135 hp Rotax 915 IS/ISC for RV-7 ?

This looks very interesting to me. Rotax is a proven manufacturer of aircraft engines and this engine will make 135 hp up to 15,000'
When I fly out of Albuquerque on an average day the density altitude is over 7,000'. My 180 hp RV6 is only making about 135 hp on take off and dropping quickly as I climb to 10,500 for cruise.
With the Rotax I would have that 135hp all the way to 15,000 and the weight of the engine is listed as 185 lbs! That's about a 100 lbs in savings.
With Van's experience with the Rotax in the 12 I wonder if we may be seeing this engine as a firewall forward option in the future.

http://www.flyrotax.com/produkte/detail/rotax-915-is-isc.html

C. Brenden said:

This looks very interesting to me. Rotax is a proven manufacturer of aircraft engines and this engine will make 135 hp up to 15,000'
When I fly out of Albuquerque on an average day the density altitude is over 7,000'. My 180 hp RV6 is only making about 135 hp on take off and dropping quickly as I climb to 10,500 for cruise.
With the Rotax I would have that 135hp all the way to 15,000 and the weight of the engine is listed as 185 lbs! That's about a 100 lbs in savings.
With Van's experience with the Rotax in the 12 I wonder if we may be seeing this engine as a firewall forward option in the future.

http://www.flyrotax.com/produkte/detail/rotax-915-is-isc.html

Click to expand...

My very first concern is C.G. With that light of an engine on the nose your playing in the aft danger zone (if not exceeding it) without ADDING weight back in. Then there is the whole re-engineering of the motor/landing mount, re-engineering of the FWF and cowling. Not that it can't (won't) be done but I don't think the -7 is the right platform for it. I'm not knocking the idea just pointing out the very obvious.

Nothing against Rotax engines but: The RV7 was designed around a 300lbs Lycoming and a metal propeller. Substituting a power package that weighs more like 150lbs less would be a serious challenge in regards to weight and balance. The engine would either have to be hung way out front or the wing would have to be moved aft. The Rotax turbo engine is also more expensive and I doubt that Van's would ever consider sucha re- design. A better alternative would be a higher compression ratio or turbo normalizing the Lycoming or clone if you want more power at altitude. By the way one of our Airpark neighbors took his 7 all the way to 28k with his normally aspirated io-360.

Martin Sutter
Building and flying RV's since 1988
EAA Technical Councelor

Interesting numbers.

The guys are right about the C of G of course, if you were to stick on a CS prop this would help.

It does make me wonder though whether we will see an RV design for this engine?

Also the implications for the Lycosaurus when Rotax develop a 160 or above.

C. Brenden said:

This looks very interesting to me. Rotax is a proven manufacturer of aircraft engines and this engine will make 135 hp up to 15,000'
When I fly out of Albuquerque on an average day the density altitude is over 7,000'. My 180 hp RV6 is only making about 135 hp on take off and dropping quickly as I climb to 10,500 for cruise.
With the Rotax I would have that 135hp all the way to 15,000 and the weight of the engine is listed as 185 lbs! That's about a 100 lbs in savings.
With Van's experience with the Rotax in the 12 I wonder if we may be seeing this engine as a firewall forward option in the future.

http://www.flyrotax.com/produkte/detail/rotax-915-is-isc.html

Click to expand...

Interesting predicament where smaller turbo engine outperforms a big NA engine on a normal operation basis. I had similar thoughts 10 years ago while standing in the Deltahawk booth.

I regards to CG, My very rough calcs show about a 30" lengthening of the engine mount. I was guessing a 30" arm on the Lyc CG. This lenghtening is more than my "gut feel" expected so if somebody else wants to challenge that, feel free.

Consider the options when building

The -7 aft CG is an issue with light weight up front, it is one of the areas for consideration if you are thinking of a light weight FP prop such as the Catto and I would guess also the composite CS props out there.

When I was building my -7 I wish I had the input and experience I have now as I would probably have considered some different options:
1. Using the 320 mount apparently would move the 0-360 2 inches forward, the cowl in the finish kit is long enough to accommodate the extra length.
2. I installed the standard Skytech light weight starter, could have put a regular heavier starter on
3. I used the standard PC680 lightweight battery on the firewall, could have installed a heavier battery.
4. A heavier motor (200HP Lyc) would also be an option although expensive

Net result is with the Catto weighing only 14lbs I ended up adding a Landol ring and the Saber heavy spacer and crush plates to put the CG back into a reasonable range to carry two up and a decent baggage load.

Compensating for 150 lbs less on the motor would be tough on the -7

One advantage of an alternative engine conversion is the ability to place the engine where you want. I'd just make a mount that bolted right on the stock RV-7 engine mount to the rotax. Then you can stick the engine further out, plus mount the battery on the new mount. An engine that is 150 lbs lighter would have to move approx 12 inches forward to compensate. With a nice tight cowl and radiator scoop it would look like a turbine . 135 hp is plenty for the RV-7. I like the idea!

Maybe the 9?

I wonder if it makes a better fit for the 9? I think they were designed as cruisers with an O 320, maybe the CG would be a bit closer? Also, I think most of them like to cruise high so they take advantage of the wing. Still have all the FWF issues of course.

Just a thought.

Geoff

The 7's tend to be tail heavy where the 9's tend to be nose heavy, from what I've gathered talking to other builders. I know when I put my IO360 and CS prop on my 9A, I had to move quite a few items aft to keep the CG where I wanted it; my battery is aft of the baggage compartment wall, and all the remotely-mounted avionics went into the mid-aft fuse area. The 9's wing also loves higher altitude cruising, so both these aspects would put the 9 as a better choice, unless the builder wanted aerobatics - in which case they will also need more horsepower than the Rotax can provide.

I can tell you that the 9A has no trouble truing 135ktas on 80hp at 17,500, using a CS prop. I do it quite a lot. That's even running lean of peak with 2400rpm, there's a little muscle still available.

EDIT - I went back and searched a couple recent flights I made, and at 16,500 I'm very stable at 151ktas , 6.4gph, 2400rpm. The percent power shows 46% on the Dynon which equates to 82.8hp from my IO360. If you've got a decent prop to put on it, the Rotax is definitely in this game for high cruise.

Bit of an old thread revival, but has anyone seen the sales numbers on the new Rotax? And do we know yet if the 185 pound installed weight includes the coolant and radiator?

Talked with the guys at Lockwood Aviation in Florida (East Coast distributor). 915 is $35,000, and by default is configured for a C/S prop and includes the slipper clutch (so if you prang the prop you only need have the gearbox inspected, no engine teardown required). That latter is a plus; however, the only prop out there right now for this engine as far as I know is the MT, so budget $10k for that.

The OP talked about this as a 135hp motor. It isn't, really, not like a Lycoming. It is limited to 5 min at max power. However, it will produce 125hp all day long and up to 16,500' @ ~8.5gph. Up high, you might actually run into your RV Vne with this motor.

The advantage of this engine is flying high; otherwise you might be better off buying a UL motor if you just want the weight savings. At 16,500', it produces more cruise horsepower than a non-turbo IO-400 (125 vs 110). If you are all about fast cross-country this may be your baby - use the extra gross weight for more fuel tankage. However, if you want similar performance without the extra complexity of a turbo you can opt for a UL520is (200hp @ 3,000 rpm, 200lbs) and a Catto fixed pitch prop for around $34k vs $45k. That 100lbs savings vs a Lycosaurus will result in good climb (probably better than the 915 at low altitude) even without a C/S prop, and $11k will buy a whole lot of gas...