LeSving wrote:
The are many aircraft with MTOW 750 kg and a Rotax already flying, Aquila comes to mind.
Indeed. But the Aquila A211, which I am learning to fly on, has an empty weight of 506 kg and MTOW of 750 kg, so the ratio is much worse than in the example posted by Jan. Actually, with me weighting 100 kg and my FI weighting around 60 kg we can barely fill the tanks full without exceeding MTOW :(
Our SAN Jodel DR1050 had an actual mass of 410.6kg without spats, and an AUW of 750kg. (Weighed at about age 45 at C of A, now Annex 2)
Cruise at over 110kts.
The one-piece wing is off and being recovered – transported on a low-loader artic. Mass will be different when reweighed after reassembly.
There are some utility SEP which boast Useful load>Empty mass:
Cherokee 235
Cherokee Six-260
Early Cessna 206
Ag planes
They may be of the stripped marketing brochure variety but I believe having this ratio was part of the selling point.
Robin_253 wrote:
So a 100hp normally aspirated Continental has an 8% more max continuous power then a 100hp normally aspirated Rotax.
No, the 912 iS has 98 hp continous, 2% 
Also, the one we have to tow glider has “big bore” mod, so it has 120 HP max, and whatever continuous max we dare to run A Rotax is not a Rotax, there are lots of variants, and lots of mods that can be done to them (big bore, EFI, EI, EI+EFI, FADEC, turbo).
Robin_253 wrote:
you would find that the first one, which is built from duraluminium sheets is actually lighter then the second one which is “all carbon”.
Maybe, I haven’t really studied them, but carbon is stronger and more rigid for the same weight. The Atec 2000 is delivered in two variants. Both have carbon composite fuselage. The Zephyr has a wooden/fabric wing, while the Faeta has a carbon wing. The Faeta has slightly more max VNE (but really not enough to mention), but +5g instead of +4g. The weight is roughly the same, I think the Faeta is 1-2 kg heavier, but has more comfortable upholstery, better seats and so on. What is gained with carbon on terms of weight is usually taken back again with more “luxury”, but the added strength and rigidity is still there.
The One has 415 kg empty weight and 750 kg MTOW. This is about the same ratio a RV-10, 4 people and cruises at 175 knots (but drinks a whole lot more fuel ) Still, this is less than the average Sonex, and it can do +6 g and is fully aerobatic. The MCR 4s is in a league of it’s own though.
It’s good we have all these different planes to chose from, one for every person.
ploucandco wrote:
Rotax 912ULS will give you 73.5kW (98HP) at 5800RPM for 5 min and 69kW (92HP) at 5500RPM max continuous. Consumption at max continuous is 25L/h.
LeSving wrote:
All carbon, double fowler flaps and so on.
The question is what compromises have been made during design process to achieve this low empty weight?
I know nothing about these types flying-wise but it takes only a casual walk around the hall at Aero EDNY to see that key components of the low weight are
Hopefully the structure is OK.
Empty weight is often a suspicious number. In the certified world it is often a figure which no actual flying specimen of that plane has. The reason for it is often avionics; in some cases the empty weight in the POH applies to configurations which have never existed.
@Robin_253, According to the operator manual, a Rotax 912ULS will give you 73.5kW (98HP) at 5800RPM for 5 min and 69kW (92HP) at 5500RPM max continuous. Consumption at max continuous is 25L/h.
The Rotax 912UL gives you 59.6kW (79.4HP) at 5800RPM for 5min and 58kW (77.3HP) at 5500RPM max continuous at 22.6L/h.
@LeSving, The ROTAX 914 has a TBO of 2000hours. It is being used in many US drones and is typically making TBO without much issues.
With a good installation (with intercooler), you get 100HP continuous till 16000ft (critical altitude) the whole day.
Robin_253 wrote:
We are mixing apples and oranges here. An O200 will give you 100 hp for as along as you need.
Well, a 912 iS will give you 98 as long as you need, a 914 will give you 105. But, the point was that the MCR 4S is recommended with constant speed prop and a 914, and only with a constant speed prop will you achieve the needed RPM to get all available HP at any given point in time, and a turbo is needed to get that HP also at alt > 0. An O-200 will not give you 100 HP, ever, unless with a constant speed prop, and then only at sea level. A Rallye has neither turbo, nor constant speed, the MCR 4S usually do.
Apples and oranges maybe, but the MCR 4S sure has remarkable performance, the Rallye 100 not so much.
Robin_253 wrote:
The question is what compromises have been made during design process to achieve this low empty weight?
None IMO. It’s simply a well designed aircraft around the principle of using a Rotax 912/914 in a 4 seat high performance cruiser. All carbon, double fowler flaps and so on.
We are mixing apples and oranges here. An O200 will give you 100 hp for as along as you need. A Rotax 912 will give you 75 hp (Do I remember correctly?) for as long as you need. That’s the difference. Of course a variable pitch prop is more flexible in terms of efficiency, but it comes at a cost: weight, coplexity, maintainance, cost….
There are lots of variable pich props for O-200, therefore if we want to compare performance of Rotax with a variable prop let’s choose a variable prop for the O-200
Robin_253 wrote:
In case of the O-200 one can firewall the throttle without time restrictions, in case of Rotax it’s a different thing. 100 hp in Continental is not equal to 100hp in Rotax
No, it’s much less. The MCR typically has a constant speed drive, thus you will get the 100 hp when needed (not continuously though). The Ralley has fixed pitch, and will make maybe 80-90 at take off depending on propeller. I have flown the Super Dimona (a seriously big and heavy motorglider, 770 kg) with a Turbo Rotax and constant speed, I think 115 hp? If it’s a terribly good idea to extract 100+ HP out of the tiny Rotax engine, is another matter though.
Even the ancient Super Cub -95 has MTOW of 680 kg with a 90 hp engine.
