Rotor does not take up speed

[Simon]

Hello everyone,

This evening I spend on the local airfield trying to get an old glider going, towed behind a car.
It's one of the earliest Bensen type gliders with a short Bensen alu rotor.
A bit like http://www.vortechonline.com/bensen/grfx/B7_flying.jpg
After handstarting the rotor looses speed when you start towing it, instead of gaining extra speed.
The hub bar is not a bar, but 3 leaf springs mounted to the teeterblock, the blades are hollow and there' s a key to prevent
them from taking an other position on the bar, so nothing's adjustable.
We even tried them in the propwash of a plane, but no luck. I have quite a bit of experience with flying gliders,
but I do not understand what's wrong here.
Anyone has some advice?

Thank you, Simon

 

[AirCommandPilot]

Is the rotor upside down? Can you post a picture of the rotor mounted on the machine?

 

[ckurz7000]

The only reason a rotor doesn't speed up is that the blade isn't seeing a suitable angle-of-attack. There are many reasons which can lead to a wrong AoA:

1) blade profile incorrect (upside down??)
2) pitch setting of the blades in the hub incorrect
3) Wrong cyclic input for the speed
etc.

-- Chris.

 

[Gyro28866]

Assuming the blades are mounted camber side up, the first thing I would check is the blade pitch. I made a set of tracking bars to be able to micro adjust the pitch on my Bensen blades.
What I found was at 1* pitch the RRPM was 425, at 1 1/2*pitch the RRPM was 375, and at 2* pitch the RRPM was 325. All the Bensen blades I have flown like 1 1/2* the best. That has been the majic number for me. Over the years, and speaking to numerous Bensen blade owners, the 375 RRPM seems to be the sweet spot.
As you stated, the "position on the bar, so nothing's adjustable" is a slight obstacle.You are going to have to figure a way to measure the pitch.
Just thinking out loud:
With a decent digital angle finder (an ap on your smart phone?) take a pitch reading on the bottom of each blade near the attach point, and see what the difference between the numbers is. Try it with the rotor in the 9-3 oclock position and in the 12-6 oclock position. Half the difference in the reading should be the pitch? for example in the 9-3 position: blade 1 is 48* and blade 2 is 45* the difference is 3* so the pitch is 1 1/2* per blade.
I don't know if this will work, I have not tried it; but it seems plausible

 

[Simon]

First, thanks you for your reactions.
I spend a long weekend in England, so sorry for my late reaction.
Yes, the rotor is the right side up. A picture is a bit difficult, because the owner took it with him to Begium. I gave him the link of this discussion so he could read
your reactions.

The pitch is no adjustable, I haven't measured it, the phone trick might do the job for at least a rough estimate.
If it is off, the only thing to do to keep the blades would be to make an adjustable hub bar or a fixed one at 1,30 degrees. The rev's will be less than 375
because it lacks the weight of the engine.

You usually speed up the towing speed from very slow to flying speed when you see the blades spinning a bit faster all the time, too much speed for the revs and
the rotor will tell you by flapping.
This blade would do nothing but slow down.

I think we should be looking at the blade angle, or an other rotor, but it will need another rotorhead as well than, if you buy those new we'd better find an
flying glider that people don't use anymore.

Thanks again, if there's news I'll let you know

Simon

 

[giro5]

My guess is you are towing too fast initially. I have Neal Carnes extruded blades on my gyro glider and I start out at about 5 mph tow speed with the blades almost level. As the blades speed up a little I slowly bring the stick back a little then after the blades speed catches up I have the tow operator speed up 3 or 4 mph. Then I keep repeating this till the blades are a blur and then the tow driver can speed up to lift off. When I first started learning we tried towing at 10 - 15 mph initially and the blades would not speed up possibly due to incorrect blade position from the control stick. Now a days if the wind is from 5 - 20 mph I can get the blades to come up to speed before even moving the gyro glider forward. For me it just took practice until I got a feel for bringing the blades up to speed. On my glider you have to push forward on the stick until just before lift off at that point the pressure changes to the opposite and your pressure on the stick goes neutral then to having to apply pressure rearward to hold the blade position and within 2 - 3 seconds you will lift off. I do not have any springs for trim on the rotor head.

 

[phantom]

If the rotor is level it won’t speed up, the correct way is to prerotate by hand and start tilting the rotor back before you start moving, if there is even a light breeze it will accelerate quickly but it’s important to have the rotor full back and turning as fast as you can get it before you start to move forward you leave it that way until the nose lifts, then you tilt the rotor so that the tail wheel is almost touching and allow it to fly off the mai s as speed builds. If there is much wind you start with the rotor level and bring it back as you speed it up.