On the lag thing. On a multi shaft engine like the PT6 you squirt in more fuel, the gas generator accelerates and produces more gas. The power turbine accelerates and the prop governor increases the pitch to maintain prop rpm. But the gas generator has to accelerate first.
On a single shaft engine when you squirt in more fuel as the single shaft tries to accelerate the prop pitch immediately increases to maintain prop rpm and hence much less lag.
Vref wrote:
There is no replacement for displacement
It is, and it’s called RPM 
It’s what makes turboprops move btw. Another replacement is boost (more torque).
A 916 iS costs around €50k. The newest and lightest Carbon Cub is built around this engine, which is pretty cool, but it also makes it very expensive.
At the end these engines when powered up further will start to cost as much as a good old IO520….
There is no replacement for displacement…I like the Risen concept though its all about aerodynamic efficiency for speed with limited power fuel burn…
Frans wrote:
What does JMB want with that? Isn’t it just a pure overkill of engine power for a UL?
The biggest benefit of the 916iS in my opinion is that it has lower fuel consumption giving it even more range. The extra power is only for take-off, in cruise the available power is about the same.
They do it because people will buy it, because more is always better :)
I think even the 915 is an overkill for an 600kg plane, but i heard that most VL3s are ordered with the 915, so from now on most will go for the 916, just because they can (except France, they still have the 100hp limit on ULs)
I think a modern 914 (EFI, FADEC) would be the best fit for a Aircraft like this, but who am i to judge ;)
I’ve heard from a good source, that JMB will introduce the VL3 with the new Rotax 316 at the Aero in Friedrichshafen. What does JMB want with that? Isn’t it just a pure overkill of engine power for a UL? Or are they finally gonna certify an EASA-VL3?
Peter wrote:
so putting in a turbine is a perfectly sensible idea
That turbine looks like a “perfectly sensible idea” for most ULs I’m afraid, as well as several experimental homebuilts An RV12, RV-9, even the -4 would be perfect with a 130 hp turbine. Not to speak of my Onex and all other Sonexes.
What is there not to like? A real turbine. Jet fuel everywhere, the right weight, the “right” power, the right sound. Fuel consumption in cruise less than 20l per h. The price tag will be around €50-100k is my guess. This will calm things down I would think. It has a heat exchanger. Will that last for the TBO of 3000h ?
The 200k+ ULs sell in very small numbers, mostly to guys who want an expensive toy, so putting in a turbine is a perfectly sensible idea It doesn’t have to have a 2000nm range.
ALL turbine engine lag when accelerated, or decelerated. And so do piston engines and props…
The simple reason is mass. More fuel for more power = mass acceleration to the required RPM. This evidently is even more pronounced with multi-turbine/compressor with multiple stages. When an N2 turbine is producing power, it is directly linked to its compressor, but only indirectly linked to the N1 compressor, which in turn is producing the real thrust, as in ~ 80%+ in modern engines. Some companies such as R&R even produce 3 rotor jet engines.
Even the small RC turbines as used in the RC world (and the Cri-Cri) show some lag.
Turbine Helicopters suffer from this lag much more as the inertia of the rotor adds to the lag of the turbine.
That is exactly the reason why helicopters do not control altitude by rotor RPM but by blade pitch (very, very simplified) and regulate power/RPM more slowly.
