Tilt Rotors - already archaic?

[Davyd]

Knowing full well how loooooooong and drawn out the development has been for the Osprey, how feasible is it to actually use jets? I mean, lots of analysts and designers have pointed out how much simpler the basic engineering and design would be to use jets instead of props since the V-22's early woes. So what is the holdup? Are we waiting for some front line flight time out of the X-35 so we can see how the lift jet functions (since i guess that may end up being a great example for this kind of use)?

Now a jet is a complex machine, to be sure. But some people have felt that the the gears and transmissions related to the props would be made unnecessary with jets. And i personally would also like to see further development (the whole reason i made the thread). I know we worked with a tilt-jet project in the late(?) 80s, think it was Bell.

So i'm just curious what some of you around here think of this possible development. Or if someone knows something i don't.

 

[rjmaz1]

Actually the next big trend will be ducted fan

A ducted fan is what is used in the JSF for vertical takeoff. Look how small the diameter is yet it provides 8200kilo's of thrust!!!! Yes thats not a typo Two of those ducted fans could lift a Osprey off the ground and they are less than a metre in diamter, compared to over 10 metres! Must more efficient.

Ducted fans dont have the risk of hittings tree's when hovering, they are also more efficient so have longer range. By having flaps under the ducts you can direct the thrust in any direction to provide high agility even at low speeds. WIthin a few microseconds you could have full 2000kilo pushing the craft sideways, backwards or forwards.

Also the big bonus is that they travel faster than a normal helicopter at over 400km/h.

Expect the next attack helicopter/attack aircraft to be a ducted fan.

DARPA are currently testing a manned ducted fan aircraft with Trek aerospace. The latest model has already exceeded 400km/h in testing.

The next step is to use the small gas turbine used in the light bell helicopter, cant wait to see that.

 

[410Cougar]

Wouldn't the engines used on the JSF be too small to use and sustain a vertical position on a bigger aircraft used for any type of troop transport? Or would we be talking about multiple engines here in different locations on the airframe, say on the wings??

Sorry if my questions are a little off...

 

[WaterBoy]

The problem with a pure jet engine would be the exhausts gases, which will probably be in the range of 500-1000 degrees centigrade, very bad for people under the aircraft. A ducted fan engine wouldn’t have exhaust gas issues but it would be very unlikely that again, anyone could work under the aircraft. Additionally it wouldn’t be able to hover over unprepared surfaces, & the FOD / visibility degradation generated by such concentrated lift columns would render ground crew & pilots effectively blind. (iirc the Osprey already suffers from this problem)

In terms of efficiency, at low speeds a propeller is the most efficient propulsion available for an aircraft. When transitioning into the transonic speed range jets then become more efficient. The larger diameter of the blades also reduce the hovering’ footprint’ of the aircraft & will allow people to work under & around the aircraft on unprepared surfaces.

As to the gear box arrangement, some interlinking of powerplants will be necessary for asymmetric control, otherwise some spectacular back flips prior to crashing will occur following an engine failure. The reason aircraft with two engines can continue to fly on one is the interlinked gearboxes (helicopters) or the aeroplane has attained sufficient airspeed to retain control surface aerodynamic control (Vmca ) using ailerons & rudders. As a side note, iirc the JSF is one of the most challenging gearboxes ever designed.

One the unique characteristics of helicopters is that whatever improvements are made to their top end speed generally comes at the expense of hovering / lifting capability & vice versa. Ducted fan jets haven’t changed that – yet! The Ospreys superior speed performance is due to the wing generating lift at ‘flying’ speed which allows a greater proportion of the available thrust to be converted into speed performance. This same wing then impedes the aircraft’s ability to hover as the propellers ‘thrust’ downward airflow onto the aircraft itself. That’s why very few helicopters have wings.

Regards, WaterBoy!

 

[rjmaz1]

A ducted fan engine wouldn’t have exhaust gas issues but it would be very unlikely that again, anyone could work under the aircraft.

Why would people be under the aircraft while the engines are running full speed?
A ducted fan could hit a side of a building or tree's on takeoff and still remain fully operational. The large 20metre rotor of a helicopter means it has to have a large space cleared for it too land, atleast 40metres by 40metres. A ducted fan aircaft could land in a 10metre by 10 metre square. Thats less than 10% the landing space compared to the same weight helicopter. The duct around the fan allows enough protection for light impacts with its surroundings, a helicopter has no such luxury.

Additionally it wouldn’t be able to hover over unprepared surfaces

To have a prepared surface 40 metres by 40metres will require alot of work. However a 10metre by 10metre section of hard surface can be found everywhere, on any side street or road or even the roof of a small building, with many objects that would be risky if the rotars.

In terms of efficiency, at low speeds a propeller is the most efficient propulsion available for an aircraft. When transitioning into the transonic speed range jets then become more efficient.

Actually in terms of speed a ducted fan sits between a prop and a jet engine. So at low/medium speeds a ducted fan is more efficient. Variable pitch blades allow it to have a much larger range of speeds.

As a side note, iirc the JSF is one of the most challenging gearboxes ever designed.

Only because when the VTOL fan is started the jet engine is already tarevling at 20,000rpm. So your effectively doing a "clutch dump", thats the best way to blow a manual transmission in a car rev to 5000rpm and drop the clutch. With a ducted fan the clutch is always connected, so it only needs a normal helicopter transmission.

One the unique characteristics of helicopters is that whatever improvements are made to their top end speed generally comes at the expense of hovering / lifting capability & vice versa. Ducted fan jets haven’t changed that – yet!

Trek aerospaces latest ducted fan design the body of the aircraft is actually designed to provide alot of lift at higher speeds allowing it to fly faster and more effeciently at that speed than any helicopter. Yet it hovering and load carrying abilities is not reduced. They are actually using variable pitch blades so the blades can provide lots of low speed high volume of air for hovering, but when traveling at higher speed the pitch changes as the air is already traveling at 300km/h when it enters the ducted fan. No helicopter has variable pitch rotars as far as i know.

The Ospreys superior speed performance is due to the wing generating lift at ‘flying’ speed which allows a greater proportion of the available thrust to be converted into speed performance. This same wing then impedes the aircraft’s ability to hover as the propellers ‘thrust’ downward airflow onto the aircraft itself. That’s why very few helicopters have wings.

Thats why a ducted fan is superior. If mounted at the ends of the wings it still gets lift from the wing in conventional flight and the wing doesn't get in the way of the thrust flowing down and hitting the wing.

Wouldn't the engines used on the JSF be too small to use and sustain a vertical position on a bigger aircraft used for any type of troop transport? Or would we be talking about multiple engines here in different locations on the airframe, say on the wings??

Ideally you'd use atleast two one on either side of the aircraft. Two JSF ducted fans alone would provide enough thrust to lift more than 90% of the helicopters in operation. Take a V-22 osprey for instance, make the wings thick and wide and mount the JSF ducted fan inside each wing, with the engines ontop like a normal helicopter. The wings provide lift as well as the ducted fans during horizontal flight, thrust is provided by the gas turbines on the top of the aircraft as well as flaps underneith the ducted fans that direct the air backwards.

So yea big things are to come from the ducted fan aircraft. In the next few decades excpect to see thousands of ducted fans aircraft. :type

 

[WaterBoy]

Why would people be under the aircraft while the engines are running full speed?

Apart from attack helicopters the army seem to have lots of people loading, entering or exiting hovering helicopters. Thats the point of a helicopter, it doesn't have to land.

The duct around the fan allows enough protection for light impacts with its surroundings, a helicopter has no such luxury.

Ballistic tolerant blades make excellent brush cutters.

To have a prepared surface 40 metres by 40metres will require alot of work. However a 10metre by 10metre section of hard surface can be found everywhere, on any side street or road or even the roof of a small building, with many objects that would be risky if the rotars.

Ever seen a Harrier air taxi off its hardened surface? Again the point of a Helicopter; no preparation & it doesn't have to land.

No helicopter has variable pitch rotars as far as i know.

ALL Helicopters have variable pitch blades, they have to. Otherwise retreating blade stall would make them uncontrollable, just like the early experiments were. The Cyclic controls the blade angle & is one of the primary flight controls.

Two JSF ducted fans alone would provide enough thrust to lift more than 90% of the helicopters in operation.

Assuming same thrust, Osprey has two 37 foot diameter rotor discs. JSF lift fans are 4 foot in diameter. The column of air (thrust) from the lift fans is around 10 times faster. Ever seen video footage of airliners blowing around cars? Same principle, as 99.9% of a turbofans thrust is produced by the fan. Additionally, the lift fans higher RPM makes it is more FOD prone, just like a jet engine.

Take a V-22 osprey for instance, make the wings thick and wide and mount the JSF ducted fan inside each wing, with the engines ontop like a normal helicopter. The wings provide lift as well as the ducted fans during horizontal flight, thrust is provided by the gas turbines on the top of the aircraft as well as flaps underneith the ducted fans that direct the air backwards.

I'd be amazed if any wing would produce lift with such disturbed airflow over & underneath it!

Regards WaterBoy!

 

[chargerRT]

say your flyin your osprey in 'plane mode',full of troops,and the tilt rotors get stuck.how are you going to land?or do you carry LOTS of 'chutes in the back...

kris