Wednesday, 8 May 2019

Twister hybrid - More details

Here are some more details regarding the Turbine GenSets currently on offer.

If anyone knows of any others please let me know in the comments section below.

First up the MiTRE from Delta Motorsports.

Their latest model has an output of 35kW with a weight of approx 50kg.

It is quite a compact unit as you will see from the below pics.

I would need double this output and if anything have a slight reduction in weight. Bear in mind that some of that 50kg is for emissions reduction for the car industry which is not necessary in experimental aviation, so with that equipment removed the weight should go down. Also more exotic materials can be used to reduce weight further, although that will drive the cost up.

The battery pack is a 'moveable ballast' so if needs be It could be relocated to another area of the airframe - perhaps under the safety cell or even split in two with part of it in the baggage area (as Pipistrel do with their electric model to retain the correct C of G).

The other turbine Generator on offer is the TurboTech from France.

Unlike the Delta MiTRE this unit is specifically designed for aviation.

They are quoting an output of 55kW with a weight of 50kg.

With 40kg of fuel (54 litres) on board this unit has the equivalent energy of 1000kg of Lithium-Ion batteries!

The size of the Turbotech unit means it will be a squeeze to fit it under the Twister cowling and to include the electric propulsion motor. So the battery pack and inverter would almost certainly have to be moved elsewhere in the airframe to make room for it.

From their Linkedin picture it seems like Turbotech are using a very similar idea (but in reverse) to the pre-cooler on the Sabre (single stage to orbit space engine). I believe the Sabre pre-cooler is made from micron-thin-walled Inconel.

More about the Sabre here:

More about TurboTech here:

And finally a link to the Delta MiTRE which is a few years old so talks about 17kW rather than their latest 35kW version:

Delta Motorsports MiTRE turbine genset.

Installed in the bottom of a car.

Compact size of the Delta MiTRE is clear from this video screenshot.

Turbotechs offering. Larger in size than the MiTRE but getting close to the figures I need for the Twister. They are quoting 15 litres per hour in the cruise - same as my current fuel consumption.

Turbotech's recuperator design seems very similar to the Sabre Hybrid rocket engine.

Monday, 6 May 2019

Twister hybrid - revised version

It's been 7 years since my first design of a hybrid Twister. (See my previous March 2012 post).

A lot has changed in that time and I've also learned a lot more about the possibilities of electric hybrid powertrains.

For the immediate future a hybrid solution is the only one that works if you actually want to tour in your aircraft (which I do).

Fully electric is only good for aerobatics and local flying of no more than 1 hour.

So with my latest design the bias is with much less batteries and a normal fuel load.

Delta Motorsports here in the UK are developing a gas turbine generator as a sustainer/range extender for Ariel's new HiperCar and for Morgans new electric model, both coming out in 2020.

Normally a gas turbine is very inefficient - typically 15%. However through the use of a recuperator and some other secret tech Delta have managed to get the efficiency of their turbine up to 30%. This makes it a game changer.

Now the generator is as efficient as a conventional internal combustion engine.

The electric propulsion motor is 96% efficient so there are hardly any losses after the GenSet.

In terms of Aerobatic performance this new hybrid Twister would have the potential to compete at the Advanced level (although the roll rate would also have to be increased, the easiest way to do this would be to dispose of the flaps and go for full span ailerons although this would lengthen the landing distance by quite some margin).

Comparing to the Extra 200 and 300, which are both quite a bit heavier, the power to weight ratio is very much in favour of the Twister. With the Extra 200 being 540kg empty, plus 120 for fuel and pilot gives a power to weight of 0.22 kW per kg. The Extra 300 fares only a little better being 682kg empty, plus 120 for fuel and pilot the power to weight ratio is 0.28 kW per kg. The Hybrid Twister with 150kW and an estimated empty weight of 330kg (+ Pilot and fuel 450kg) would have a power to weight ratio of 0.33 kW per kg. This would give it a phenomenal climb rate of at least 3,500 fpm.

With that kind of climb rate then the lower VNE becomes less of a problem as the display/sequence can be flown in a smaller space and at a slower speed as there is less need to 'wind up' to a high speed to gain energy before starting a vertical figure.

Watching the new GenPro recently on YouTube it has shown me that with a superior power to weight ratio you do not need to fly so fast and take up so much room in the box. 

This suits the Twister as it has a lower VNE and G limits than the Extras. (160kts vs 217kts and +6-4 vs +-10G)

As you can see below the turbine would be housed in it's own heat proofed section of the cowl. The exhaust can be made parallel to the airstream for a little extra thrust. The turbine would be fed either by a dedicated NACA duct on the side (shown) or taking part of the air from the LoPresti inlets at the front - if there is some spare capacity left over from the liquid cooling requirements.

Hopefully the existing cowling and oil cooler scoop can be used.

With the small battery pack you would have 2 mins of max power - 201hp. Then in the cruise the power output of the GenSet would slightly exceed the propulsion motor requirements and therefore it could be used to charge up the batteries.

There is also some nice redundancy here. If the turbine GenSet fails after take off then you still have approx 4 mins of battery power to get around an abbreviated circuit and back on the ground.

Cost is a big unknown at the moment as the turbine is not ready for market. This design would also need a new variable pitch prop to deal with the greater power.

The existing fuel system can be used and with 30% efficiency from the turbine it should mean that the range is not compromised. Jet A-1 is less than half the price of AvGas in the UK too (and if you can buy at the corporate rate then it is even better as it is approx 20% of the price of AvGas). 

See the below illustration for details and layout.