Rotax 912iS or 912ULS for Sling 2 ???

[cam737]

^^^^^^^^^^^^^^^^^^^BOOM!^^^^^^^^^^^^^^^^^

Thanks brother Tim! I was really feeling the heat.



So, back to my original question....

ULS or iS?

[FastEddieB]

So, back to my original question....

ULS or iS?

Probably.

[TimTaylor]

If the iS with full fuel has enough useful load for my mission, then it becomes a question of engine cost. If the added cost of the iS is within my budget, that's what I would go for.

If the iS with full fuel is too heavy for the mission, and I have to go with less than full fuel, then I might as well get the ULS and go with full fuel. Range would probably be about the same, so why pay for the more expensive engine?

EDIT: If my missions are varied, and I can frequently take advantage of the iS and full fuel, I would go for the iS. There would be no disadvantage to flying with less than full fuel when the airplane is heavily loaded (versus ULS), except for engine cost.

[WDD]

The Tim Taylor post is probably the path forward to determine the answer.

Does this spreadsheet help?

----------------------------------------------------------------------------ULS-------------IS
Useful Load ..............................................................xx..............xx

Common "Human" Payload............................................. xx.............xx
(will you fly 90% with one person, 2 people, etc.?
Is there a "must have" of 2 people? if so, use that)

Common Baggage (How much will most frequently carry?........xx............xx
Is there a "must have" of a certain capacity? If so, use that)

Fuel Capacity in lbs (Load-"Humans"-Baggage)......................yy............yy

Range ((fuel Lbs/6)-Unusable)/Fuel Burn)-1) * Cruise Speed....zz............zz
( -1 is to allow for reserve. Remember that Fuel burn
will be different between IS and ULS)
How much range do you need / want?

IF the capability is acceptable with the extra weight of the IS, I would say get the IS.
IF not, but the capability with the ULS is acceptable, get the ULS
IF Neither are acceptable..... get a different plane LOL.

[MrMorden]

Not trying to stir up arguments, but I have seen the "you have to start every flight with full fuel" idea expressed quite a bit, and I disagree with it. I agree with the *intent*, which is that you should always have more fuel that you could reasonably expect to burn for a given flight; where I disagree is in the execution of that mindset.

In my mind, it really depends on what you are doing, fuel capacity vs. burn rate of the engine, range, etc. Most LSA have about 18-25 gallon fuel capacity and burn 5gph or a little less (assuming a Rotax 912 engine, it's more for a Continental or Lycoming). Given that, do really need full fuel to take your buddy from work on a 45 minute local sightseeing flight? Or is 15 gallons enough, or even ten?

If your answer is "no, always carry full fuel", then in many cases that means not making the flight. In the above example, if your airplane has a useful load of 500lb, and you and your buddy weigh 200lb each, that leaves 100lb. If you have 16lb of gear (headsets, papers, cables, tiedowns, etc) in your airplane, you have 84lb left for fuel - 14 gallons. It's way more fuel than you are planning to use, more than double required VFR reserves. So do you cancel and stay on the ground because you can't carry full fuel, or make the flight?

Let's put it another way: If you have two 1320lb gross airplanes with the same fuel burn, one with 18 gallon tanks and one with 25 gallon tanks, why would it be safe to make an hour long flight in one with 18 gallons but not the other with 18 gallons? Tank size is arbitrary...if you had an airplane with 100 gallon tanks and 600lb useful load, would you never fly the airplane because you can't take off legally with full tanks?

In my case, my airplane has 34gal fuel capacity with 33gal usable - big tanks. I burn 4.0 - 5.0gph depending on throttle setting. How much I will carry will depend on what I'm doing. If I'm making long cross country flights, I fill it up, USUALLY. But I made a flight from Georgia to Michigan with my wife last summer, and loaded 24gal fuel on each leg so that we'd have spare weight for our bags. It worked out great and we never landed with less than ten gallons. I wasn't going to not make the flight because I could "only" carry 4.5 hours worth of fuel per leg!

Most of my local flights are an hour or less, and I feel NO need to carry fuel fuel, and usually have 16-22 gallon aboard for those flights. The airplane performs better with less fuel weight, and that is a big plus especially on hot days or at high altitude airports. There is no such thing as a free lunch, and carrying extra fuel is no different. There are trade-offs! I do have a "no take off" minimum fuel of five gallons in each wing tank. That is primarily due to my CT's quirky fuel system than can cause an engine stoppage if one tank is empty and you make an uncoordinated turn with the other tank low on fuel. If I had a single tank with good pickups, my minimum might well be 7gal (I probably would not take off with less than 1.5hr of fuel unless it was a dire circumstance).

The point to all this rambling is that I think you are best served by making airplane purchase decisions based on what *you* want to do with the airplane, not not based on the designer's decision of fuel tank capacity.

[FastEddieB]

Posted before, but on point...

Even with 18 gal capacity, I need to watch my fuel load if I want to carry a passenger (the bottom portion is my “cheat sheet”):



I generally keep my Sky Arrow at about 1/2 fuel, and top off only as allowable per the above. And I’m OK if a passenger weighing up to 220 lbs show up and wants to fly.

[jetcat3]

To me this is like the situation between the iPhone 8 Plus vs iPhone X. The iPhone 8 Plus is a refined (albeit older design by today’s standards) evolution of a phone released 4 years ago. It’s rugged, reliable, and fast.

The iPhone X is cutting edge and gives you a new experience that pushes the boundaries of what is to come in the future. It will evolve and continue to improve through software updates.

The 912 iS is the iPhone X in this case. It’s truly a state-of-the-art engine and definitely is the future of Rotax. That’s the direction they will be heading and that engine is only going to get better through software updates as well and continued use in the field.

If it were my choice I would most likely spring for the 912 iS. The smoothness and throttle response is quite impressive in my opinion.

[cam737]

Thanks gentlemen! There are a lot of big-brain pilots here (well...bigger than my brain) and very helpful pilots!

FastE...You're funny, and, I copied your cheat sheet months ago when I was considering Sling 2 LSA! It became obvious to me that to get the Sling 2 with all the bells and whistles that I want...EAB was the way to go at 1540 gross.

MrM...great thoughts. At the airline, we always carry just enough fuel for the mission...UNLESS...we decide to TANKER fuel. 'Tankering' fuel means we might carry extra gas, because the higher price of gas at our destination justifies the extra fuel burn cost to tanker in. One of the many things that I like about the Sling 2 are the 39.6 gallons of fuel capacity. Local hops...who cares. But, this allows 'tankering' fuel (cheap car gas?) on say an 800nm round trip with the ULS, or on a 1100nm round trip with the iS. The convenience of not having to re-fuel is worth something as well. I'm thinking...flying FL to Bahamas and back without even thinking about where to re-fuel.

JC3...I have iPhone 10! ??????????????? I like it.

[MrMorden]

MrM...great thoughts. At the airline, we always carry just enough fuel for the mission...UNLESS...we decide to TANKER fuel. 'Tankering' fuel means we might carry extra gas, because the higher price of gas at our destination justifies the extra fuel burn cost to tanker in. One of the many things that I like about the Sling 2 are the 39.6 gallons of fuel capacity. Local hops...who cares. But, this allows 'tankering' fuel (cheap car gas?) on say an 800nm round trip with the ULS, or on a 1100nm round trip with the iS. The convenience of not having to re-fuel is worth something as well. I'm thinking...flying FL to Bahamas and back without even thinking about where to re-fuel.

I agree, if you have the weight to spare and want to avoid refueling, or using more expensive gas, then that makes sense. After all, even a "quick" refueling stop will cost you 15min or more, so you really lose ground if you are trying to get somewhere fast.

Don't get me wrong, I'm not opposed to carrying more fuel, I just don't feel like I have to top it off before every flight to be safe.

[TimTaylor]

The point I was making is that if your mission is usually a heavy load (such as 2 people), and the iS engine is too heavy to carry full fuel, maybe it would make more sense to save the money and go with the lighter ULS engine. In that case, you COULD carry full fuel if you wanted. That doesn't mean you HAVE to carry full fuel.

[TimTaylor]

Maybe another way to look at this is to ask, "is there anything the iS can do the ULS cannot and vice versa"? I think the answer is yes, the iS can fly further and longer with full fuel than the ULS. If that is worth the difference in price, go for it. It would be for me since I usually fly solo.

[Warmi]

Maybe another way to look at this is to ask, "is there anything the iS can do the ULS cannot and vice versa"? I think the answer is yes, the iS can fly further and longer with full fuel than the ULS. If that is worth the difference in price, go for it. It would be for me since I usually fly solo.

That and the fact that it will start in cold weather and basically require much less babysitting much lik a car ...that’s worth something in my book.

[WDD]

Wait - sorry I missed it.

You're going EAB with 1540 gross?

Get the iS.

[cam737]

THANKS TO ALL for the great inputs! I really appreciate it and I actually learned some things. None of you are wrong...except the grammar police person.

So, after much debate, much hem-and-hawing, honest self-assessment of mission requirements, group therapy sessions (not really), phone calls to The Airplane Factory in California and indirectly to The Airplane Factory in South Africa...I have made my decision...

Both engines are great choices. But for me........

Drum roll please.........................

Rotax 912ULS.

Applause please.........................

https://www.savvyaviation.com/wp-conten ... x-912).pdf

[3Dreaming]

THANKS TO ALL for the great inputs! I really appreciate it and I actually learned some things. None of you are wrong...except the grammar police person.

So, after much debate, much hem-and-hawing, honest self-assessment of mission requirements, group therapy sessions (not really), phone calls to The Airplane Factory in California and indirectly to The Airplane Factory in South Africa...I have made my decision...

Both engines are great choices. But for me........

Drum roll please.........................

Rotax 912ULS.

Applause please.........................

https://www.savvyaviation.com/wp-conten ... x-912).pdf

Wow, I have never been called the grammar police before. With all the...... ( ),,, I wasn't sure what happened when.