hub bar lengths?

[13brv3]

I thought I remembered seeing some discussion about this, but I can't find it. As an example, let's say you need a 25 ft rotor. If you compare 10.5 ft blades with a 4 ft hub bar, to 12 ft blades with a 1 ft hub bar, would there be any significant difference in performance? I have to believe the speed of the inboard end of the blade is so low as to not really produce much useful lift. On the other hand, the typical rectangular hub bar likely creates more drag. I'm sure there's some difference, but would it be enough to notice?

Thanks,
Rusty

 

[Resasi]

Been wondering that myself.

 

[Tyger]

Let's say the rotor is spinning at 350 rpm. That means that 4 feet out from the hub the blade is moving at 8 x pi x 350 = 8796 fpm = 100mph.
I'd say that's probably generating some reasonable lift, in the "driving region". Three feet out it would be 75mph, etc.

 

[13brv3]

Let's say the rotor is spinning at 350 rpm. That means that 4 feet out from the hub the blade is moving at 8 x pi x 350 = 8796 fpm = 100mph.
I'd say that's probably generating some reasonable lift, in the "driving region". Three feet out it would be 75mph, etc.

My reference to a 4 ft (total length) hub bar would be 2 ft out, so only 50 mph. Another way of looking at it is to imagine a 48" 2 blade prop, which is really short. Now imagine it at only 350 RPM, or half the idle speed of many aircraft engines. Not going to get much thrust out of that.

As a side note, I wonder what the realistic max length for a hub bar is. I'm pretty sure I've heard of 4 ft, but don't know if that's as long as they get.

 

[JETLAG03]

How about the same blades of hub bars of differing lengths. I have two hub bars one 60cm (24") longer than the other. I notice a difference in max spin up (electric) but how would the two hub bars change the lift/drag?

 

[XXavier]

How about the same blades of hub bars of differing lengths. I have two hub bars one 60cm (24") longer than the other. I notice a difference in max spin up (electric) but how would the two hub bars change the lift/drag?

If, with the same blades, you switch from a short hub bar to a longer one, the effective length of the blades is increased, and –for the same angular velocity– the lift and drag of the rotor will be higher. (To a first approximation, and in the absence of inflow, the lift (and the drag) of a gyro spinning blade grows with the cube of its length...).

Hence, if you increase the rotor diameter, and for the same input power, the maximum angular speed reached by the electric spin-up will be lower...

 

[Tyger]

If it's a question, then, of how much of the first two feet from the hub is blade vs bar, I think we have shown that any blade that close to the hub will have very little lift (with the bar having zero lift). So it seems to me the difference would be de minimis.
I take the point that putting the same set of blades on a longer hub bar essentially adds blade further out, which would make some significant difference.

 

[JETLAG03]

With the short hub bar I have 7.7m and 8.3m with the long hub bar.

As @XXavier mentioned I noted a lower rotation speed with the long bar and a significant difference in the float on round out