Fuel efficient low observability (stealthy props or ducted fans)?

[Avimimus]

Hello,

I was thinking about the drive for endurance platforms, as well as low observable transport aircraft, and how to attain reasonable fuel efficiency...

Obviously one could use fans buried in the fuselauge at the expense of internal volume. In this case I wonder a lot about the ideal form to maximise internal volume while maintaining a low frontal cross-section and stealthy characteristics.

Another possibility is an external prop. I was wondering about the possibilities to lower RCS on a propeller. A shroud around the blades from the sides (where relative velocity would act as a give-away). In addition, it might be possible to use the shroud to provide anchoring for the blades, allowing the blades themselves to be made of less stiff materials that are more transparent to radar.

What do you think of this approach - could it work? What are the challenges?

 

[Kryptid]

Propellers present many challenges for low observability. Making them radar transparent or coating them with RAM alone won't be good enough. Standard propellers certainly don't conform to planform alignment. Trying to shape them in such a way that they appear swept when viewed from the front or rear hemispheres would cause problems, especially since centrifugal force would want to force the blades into a standard "flat" configuration. Strengthening the blades against that would cause a weight penalty. Shaping for stealth will probably be at odds with shaping for efficiency as well. One possible solution might be to mount the propeller in a shrould (as you stated before) with F-117-style grids on the front and back to keep radar from seeing the propeller. However, that would result in an increase in weight and wetted area compared to a normal propeller.

The only way I can imagine this working would be to completely hide the propeller from radar. Either in radar-blocked nacelles or in S-ducts in the fuselage (as you pointed out). I don't believe you are going to get true stealth with any kind of practical propeller design exposed, regardless of shaping and materials of the prop. This would make the propeller similar to a ducted fan. Unfortunately, propellers are more efficient when their blades are long. Hiding props in nacelles or the fuselage would limit the size of the props. One might possibly be able to get around this by having multiple propellers powered by only one or two engines (similar to the distributed propulsion systems seen in future concept designs).

 

[jsport]

Propellers present many challenges for low observability. Making them radar transparent or coating them with RAM alone won't be good enough. Standard propellers certainly don't conform to planform alignment. Trying to shape them in such a way that they appear swept when viewed from the front or rear hemispheres would cause problems, especially since centrifugal force would want to force the blades into a standard "flat" configuration. Strengthening the blades against that would cause a weight penalty. Shaping for stealth will probably be at odds with shaping for efficiency as well. One possible solution might be to mount the propeller in a shrould (as you stated before) with F-117-style grids on the front and back to keep radar from seeing the propeller. However, that would result in an increase in weight and wetted area compared to a normal propeller.

The only way I can imagine this working would be to completely hide the propeller from radar. Either in radar-blocked nacelles or in S-ducts in the fuselage (as you pointed out). I don't believe you are going to get true stealth with any kind of practical propeller design exposed, regardless of shaping and materials of the prop. This would make the propeller similar to a ducted fan. Unfortunately, propellers are more efficient when their blades are long. Hiding props in nacelles or the fuselage would limit the size of the props. One might possibly be able to get around this by having multiple propellers powered by only one or two engines (similar to the distributed propulsion systems seen in future concept designs).

for quellish to read

 

[TomcatViP]

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Fury is a long-endurance, survivable Unmanned Aerial System that brings strategic capabilities to the tactical warfighter. With the largest payload capacity and volume among runway-independent UAS, and its advanced heavy fuel propulsion system, best in class power generation, and low signature design, Fury is capable of supporting any Intelligence, Surveillance Reconnaissance and IEW requirement. Fury – The Best Value Aircraft for ISR and IEW.

Exposed propeller does not constitute a major increase in RCS even in doppler mode in the sense that signature reduction stays manageable.

 

[riggerrob]

All wood propellers are almost invisible to radar.
Consider the Mosquito bomber which was almost invisible to German radar .... until its wooden structure got water-logged!

 

[Avimimus]

One of the benefits of having a buried high bypass system would be to use some of the additional airflow to spread out the exhaust (and make the IR signature/wake a bit weaker).

One thing I worry about though - radar penetrating the skin of the aircraft from other angles (than directly in front).

 

[jsport]

Propellers present many challenges for low observability. Making them radar transparent or coating them with RAM alone won't be good enough. Standard propellers certainly don't conform to planform alignment. Trying to shape them in such a way that they appear swept when viewed from the front or rear hemispheres would cause problems, especially since centrifugal force would want to force the blades into a standard "flat" configuration. Strengthening the blades against that would cause a weight penalty. Shaping for stealth will probably be at odds with shaping for efficiency as well. One possible solution might be to mount the propeller in a shrould (as you stated before) with F-117-style grids on the front and back to keep radar from seeing the propeller. However, that would result in an increase in weight and wetted area compared to a normal propeller.

The only way I can imagine this working would be to completely hide the propeller from radar. Either in radar-blocked nacelles or in S-ducts in the fuselage (as you pointed out). I don't believe you are going to get true stealth with any kind of practical propeller design exposed, regardless of shaping and materials of the prop. This would make the propeller similar to a ducted fan. Unfortunately, propellers are more efficient when their blades are long. Hiding props in nacelles or the fuselage would limit the size of the props. One might possibly be able to get around this by having multiple propellers powered by only one or two engines (similar to the distributed propulsion systems seen in future concept designs).

for quellish to read

for everyone to read again.