New Bell tiltrotor idea
 

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Rotorhead friends,
I posed a question in military aviation, which no-one seems to be able to answer yet - wondering if you all might have any insights.
Above is Bell's latest concept for a high speed jet tiltrotor.
No doubt if they're floating this idea - they've answered the question I have, but I've reproduced it below to see if any of you can explain?
I hope my question is clear enough.

Translation from stowed high speed flight to rotorborne flight could be interesting.
The Osprey doesn't fold it's rotors - and it transitions from a mid-speed turboprop to a helicopter by trading thrust vector for lift - right?
But this thing would transition from pure jet, through turbo-prop to helicopter.
What speed would you need to decelerate to, to be able to fully deploy (presumably) feathered rotors into the airstream, yet still support the vehicle's weight in wingborne flight?
Actuators would need to be very powerful to deploy long rotors into an airstream that would have to be fast enough to support wingborne flight at vehicle MAUW and the rotors would have to be damn stiff and strong to withstand that airload.
Is it then a gradual transition from turbojet to turboprop mode - increase in pitch - thrust from jet exhaust transitioning to propeller thrust?
Am I missing something here?
Can't quite picture how the transition between modes of propulsion would work... but then I'm no aerospace engineer.

The Osprey can fold it's rotors, just not in flight.

Maybe if the nacelles are capable of rotating slightly rearwards whilst effectively gliding with zero thrust the airflow would help deployment of the blades, which would then be capable of assisting quite rapid deceleration? Kind of bypassing the turboprop mode when slowing down. Just a thought.

If I understand you correctly - similar to the Harrier transitioning from forward to vertical flight - the nozzle thrust gradually assumed the load lost by the wings as the jet slowed.
Only issue here is there's only two vectors of jet thrust aft of the C of G - the Harrier had four distributed evenly around the C of G.
This is really doing my head in.
Where's NickLappos Nick Lappos when you need him...?!

Perhaps they only need to deploy enough to clear the wing, and then CF does the rest. From high speed to hover, reducing RPM & CF would put the blades on the deploy stop, and then they are slowly retracted fully.

My question is how can a jet engine be both a turbo-prop and a pure jet engine? Sure, a turbo-prop develops a bit of thrust, but most of the combustion energy is converted to torque by the last stage turbine blades, which are certainly a non-removable part of the engine. This idea seems to have much built in inefficiency and/or extra weight. Am I missing something? Do they intend to fly extra engines just for landing?

Easy, just feather the power turbine blades when not required. 😁
I'll get me coat.

Very droll.
Well, if you lot can't explain it to me - I'm prepared to call bullsh!t on Bell's concept until proven otherwise...

That's a safe bet!

I was joking, honest...

Variable Turbine Blades.

Jaysus - well there you go - you learn summit every day.
That would have to be some kind of actuation system to work at the speeds and temperatures involved.
Still doesn't solve Bell's problem though... ;)

Variable geometry turbines have been "in development" for over 40 years. They are still 'in development' - as it's rather difficult to things that can survive at those sorts of temperatures for several thousand hours between overhauls. The LP might be possible - a lot cooler in that part, but still a difficult engineering problem. Plus, the whole reason we went from pure jets to fan jets is that pure jets are not efficient. So you can go fast, but not very far...
In short, I'll believe it when I see it.

It's concept art, dreamed up by the marketing department at happy hour. "Wouldn't it be cool to have a tiltorotor that folds the proprotors back and becomes a jet?"

Yep - that's increasingly what I'm thinking.
Cos them wings also look pretty thick and fat to get up to jet speeds.
And in addition, the rotors are very short and stubby, imagine the disc loading in the hover at MAUW!

Folding tiltrotor rotor technology was demonstrated in wind tunnels at sub-scale and full-scale (25’ diameter) in the 1970’s. The clutchable lift fan on the F-35B is another way to turn a turbo shaft engine into a turbofan. There are challenges to be sure, but it’s more than marketing BS. Look at the patent traffic over the past 5 years… solutions are being found for the issues identified years ago.

Aha:
https://smartech.gatech.edu/bitstrea...00908_mast.pdf
Page 15 of above for a diagram of the mechanism... and page 24 onwards for detailed analysis of transition sequence.

The correct terminology for the concept shown at the beginning of the thread appears to be a stop-fold tiltrotor.
Apparently Bell did indeed test a full 25 foot nacelle folding rotor in the 40x80 foot Ames wind tunnel in Feb 1972.
The photos I can see look remarkably like the drawings of the power unit above.
Boeing, Bell and ONERA in France were involved.
They tested stop-fold proprotors up 0.85 Mach.
Wow. There's nothing new under the sun...

and more here:
https://hushkit.net/2021/08/03/bell-...-dr-ron-smith/
A report post the Bell 1972 tests is linked to in the above article - fascinating reading.
https://ntrs.nasa.gov/api/citations/...9720022363.pdf

So, thinking this through.
For transition from aeroplane prop driven mode to jet driven mode, presumably you reach a prop driven speed where wing borne flight driven by turbofan thrust can assume vehicle load - and that still won't exceed airloads on the prop.
Optimal transition speed is between 150 and 175 knots.
At that point - you progressively reduce torque, increase turbofan thrust, freewheel the prop, feather it - it stops in the airflow, is locked and retracted flush with the nacelle.
You are then free to accelerate further - limited only by jet thrust and fixed wing airframe considerations - up to 400 knots apparently.
Transition back is the reverse, feathered prop deployed into airstream (actuators need to be powerful, but still doable) decelerate while still driven by fan thrust, progressively transfer power to engage prop, increase pitch, and you're back to turbofan driven prop mode.
Controls very similar to the Osprey - cyclic and collective that progressively wash out or back in according to mode - and beep-able nacelles.
Corrections or more sophisticated explanations welcomed.

Looks unnecessarily complex to me with so many potential failure modes to deal with e.g. I guess you would need to test rotor stow/unstow during preflight rather than find out about it later. Sounds like it will also have high maintenance and serviceability challenges. Perhaps building it will lead to further innovations elsewhere. At least they are not proposing to use drone style tiny electric fans.

Making it fly is sometimes the easiest part.
 

After watching videos of the F-35 STVOL takeoff and land, I don’t see any reason why the Bell concept would not be technically feasible.

The real challenge for Bell will be proving reliability and cost effectiveness.

You Tube

I concur.
Technically much more feasible now than in the 70s.
Advances in materials science, composites and fly-by-wire, FADEC etc.
A little more complex than the Osprey, but not much.
Big expansion in range circle... would offer some interesting capabilities for many different types of mission.
There's an added benefit pointed out in the Bell paper - in the event of an engine out, or both out, this thing could perform a conventional run-on fixed wing landing - albeit at high speed.

https://news.bellflight.com/en-US/20...ehpUijDRoE7E1w


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