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Old 08-24-2023, 08:09 AM
hk1232's Avatar
hk1232 hk1232 is offline
Join Date: Jan 2022
Location: Minneapolis, MN
Posts: 13
Default Rotax 915iS + Extra weight to change CG

Someone tell me why the Rotax 915iS in an RV-9 (like the Lockwood demo) always propose extending the forward position rather than adding weight to align more closely with the heavier Lycoming.

I understand lighter is better, but if there are downsides of the lighter engine isn't this a reasonable approach to normalizing the CG?

The Rotax is 185 lbs and a Lycoming 320 is 244 lbs. A 2" x 8" lead disc would come close to zeroing it out. I haven't done any FW forward work yet but that doesn't seem like it would be a space issue.
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Old 08-24-2023, 09:32 AM
CaptPausert CaptPausert is offline
Join Date: Sep 2022
Location: Oklahoma City
Posts: 77

I always wondered if you could balance a lot of the weight out with an AC system.
Brian Walters

USAF T-6 Structural integrity program and Deputy corrosion program engineer (civilian)

RV-9A Empennage kit 90% complete. Wing kit received and inventoried.

Dues paid Nov 2023
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Old 08-24-2023, 09:43 AM
Diego9 Diego9 is offline
Join Date: Aug 2018
Location: Oegstgeest, The Netherlands
Posts: 8

I am also looking at the 915 for my RV9...
Indeed Lockwood solution looks very extreme and I understand it needs changes to the aft part of the plane...
I was wondering if adding some weight (as the OP is proposing) or moving the propeller just a few inches (4?) plus moving the battery fwd a bit plus adding something else (not necessarily dead mass), would be a better solution.
Has anybody discussed the selected choice with Lockwood to understand the logic of their design solution?
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Old 08-24-2023, 11:58 AM
3dmichi66 3dmichi66 is offline
Join Date: Jan 2022
Location: Austria
Posts: 1
Lightbulb Concepts for Rotax 915iS

Hi together

I did also some work on concepts for an RV with 915iS. It was based on the RV-7 plans, however I am building an RV-9 now and the engine bay is very similar.

The idea is to keep the aircraft close to the standard configuration and cowling with a small extension and using a hydraulic propeller and the external generator to get the aircraft more nose heavy. Additionally a weight at the gearbox would be required. Weight here is not the nicest solution from an engineering perspective, but it's the simplest solution and the powertrain is anyway lighter than standard. Cooling would be with the standard air intakes for intercooler and coolant cooler. The oil cooler placed in the center.

Here are some pictures:

If anyone is interested for an exchange of ideas, feel free to contact me.

edit: This is my first post here, so hello to everybody and thank you for this great community.

Kindest regards
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Last edited by 3dmichi66 : 08-24-2023 at 12:16 PM.
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