited by placard to 2850rpm.

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1 hour ago, Tom Baker said:

The Carbon Cub is draggy enough that they don't have to worry about speed, and that is not the reason for the RPM limit.

Not the point Tom. I was simply saying that the FAA has allowed this by placard in some cases. Since I don't know the stall speeds on this plane I also don't know if that would be enough.

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I am VERY anxious for a Bristell LSA with the 915 IS!

One thing to consider is that reducing RPM for cruise caused an abnormally high number of  Rotax engine failures on the early CTs. The aircraft was very slick and the props were pitched for performance. Pilots would often reduce RPM to fly slower. At first Rotax was very perplexed as to the cause of the problem. (Please correct me if I am mistaken or if there were extenuating factors.)

Rotax motors are are happiest cruising at high RPMs. They are not happy lugging at low RPM cruise with an aggressively pitched prop stressing the engine.

On the other hand, the 915 IS is strengthened considerably throughout for the higher HP, and actually runs on less compression. So maybe stresses would be better tolerated.

So if Bristell or any other LSAs pitch the prop on a 915 IS within LSA cruise parameters, the prop shouldn’t be too aggressive. Yet when you go to full-throttle climb there is plenty of HP!

I believe the early performance specs posted here from Dan Johnson were with an adjustable prop.

I’d be very happy with 2,000 ft/min climb and a solid 120 kts cruise.
Keep it LSA legal and let the turbo give you consistent 120 kts at altitude! 👍

I’d be interested to hear what Rotax says about running the 915 IS at lower RPMs in cruise.

 

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27 minutes ago, designrs said:

I am VERY anxious for a Bristell LSA with the 915 IS!

One thing to consider is that reducing RPM for cruise caused an abnormally high number of  Rotax engine failures on the early CTs. The aircraft was very slick and the props were pitched for performance. Pilots would often reduce RPM to fly slower. At first Rotax was very perplexed as to the cause of the problem. (Please correct me if I am mistaken or if there were extenuating factors.)

I don't think you have your facts straight. Early on Flight Design had their props over pitched. Reducing RPM was not needed, because the RPM could barely go over 5200 full throttle level flight. This excessive prop pitch is what led to failures. Rotax recommends the engine turning a higher RPM for take off, they don't like lugging the engine. The problem with a slick airplane like the CT is that if you set the propeller for the RPM that Rotax wants for take off, it will easily over speed in cruise. Years of experience have shown that setting the RPM so that you can achieve 5500-5650 full throttle level flight at the altitude that you want to fly will give the overall best performance for a CT. I suspect other slick airframes will be similar. 

Also the early Rotax engines had a lighter crankcase that was more prone to cracking. Since Flight Design was one of the early light sport producers they used more of the early engines with the light crankcase. Larger production numbers, the early crankcases, plus how they set their props gave the appearance of more failures. In reality I don't think the percentage of failures was any higher. The heavier crankcases have not had any issues that I am aware of.

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