Can you turn a constant speed prop into a fixed pitch?

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Re: Can you turn a constant speed prop into a fixed pitch?

Postby drseti » Wed Jul 29, 2020 2:57 pm

I believe you're thinking about the Rotax 915iS, Ed. Like most turbocharged engines, it performs best driving a constant speed prop, and can be FADEC controlled (single lever). Certainly not intended for the LSA market, since the 915 is a very heavy engine.
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Re: Can you turn a constant speed prop into a fixed pitch?

Postby 3Dreaming » Wed Jul 29, 2020 3:01 pm

There are 912is powered airplanes that are set up that way, but they are not LSA.

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Re: Can you turn a constant speed prop into a fixed pitch?

Postby fatsportpilot » Wed Jul 29, 2020 5:10 pm

drseti wrote:I believe you're thinking about the Rotax 915iS, Ed. Like most turbocharged engines, it performs best driving a constant speed prop, and can be FADEC controlled (single lever). Certainly not intended for the LSA market, since the 915 is a very heavy engine.

It's heavy but some manufacturers have made it work like the Bristell 915 with useful load to spare, even if it wasn't intended.

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Re: Can you turn a constant speed prop into a fixed pitch?

Postby drseti » Wed Jul 29, 2020 5:21 pm

fatsportpilot wrote: the Bristell 915 with useful load to spare,


Even John Rathmell, the marketing manager for Bristell USA, said in a recent AOPA Pilot interview about that aircraft, "It's too heavy."
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Re: Can you turn a constant speed prop into a fixed pitch?

Postby fatsportpilot » Wed Jul 29, 2020 6:16 pm

drseti wrote:
fatsportpilot wrote: the Bristell 915 with useful load to spare,


Even John Rathmell, the marketing manager for Bristell USA, said in a recent AOPA Pilot interview about that aircraft, "It's too heavy."

The useful load is 500 lbs which is more than some LSAs with the 912ULS.

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Re: Can you turn a constant speed prop into a fixed pitch?

Postby Warmi » Wed Jul 29, 2020 8:34 pm

fatsportpilot wrote:
drseti wrote:
fatsportpilot wrote: the Bristell 915 with useful load to spare,


Even John Rathmell, the marketing manager for Bristell USA, said in a recent AOPA Pilot interview about that aircraft, "It's too heavy."

The useful load is 500 lbs which is more than some LSAs with the 912ULS.


There are no miracles and they have to work with commonly known materials just as everyone else so the only explanations is that they are cutting corners somewhere ( for instance more fragile landing gear or some other parts ) and since the plane is relatively new and with relatively few of them out being there , we just don’t have enough data - it may very well be that their trade offs are actually reasonable and will not result in any serious issues ... who knows ...will see.
Flying Sting S4 ( N184WA ) out of Illinois

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Re: Can you turn a constant speed prop into a fixed pitch?

Postby fatsportpilot » Wed Jul 29, 2020 8:42 pm

Warmi wrote:
fatsportpilot wrote:
drseti wrote:
Even John Rathmell, the marketing manager for Bristell USA, said in a recent AOPA Pilot interview about that aircraft, "It's too heavy."

The useful load is 500 lbs which is more than some LSAs with the 912ULS.


There are no miracles and they have to work with commonly known materials just as everyone else so the only explanations is that they are cutting corners somewhere ( for instance more fragile landing gear or some other parts ) and since the plane is relatively new and with relatively few of them out being there , we just don’t have enough data - it may very well be that their trade offs are actually reasonable and will not result in any serious issues ... who knows ...will see.

I think I read somewhere that they are using really thin aluminum and compensating by adding more flush rivets.

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Re: Can you turn a constant speed prop into a fixed pitch?

Postby drseti » Wed Jul 29, 2020 9:43 pm

This aircraft is 115 pounds heavier than the 912-powered Bristell. That is a non-trivial difference, considering that the Bristell is noted for its excellent useful load. 70 pounds of that weight increase is just the engine. The other 45 pounds is the 4-bladed prop, plus what they put in to support that engine. This from AOPA Pilot, April 2020:

"First of all, it’s too heavy,” said John Rathmell, national sales manager for Bristell Aircraft in Lititz, Pennsylvania, a U.S. dealer for the Bristell.


See the whole article at:

https://www.aopa.org/news-and-media/all-news/2020/april/pilot/bristell-915
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Re: Can you turn a constant speed prop into a fixed pitch?

Postby fatsportpilot » Thu Jul 30, 2020 9:22 pm

Ok I see. I thought "too heavy" you meant too heavy to be legal. The useful load goes way down but it could still fit a rather large pilot and a child or maybe two small pilots. There are some LSAs with 300 or 400 lbs useful load which is incredibly low.

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Re: Can you turn a constant speed prop into a fixed pitch?

Postby drseti » Thu Jul 30, 2020 9:41 pm

fatsportpilot wrote: There are some LSAs with 300 or 400 lbs useful load which is incredibly low.


There are ASTM rules that set minimum useful load requirements. For 2-place SLSA, I seem to recall the minimum useful load requirement is something like two 180 pound occupants, and enough fuel for 1 hour of flight at normal cruise power, plus a 30 minute day VFR reserve. For a Rotax 912 powered aircraft that burns 5 GPH, that required useful load comes to (2 x 180#) + (6#/gal x 7.5 gal) = 405 pounds. So, only a 1-place LSA could get away with a lower useful load.

SEE CORRECTIONS BELOW
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Re: Can you turn a constant speed prop into a fixed pitch?

Postby drseti » Fri Jul 31, 2020 6:19 am

Actually, I was mistaken about the details of the ASTM useful load rule. It's a little worse than I thought - this from Dan Johnson:

Currently that requirement is 430 pounds for fixedwing airplanes (land). That number was arrived at by allowing 190 pounds for each seat occupant (380 pounds for two-seat S-LSA), and one-half the horsepower in pounds (for example: with a 100-hp engine add 50 pounds).

Accordingly, any fixed-wing S-LSA that has an empty weight of more than 890 pounds (with all options added) is being manufactured and operated in violation of the ASTM standard. Buyers of S-LSA are advised to determine whether an S-LSA meets this standard before purchasing or operating it. Bottom line: Don’t buy a new S-LSA that has an empty weight greater than 890 pounds.


So by that formula,, the Bristsll 915 (which has an engine that can produce 141 HP) must have a useful load of at least 450.5 pounds, and a max empty weight of 869.5 pounds. The Bristell 915 reviewed in AOPA Pilot meets that spec - but if you want to order one with any additional options, you may be out of luck.

(Nof that I'm in any danger of buying one, at $296k!)
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Re: Can you turn a constant speed prop into a fixed pitch?

Postby 3Dreaming » Fri Jul 31, 2020 8:41 am

drseti wrote:Actually, I was mistaken about the details of the ASTM useful load rule. It's a little worse than I thought - this from Dan Johnson:

Currently that requirement is 430 pounds for fixedwing airplanes (land). That number was arrived at by allowing 190 pounds for each seat occupant (380 pounds for two-seat S-LSA), and one-half the horsepower in pounds (for example: with a 100-hp engine add 50 pounds).

Accordingly, any fixed-wing S-LSA that has an empty weight of more than 890 pounds (with all options added) is being manufactured and operated in violation of the ASTM standard. Buyers of S-LSA are advised to determine whether an S-LSA meets this standard before purchasing or operating it. Bottom line: Don’t buy a new S-LSA that has an empty weight greater than 890 pounds.


So by that formula,, the Bristsll 915 (which has an engine that can produce 141 HP) must have a useful load of at least 450.5 pounds, and a max empty weight of 869.5 pounds. The Bristell 915 reviewed in AOPA Pilot meets that spec - but if you want to order one with any additional options, you may be out of luck.

(Nof that I'm in any danger of buying one, at $296k!)


It's not quite that bad :evil: , you actually would have 3 more pounds of useful load. The formula is based on maximum continuous power, so you would use 135 horsepower. This is the reason that the Carbon Cub has a maximum continuous power of 85 horsepower, not speed as everyone seems to think.

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Re: Can you turn a constant speed prop into a fixed pitch?

Postby drseti » Fri Jul 31, 2020 12:04 pm

3Dreaming wrote:It's not quite that bad :evil: , you actually would have 3 more pounds of useful load.


Wow, that makes all the difference in the world! :wink:
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Re: Can you turn a constant speed prop into a fixed pitch?

Postby 3Dreaming » Fri Jul 31, 2020 12:26 pm

drseti wrote:
3Dreaming wrote:It's not quite that bad :evil: , you actually would have 3 more pounds of useful load.


Wow, that makes all the difference in the world! :wink:


A half gallon of fuel.

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Re: Can you turn a constant speed prop into a fixed pitch?

Postby foresterpoole » Mon Aug 03, 2020 12:35 pm

3Dreaming wrote:
drseti wrote:Actually, I was mistaken about the details of the ASTM useful load rule. It's a little worse than I thought - this from Dan Johnson:

Currently that requirement is 430 pounds for fixedwing airplanes (land). That number was arrived at by allowing 190 pounds for each seat occupant (380 pounds for two-seat S-LSA), and one-half the horsepower in pounds (for example: with a 100-hp engine add 50 pounds).

Accordingly, any fixed-wing S-LSA that has an empty weight of more than 890 pounds (with all options added) is being manufactured and operated in violation of the ASTM standard. Buyers of S-LSA are advised to determine whether an S-LSA meets this standard before purchasing or operating it. Bottom line: Don’t buy a new S-LSA that has an empty weight greater than 890 pounds.


So by that formula,, the Bristsll 915 (which has an engine that can produce 141 HP) must have a useful load of at least 450.5 pounds, and a max empty weight of 869.5 pounds. The Bristell 915 reviewed in AOPA Pilot meets that spec - but if you want to order one with any additional options, you may be out of luck.

(Nof that I'm in any danger of buying one, at $296k!)


It's not quite that bad :evil: , you actually would have 3 more pounds of useful load. The formula is based on maximum continuous power, so you would use 135 horsepower. This is the reason that the Carbon Cub has a maximum continuous power of 85 horsepower, not speed as everyone seems to think.



Irregardless, I'd say it's pushing usable at those weights, I mean you basically have no range or no passenger weight. Thanks for the correction on the engine, it appears I confused the engine model. This is all academic anyway, it's still not legal under the current standards. Possibly in the future we will see a re-write more in line with European ultralight standards which allow retractable gear and constant speed props. I'm not holding my breath considering any federal regulation changes seem to move at the speed of an out of breath snail....
Ed


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