And if the 915 engine longevity is a good as the 912 then it will be good for 5000 hours.
I don’t know. I have flown several O360 that were padt 5000h, but I would not call them “good for 5000 hrs”. Likewise I know plenty 912S with big downtimes due to technical problems and not one 914 that made TBO withoug bigger maintenance. The 915 already is grounded by an Emergency AD.
I don’t think that you can and should calculate with these kind of lifetimes, even if the lower powered models do occasionaly reach them. Like with the Lycoming and Contis, too.
If you calculate for flight schools/AOCs, calculate with .8 TBO and be happy about any hour you get additionally out of the engine.
Agreed 100%.
The Rotax AD has surprised not a few people…
I would also add that there is a massive amount of disingenuity posted about how fast, how economical, reliable, trouble free, etc, some of these engines (and planes) are. I am always surprised at how someone pays 100k+ for a Rotax plane “because it gives me cheap flying”. In reality there is no free lunch in physics, ever. Any gain comes at a loss somewhere else, so if e.g. you want 150kt IAS from a “320”, say 70% power, peak egt, you will end up with 2 seats + a backpack, and the occupants better know each other quite well 
A genuine 140kt IAS on a Rotax engine, not running flat out, will come in a tiny single seater cockpit (possibly a tandem 2-seat, so they won’t get to know each other too quickly 
). If you want cheap flying you have to fly a small plane, not a normal size plane with a small engine
The DA42 is by all accounts a good plane, nowadays, after all the years, finally.
The downtime stories on some SEP diesel retrofits, or factory installs in normally-avgas models, are sadly quite common, and not good.
For an FTO, the diesel business case is really compelling. Speak to any busy FTO which runs a fleet of DA42s and work out how much profit each DA42 contributes each year. It’s enough to buy a new one every 2 years.
Cheap flying time (£/hour) is mainly to do with the geometry of the aircraft (long wing span, low wing loadings, low weight capability) rather than engine design, so can completely run on those Rotax engines flying sub-FL60 on avgas/mogas
If you want cheap touring (£/nm), that is when you sacrify aerodynamics and you plug a powerful engine runing on diesel fuel with reliable instrumentation and you start looking for flying at FL200
Not much different from car compassion, pointless unless you know you know if it is in city or countryside?
You will be surprised how much “expensive a Rotax” if you plug it to a PA18 airframe instead of a HK36?
100% disagree.
All be it on a sample size of 3. Every 100hp Rotax engine I know fitted to a certified aircraft ran to its TBO of 2000 hours on nothing but routine maintenance.
On the microlight front schools initially started pushing the 80hp out to 3000 hours again only on a small sample of about half a dozen they all did this on nothing but routine maintenance.
Since then I’ve known schools to push it out to 5000 hours and then simply put another one in.
Lyco’s and conti’s simply don’t do that. They need new cylinders, new mags, new camshaft. They need shipping to the states for overhaul. A mixture of 80 and 15/50 oil with camcard, regular oil samples and a 2.5 grand spare set of mags on the shelf
long wing span, low wing loadings, low weight capability
I am not sure those three are primary for fuel flow per knot. I think the main ones are
The 1st is the cockpit volume, mainly. In the low subsonic regime, streamlining doesn’t have a big effect.
The 2nd suffers from most GA flying way above Vbg and has a lot of parasitic drag. So the real solution is to fly slowly And, sure enough, the planes with long thin wings and low wing loading are all types which fly very slowly. The U2 flew very slowly too; just above Vs at FL600+ which was ~ 500kt TAS.
There is no “magic” anywhere.
All be it on a sample size of 3. Every 100hp Rotax engine I know fitted to a certified aircraft ran to its TBO of 2000 hours on nothing but routine maintenance.
I don’t doubt this at all but I reckon that one could easily find another sample of 3 where a busy school gets TBO every time on Lycos, flown daily.
Peter wrote:
The 1st is the cockpit volume, mainly. In the low subsonic regime, streamlining doesn’t have a big effect.
Yes, I agree the big factor is cross-section of the cockpit (glider style, but no one can see you on head-on collision) and slow flying close to min sink rate, but this gives you the other variables
Peter wrote:
The 1st is the cockpit volume, mainly. In the low subsonic regime, streamlining doesn’t have a big effect.
This is often repeated, but can somebody put real numbers? I.e. cockpit vs wing vs tail drag? 100% wise? Ideally both for cruise speeds and for Vbg..
There are designs that are clearly cleaner than others, even with comparable frontal area (e.g. Pipistrel Virus SW, but also Vari-eze and related). No magic/free lunch/100% improvement, but solid 20-30% difference.
Certainly you can achieve 20-30%, between a dirty design and a clean design.
esteban wrote:
This is often repeated, but can somebody put real numbers?
The cockpit cross-section is a main part in the drag equation, double it and you double drag forces
Streamlining/surface goes into the drag coefficient,
- Shape has some effects: sharp cone has 0.5 a round sphere has 0.45 and a cube has 0.8, so 10% to 40%
- Surface will help as well: smooth fiberglass vs dented metal will give you something around 30%
Speed does not matter as long as you are in the laminar regime on the left of the drag curve, you should see IAS^2
@Ibra:
- effects of shape, if I look at wiki, it shows even 0.04-0.09 for streamlined body, that is much more improvement than 10 – 40%, compared to a basic shape (say long cylinder or sphere)
- wings and tail surely have also their own drag, how much is that compared to the cockpit? (that is my question!)
– in the extreme, extra large wing with extra small cocpit the drag will be determined by the wings, with small wings and large cockpit it will be the other way around
– what are the numbers in real airplanes (GA, motorgliders, gliders…)
- linear vs turbulent flow will also make quite a difference (I guess relevant only for twins/pushers, with tractor it is turbulent from the very beginning)
