Mast height

[asra]

A question for Greg or Tim, or?I am reconstructing a replica Air Command to make it a little safer, ie. heavier mast and clt or closer than the original measure of 10 inches. The question is, will the machine be more stable with a slightly longer mast, I only say that, as with the original height, say from the thrust line, the seat is higher so the rotor seems a bit close for comfort. Is prop clearance the only factor? My new configuration is similar to the butterfly.thanksJohn

[NiqueNaque]

John,Making your mast longer means that the mass of the rotors is moved further away from the COG, vertically and slightly to the back. If you have a look at SamL's post on the COG for his dragonfly and my similar photo's the vertical displacement will affect the position of the COG with respect to the thrust line if you are not going to move the thrust line. If your machine's thrust line is above the COG then moving the rotor up moves the COG up a little, that will be a little improvement as it moves the COG closer to the thrust line. As a proportion of the Gyro's weight the rotor system might account for say 50 kg out of about 300, it might be 1/6 th of the weight and depending on the weight of other items making it a low centre of gravity gyro and their distance from the COG a small change in the mast may make a big difference. That will be the static stability.I had an idea - if you have the rotor from the cheek plates unbolted and the control rods disconnected, you could jerry up a drum or similar of the same weight as the head and rotor assembly on a piece of timber that the drum was firmly fixed to. Do your vertical COG photo's as Sam did and see how the stability comes out for various positions, then determine your best height. Don't forget that you have to be able to get the rotors on and off, and transporting a high object can be a problem on the roads,never mind spinning them up for prerotation.Have a look at the rotor system generating drag and the turning moment (pitching up) that will generate. When you apply power to increase thrust there will be a greater nosing up moment due to the greater displacement of the drag from the rotors away from the COG, your machine will nose up more than another one with a shorter mast would, that may be good or bad depending on where the centre of drag for the aircraft is.It is better to consider reworking the positions of everything to achieve as close to CLT as possible, making the machine as short as possible, failing that, use a big stabiliser.Hope this helps,Nick.

[asra]

NickThanks for that, Making up a dummy head is a good idea. Another factor is response times on the pendulum effect, presumably a long mast would be slower and it would raise the center of drag...is this a good thing or does one benefit cancel out the other?CheersJohn

[NiqueNaque]

John,Sorry, I shortchanged you on the idea of the dummy head.What I was thinking of was a piece of 4 x 2 timber cemented into a bucket such that the bucket of cement weighed as much as your rotor assembly, torque bar, etc, including the cheek plates. You could do the same with scrap steel or what ever, just don't drop it on yourself or the gyro when you've got it up there. G Clamp the timber onto the mast, being careful not to crush the mast with the clamp, it can happen if you don't put a plate over the mast section between it and the G clamp. Have a fiddle with this and see how it pans out, I've not tried it but would be interested to see what the outcome is of moving the mass up in 25mm increments.As to the pendulum effect and response times, I've not been flying for some time so I'm out of touch, post an email to Phil Wright from the HVGC as he flies the two seat Rossco machine, as does RossB fly a single seater Rossco, both have long masts and a high thrust line, they may be able to give you an opinion on how this works. Ross would be my first call because he has two machines, one with a short mast and near CLT, the other the long mast Rossco, he'd be able to give you a comparison but it will be biased for the Rossco because of its amazing performance and fuel consumption.Worthy of consideration is what the pendulum effect is swinging on... you might be thinking of a pendulum on a clock, which swings on the clock mechanism a fair distance above, our gyros hang off that, yes, but they have another master, the centre of gravity. When the drag on the rotor disappears, even momentarily, what will stop the craft from pitching forward if it has a high thrustline? The stab (dependent on area and moment arm length) is one, the mass inertia of the rotor, the gyroscopic inertia of the rotor and the centre of drag for the fuselage and air frame if it is above the thrust line, if the centre of drag is below the thrust line you have issues.Hope this helps,Nick.