GURPS 3e: Making a duct-rotor helicopter

[Fred Brackin]

Originally Posted by GURPS Fox

The thing was, they looked like regular rotors, just in ducts. :\ So sorry for the hiccups that makes.

At least I've resigned to simply using multiple main rotors for intermeshing rotors helicopters...

They do look very large for ducted fans. In the Real World this might give you better lift efficiency (which ducted fans suck at) but it'd come at the expense of forward speed.

What Ve2 _really_ loves is a lifting body with vectored thrust jet engines. That's where I always ended up when tryign to build this sort of thing.

Oh, and I agree about the "Kaman" style helicpters.

[GURPS Fox]

Originally Posted by Fred Brackin

They do look very large for ducted fans. In the Real World this might give you better lift efficiency (which ducted fans suck at) but it'd come at the expense of forward speed.

What Ve2 _really_ loves is a lifting body with vectored thrust jet engines. That's where I always ended up when tryign to build this sort of thing.

Oh, and I agree about the "Kaman" style helicpters.

To be honest, when I saw the 'Hummingbird' -the world's second military production helicopter... made in WW2 by the Germans- I was enamored with them. I mean, look at the thing, it's so adorable (for a military machine).

[Anthony]

A more realistic treatment of fans in Vehicles would allow you to specify fan area and fan power separately; the reason helicopters have high lift efficiency is just because they have extremely large fans (lift is actually proportional to the 1/3 power of area and 2/3 power of fan power). The SA-2 Samson has similar fan size to area to a V-22 Osprey, which is pretty underwhelming as a helicopter.

The main problem with ducted rotors for helicopters is the 'why' problem -- it's a bunch of weight that serves no purpose in normal flight. It's useful for microdrones because it reduces damage (to both vehicle and object struck) when colliding with environmental objects, but at the sizes and speeds of full scale aircraft it will just be slightly differently shaped wreckage.

[thrash]

Originally Posted by Anthony

The main problem with ducted rotors for helicopters is the 'why' problem -- it's a bunch of weight that serves no purpose in normal flight.

Ducted fan blades are usually made of stiffer material and therefore able to run at higher RPMs than an equivalent rotor system, making them smaller for the same overall power. Done right, the blades should be lighter as well for being supported on both ends (and/or from not having to rotate some of the structural mass). Because they are self-contained, they can be vectored in ways that a conventional rotor system has trouble duplicating. I suspect there is also a gain in efficiency from eliminating rotor tip vortices, but I haven't seen the math on that.

That's not to say ducted fans don't involve a lot of additional weight and complexity, just that there are compensating gains.

[RyanW]

Originally Posted by thrash

That's not to say ducted fans don't involve a lot of additional weight and complexity, just that there are compensating gains.

On the other hand, helicopter rotors generate lift like a wing when the helicopter is moving, requiring much less down thrust (and thus power) than when hovering. A helicopter operating at the edge of its capacity can even stall (fully loaded Hinds in the high altitudes of Afghanistan had to make rolling takeoffs). If a ducted fan gains any lift that way, I'm sure it is greatly reduced.

[thrash]

Originally Posted by RyanW

On the other hand, helicopter rotors generate lift like a wing when the helicopter is moving, requiring much less down thrust (and thus power) than when hovering.

They don't, really, except for specialized configurations (like X-wings). What is happening is that the rotor in forward flight is working on undisturbed air, rather than eating it's own downwash at a hover. This would apply to ducted fans as well. The benefit happens at relatively low airspeeds (~ 35 knots), and is called "effective translational lift."

A helicopter operating at the edge of its capacity can even stall...

It's called "retreating blade stall." It occurs when the forward airspeed gets close to the rotational velocity of rotor system. The advancing blade gets extra lift; the retreating blade has to increase its pitch to compensate for the loss of lift until it stalls. The helicopter pitches up (from gyroscopic precession -- the effect is felt 90 degrees out of phase) and rolls over. Note that if the rotor really acted like a fixed wing as high speeds, this wouldn't happen.

...(fully loaded Hinds in the high altitudes of Afghanistan had to make rolling takeoffs).

Same thing happened to Cobras during summer gunnery at Fort Carson, Colorado (5,874 ft above sea level at the airfield; higher at the range). This is just due to the reduction in effective lift at high altitude on hot days.

Another effect of high-hot conditions is loss of tail-rotor effectiveness at a hover. The Cobra operator's manual says that they aren't subject to this problem, but I can assure you that they are.

[Anthony]

Originally Posted by thrash

They don't, really, except for specialized configurations (like X-wings). What is happening is that the rotor in forward flight is working on undisturbed air, rather than eating it's own downwash at a hover.

While it's not precisely the same mechanic, the math actually works out the same -- either a wing or a helicopter rotor has the net effect of causing downward deflection of an incoming stream of air, generating lift and causing drag. The equations mostly diverge at less than stall speed for a fixed wing of equal wingspan.

Which presumably would happen with a ducted fan as well, but you'd want to make sure that the ducting isn't interfering with the airstream, which I suspect it would do for designs that look like the SA-2.