Changing to the Rotax 915 would essentially be the same as using the original Thielert 135 HP engines, which are still marketed. Slimmer cowlings than the Austro engines, lower weight. But JetA.
I think @Emir did the right thing; go for the 155 HP ‘Thielerts’, and effective scimitar props. No weight increase, only a minuscule increase in frontal area compared to the 135 HP version
esteban wrote:
Diesels should in theory be easier to cool (better thermal efficiency = less cooling needed), but those engines are simply too large & heavy
It’s not easy making something intrinsically heavy into something that flies, especially on a tight budget. I understand the design process for the aluminum engine block was simply to copy the iron Mercedes block in aluminum without much in the way of analysis. By some miracle it worked structurally but it gives the inpression that the whole thing being mostly the product of trial and error.
aart wrote:
Changing to the Rotax 915 would essentially be the same as using the original Thielert 135 HP engines, which are still marketed.
Uhmm, looking here , Continental CD135 are almost 300pounds, AE300 well over 400, Rotax915is about 180. That is quite a difference, especially on a twin.
In Europe, the fuel cost dominates the direct operating cost, so a diesel will rapidly beat a Rotax. A busy FTO makes much more money out of a diesel DA42 than it would make out of an avgas DA42.
Another factor is that, at the height of the Thielert saga, an FTO could have run a TBM700 for what it was costing them to keep a DA42 flying (I was told that would have been literally true), so no way would any of them touch a new engine with a 20ft bargepole. And since Diamond nearly went bust due to this, they would likewise not take the risk. The only thing which prevented Diamond’s bankrupcy was that they could walk away from a “whole aircraft” warranty, by having separate engine and airframe warranties.
I’m not so sure there Peter maintainance costs are and the 912 runs and runs and runs on nothing but routine maintenance. I know examples approaching 5000 hours.
And although the UK insist that all aircraft used for flying instruction have to have their engines replaced at TBO (2000 hours for the 912), other countries in Europe are happy for schools to run on condition.
Peter wrote:
so a diesel will rapidly beat a Rotax.
Depends where. In Norway there are road tax on fuel. But if the fuel is not used “on the road”, you get that tax back. Mogas is therefore comparable to Jet A1 or diesel (when used off the road). Thus the only expensive fuel for aviation is avgas/UL91/etc. A diesel will certainly not beat a Rotax, because a Rotax runs on any mogas available. I’m sure other countries have similar arrangements.
Who can tell what “beats” what with technology and fuel selection being driven all over the map by taxes, versus fundamentals? AVGAS with a reasonable tax level applied costs €1 per liter.
Re: DA42 with Rotax:
There are several aspects:
1) while mogas might be readily available on UL fields, in places DA42 would tend to visit, Jet-A would be much more convenient. Still, mogas should be much cheaper than avgas.
2) FTOs might be scared touching a new unknown engine, however Rotax is by now well accepted/respected one. 912is had its toothing problem, so would 915is have … but nobody (well, few) expects or fears Thielert-like fiasco.
3) less weight and less drag just means more efficient airplane (= faster or less fuel burn = cheaper to run). 100+kg (wrt to CD-155, much more wrt to AE300) makes quite a bit of difference – this also results in slower stall speeds and safer aircraft. Things compound …
And yet the DA42 with its diesel engines outsells the much cheaper twin Rotax engined Tecnam 2006. Why do you think that is?
50% more range, TKS and cheap Jet A when you land – the tecnam is designed for multi training – the diamond is a tourer
