All i need now is the balls to try it
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to teather or not to teather
I completely agree with the advice given by Gaseous. Do not teather. If the main rotor and tail rotor react as expected while light on skids then hovering will not be a problem. Also, don't be afraid to slam that collective down if any rolling starts to occur. The landing gear won't mind. I have about 900 hours in Rotorways and what Gaseous posted is excactly what all the Rotorway pilots are instructed to do with their own helicopters. Also, the belt driven tail rotor was very reliable. I would not have any problems with the belts whatsoever for as you mentioned they last hundreds of thousands of miles and are even used on farm equipment with far more stress. Easy to inspect and replace, they are a great choice.
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It might be easier to do some dynamic testing by bolting the machine on top of an old truck and racing up and down the runway or open field.
This has been done with fixed wings of odd design, such as canards, to test the limits.
Just an idea.
This has been done with fixed wings of odd design, such as canards, to test the limits.
Just an idea.
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Re:To thether or not to tether.
Southernweyr, not trying to counter your thoughts here about avoiding tethers. My post was not very clear - i'll see if i can tidy it up. 
What i meant to say is that the single main problem with tethers is that they apply loads to the airframe which do not pass through the C of G and thus produce a torque high enough to overpower any cyclic input. This will be even worse if the tether suddenly becomes taught as dynamic rolloever is sure to follow. I'm making the assumption here that BugDevHeli wants to find a way to accumulate time and test the machine with minimum risk to pilot. The best method is to have the machine fixed in such a way that the collective cannot alter the machines height, with the simplest being to bolt it to a trolley.
What i was trying to figure out is if there is a method of applying linkages between a mobile ground run trolley and airframe to simulate in flight loads and attitudes but keep machine in check in case of any problems. This would give Bug a feel for the machine as well as expose parts to realistic loads. If there was any problem the machine is still on the ground so there would be no complications.
The only practical way i can think to do this is to use 3 or 4 links all angled in to pass through C of G. This way collective and cyclic can be flown for realistic flight attitude, hence dynamics, hence loads with machine all the while being well supported. The problem though is that if the machine started to fall over (for whatever reason) then the links would now be pointing the wrong side of the C of G and would not stop the machine falling over. A simple solution is to have the virtual apex (where all links intersect) above the C of G, so that machine has to rotate some way before C of G passes outside of virtual pyramid created by linkages. An even safer solution would be to have a sprung centreing mechanism that would overcome any moment as the machine tried to fall over.
Bug, please comment whether any of this is any help. I'll try to clarify more if needed.
What i meant to say is that the single main problem with tethers is that they apply loads to the airframe which do not pass through the C of G and thus produce a torque high enough to overpower any cyclic input. This will be even worse if the tether suddenly becomes taught as dynamic rolloever is sure to follow. I'm making the assumption here that BugDevHeli wants to find a way to accumulate time and test the machine with minimum risk to pilot. The best method is to have the machine fixed in such a way that the collective cannot alter the machines height, with the simplest being to bolt it to a trolley.
What i was trying to figure out is if there is a method of applying linkages between a mobile ground run trolley and airframe to simulate in flight loads and attitudes but keep machine in check in case of any problems. This would give Bug a feel for the machine as well as expose parts to realistic loads. If there was any problem the machine is still on the ground so there would be no complications.
The only practical way i can think to do this is to use 3 or 4 links all angled in to pass through C of G. This way collective and cyclic can be flown for realistic flight attitude, hence dynamics, hence loads with machine all the while being well supported. The problem though is that if the machine started to fall over (for whatever reason) then the links would now be pointing the wrong side of the C of G and would not stop the machine falling over. A simple solution is to have the virtual apex (where all links intersect) above the C of G, so that machine has to rotate some way before C of G passes outside of virtual pyramid created by linkages. An even safer solution would be to have a sprung centreing mechanism that would overcome any moment as the machine tried to fall over.
Bug, please comment whether any of this is any help. I'll try to clarify more if needed.
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all i need now is the balls to try it.
Thanks for your suggestions. I had considered tethers, mechanical restraints of some kind, or possibly just going for getting skids light and then an inch or two clearance etc. Having had a previous machine do a 180 in the blink on an eye , i am considering short tethers for initial getting light on the skids and confirming tail rotor authority and then if all is well proceed to small hops foreward etc. The device the young lady is holding is a test model for my ring rotor. This enables me to acquire great inertia without the penalty of heavy blades or rotor hub. the outer ring constrains the inertial forces (depending on its elasticity) and also allows me to put the weight where i want it. It does however produce some unexpected flexation modes within the blades themselves. FDI analysis indicates an improvement in lift especially around the tips.The improved performance on the tip loss factor overcomes the additional drag of the ring itself Thanks Bug
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Bug,
Have you worked out the effective hinge offset with the ring rotor? I ran into headaches using standard Prouty equations. The easiest method is to compare rotating flapping frequency with rotational frequency then use:
Lead angle = 90 * (rpm/60) / frequency.
Rotating flapping frequency will be a standard output from your FEA (or test).
I'm sure this is old hat...
Have you worked out the effective hinge offset with the ring rotor? I ran into headaches using standard Prouty equations. The easiest method is to compare rotating flapping frequency with rotational frequency then use:
Lead angle = 90 * (rpm/60) / frequency.
Rotating flapping frequency will be a standard output from your FEA (or test).
I'm sure this is old hat...
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All i need now is the balls to try it
Graviman. The answer is i have no idea as yet of what the optimum offset should be. Because we have a situation where each blade is influenced by the preceeding and following blades. All data to date has been acquired by (a) running a test rig where the rotor is ginbal mounted and has been free to teeter, or (b) FDA carried out by a university for me, where it was treeted as a simple rotating fan. I will eventually have to resort to my usual method which is make it, try it, if it does not work make it again. I have found that using this method the third attempt is usually somewhere near the mark. Thanks Bug.
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This may be a silly suggestion, but instead of worrying about tethering the thing, why not just give it some training wheels?
Make a nice light frame out of relatively thin but sturdy metal, and extend the landing gear by 10 feet in all directions. You are then assured that it cannot tip over unless you are greater than 10 feet from the ground. If anything goes wrong whilst in the hover you can cut the power and it will just sit back down.
Ovciously weight is an issue but if you planned it carefully I think it would be fine.
Make a nice light frame out of relatively thin but sturdy metal, and extend the landing gear by 10 feet in all directions. You are then assured that it cannot tip over unless you are greater than 10 feet from the ground. If anything goes wrong whilst in the hover you can cut the power and it will just sit back down.
Ovciously weight is an issue but if you planned it carefully I think it would be fine.
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Some helpful tips (i hope).
Skittles,
Training wheels - not a bad idea. Will minimise risk of dynamic rollover. Actually looking at some fixed wing gear would help. Some tail-dragger main gear designed for operation on rough strips hinges so that tyre contact patch moves radially about C of G (ie high roll centre suspension). The benefit is that cross-wind landings do not risk rolling the aircraft on landing.
Designing skids to flex about C of G will give the same advantage. Bug seems to have already considered this. I like the curve in the struts too, since this allows compliance in direction of C of G too (as well as a little plastic strain energy absorbtion
).
----
Bug,
If teetering: the control lead angle will be 90 degrees (less delta3 for coning, wee-wa, and/or inflow roll). Actually, i would seriously consider Dave Jackson's unihub design to minimise shaft vibrations, hence fatigue alternating loads, for your ring rotor.
If hingeless: you only need to know the first in plane bending mode frequency of the full disk. If this was checked in FEA with centrifugal accel applied then you can find control lead angle directly from my formula above. If you only know in plane bending frequency when static then a reasonable first estimate may be found using:
Freq-rotating < SQRT( Freq-static^2 + (RPM/60)^2 )
Where Freq-rotating then gets put into the equation for lead angle.
This approach works just as well for a ring-rotor as for individual blades because any fully teetering disk would remain in plane as you move the axis, in just the same way that individual blades do.Thus you can add centrifugal accel and bending hinge moments.
Assuming this hasn't totally befuddled you, i'd be happy to clarify a little more...
Training wheels - not a bad idea. Will minimise risk of dynamic rollover. Actually looking at some fixed wing gear would help. Some tail-dragger main gear designed for operation on rough strips hinges so that tyre contact patch moves radially about C of G (ie high roll centre suspension). The benefit is that cross-wind landings do not risk rolling the aircraft on landing.
Designing skids to flex about C of G will give the same advantage. Bug seems to have already considered this. I like the curve in the struts too, since this allows compliance in direction of C of G too (as well as a little plastic strain energy absorbtion
).----
Bug,
If teetering: the control lead angle will be 90 degrees (less delta3 for coning, wee-wa, and/or inflow roll). Actually, i would seriously consider Dave Jackson's unihub design to minimise shaft vibrations, hence fatigue alternating loads, for your ring rotor.
If hingeless: you only need to know the first in plane bending mode frequency of the full disk. If this was checked in FEA with centrifugal accel applied then you can find control lead angle directly from my formula above. If you only know in plane bending frequency when static then a reasonable first estimate may be found using:
Freq-rotating < SQRT( Freq-static^2 + (RPM/60)^2 )
Where Freq-rotating then gets put into the equation for lead angle.
This approach works just as well for a ring-rotor as for individual blades because any fully teetering disk would remain in plane as you move the axis, in just the same way that individual blades do.Thus you can add centrifugal accel and bending hinge moments.
Assuming this hasn't totally befuddled you, i'd be happy to clarify a little more...
Last edited by Graviman; 20th Jan 2010 at 17:54. Reason: Because it gives me quick break from fatigue calcs...sigh.
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This is one of the most interesting threads on the Net, its giving many people time and room to think about many aspects that we are all involved with, not just in flying Helis but also in the engineering dept also, its the first thing I read when on the net!
Peter R-B
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Peter R-B
Vfr
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Same here, I'm thoroughly enjoying it. I'm a little glad I'm not going to be the one testing the machine, but very much looking forward to seeing this get airborne. This thread makes good reading and is inspiring too!
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all i need now is the balls to try it
Many thanks for you supportive and constructive comments. I should make it clear that i lay no claim to being a qualified pilot or for that matter particularly clever. Having to make your own helicopter stems from having a passion about something and not money to persue it in the normal manner' Regarding the machine, a new sprag clutch is on order and hopefully within the next week or so i will be attempting to get it off the ground. Looking at the Rotorway video i feel tethers are going to be my first choice. Having pulled it light on the skids already i know my brain was buzzing a bit, and i think i may not handle the full monty if anything happened that pushed me past my limits. Thank you. Bug
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All i need is the balls to try it
Just found a few piccies. The one in tethered hover was Bug2. In answer to questions. No ppl, and in the UK i am informed you can test on tethers but you cant do what i am goin to do shortly
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Some constructive thoughts...
Bug,
Just make sure that when the tethers are taught they line up with the C of G.
The reason the rotorway went over is that skids do not allow any compliance (in comparison with oleos). So the skid touches and applies a force that does not pass through the C of G - this is called a torque and clearly overpowers any cyclic input. If you are lucky and dump the collective then the aircraft has not accumulated enough inertia about the longitudinal axis to roll over. Interesting to note that ship based helicopters allow negative pitch to apply a downwards force, which would help recover.
Allow as much compliance as you can in the tethers - bungee chord maybe. The more gently you deccelerate the airframe the lower the forces will be.
Ideally, tether the fuselage and not the skids to minimise effect of angular movement (ie CG moving off thether axis). Hard to achieve and be sure that tether does not foul the skids too (which could be a disaster). This may be why a lot of the pioneers used widely spaced oleos to allow tethers freedom of movement.
I still think you might be better off replacing skids with ball end links and do initial ground runs on a wide base trolley overweighted to overcome any lift the rotor could develope - including transition. Have links forming corners of a pyramid with apex just above CG. Diagonal crosses with bungee chord will overcome any tendancy to fallover. If not a scissor link under the fuselage then use a Watt Z-linkage on tail boom to stop any yaw without affecting pitch and roll. This will let you "fly" the machine to get a feel for its dynamics, albeit slowed down a little, as well as gain confidence in the design.
I'm pretty busy, but would be happy to talk off-line.
Still in Cambridgeshire but don't mind a day trip.
Mart
Just make sure that when the tethers are taught they line up with the C of G.
The reason the rotorway went over is that skids do not allow any compliance (in comparison with oleos). So the skid touches and applies a force that does not pass through the C of G - this is called a torque and clearly overpowers any cyclic input. If you are lucky and dump the collective then the aircraft has not accumulated enough inertia about the longitudinal axis to roll over. Interesting to note that ship based helicopters allow negative pitch to apply a downwards force, which would help recover.
Allow as much compliance as you can in the tethers - bungee chord maybe. The more gently you deccelerate the airframe the lower the forces will be.
Ideally, tether the fuselage and not the skids to minimise effect of angular movement (ie CG moving off thether axis). Hard to achieve and be sure that tether does not foul the skids too (which could be a disaster). This may be why a lot of the pioneers used widely spaced oleos to allow tethers freedom of movement.
I still think you might be better off replacing skids with ball end links and do initial ground runs on a wide base trolley overweighted to overcome any lift the rotor could develope - including transition. Have links forming corners of a pyramid with apex just above CG. Diagonal crosses with bungee chord will overcome any tendancy to fallover. If not a scissor link under the fuselage then use a Watt Z-linkage on tail boom to stop any yaw without affecting pitch and roll. This will let you "fly" the machine to get a feel for its dynamics, albeit slowed down a little, as well as gain confidence in the design.
I'm pretty busy, but would be happy to talk off-line.
Still in Cambridgeshire but don't mind a day trip.
Mart
Last edited by Graviman; 24th Jan 2010 at 19:41. Reason: Just passing though...
