[Keyrigger]

For a lot of new (yes, sometimes used and abused) scale flyers, the concept of mechanically phasing a head does not sink in. No matter how many pictures you take of assembled and working helicopter heads to show, there is always someone that still "Doesn't Get It". I think that there is way too much to look at and decipher in a picture of an actual head so I made up two pictures, one from the top without the rotor head in place but the grips and swash, and the other to show the side view with the angled links from the side, both directions of rotation. There are a few that have chosen to make a counter-rotating head for their first scale bird so I might as well do both directions. There is only one blade and grip shown and the relationship of the ball on the swash is irrespective of the number of blades on the head.

Our modern two blade head systems now have really long arms to bring the ball link on it directly in line with the main shaft so that all you need to do is to move the driver (some heads this is not even needed as the driver and rotor are one piece) so that it puts the link directly in line with the main shaft. With the vast number of newbies going right to flybarless, the companies and forums were getting inundated with questions about the latest technology (more so than any before) and they ended up making the head and driver one piece as not very many pilots are good mechanical techs. If you can't just slap it together and take off, they're lost. So for those and as a reference for us all, here are the two pictures. Take care.

Don

[pittsflyer111b]

Thanks for showing this. I do have one question. What do you do if the blades are trailing edge controlled instead of leading edge controlled as shown in your diagram? Does one just flip the blade grip over and make it LE controlled?

I had a 3 blade head that I could never get to fly right and it was trailing edge controlled.

Thanks
Bob

[Keyrigger]

Most scale heads tend to be leading edge control, not to say that it can't be done the other way, but it does have a lot to do with the amount of power needed to move the trailing edge of the blade. However, you still need to move the ball 90 degrees ahead of the grip which leads to very long links and can cause a lot more flexing of the link which also leads to poorer performance. It also will take a lot more room away from the swash driver and that can be a real hassle. If you still have that head, you could flip the grips and make it work. If you took that head and used it with one of today's adjustable phase gyros (or radios), you could let the links go straight to the swash plate, which would give you trailing edge control (some swear it is softer and easier to handle) and let you run it in clockwise rotation. I think you will be near the maximum correction because of the arm being behind the blade and if it is not enough electronic correction, you could use a slightly pushed forward swash ball in conjunction with the electronic phasing. It can be done. Take care.

Don

[Keyrigger]

Yesterday, I tried an experiment, since my AS350 is down for a repair to the 720, where I used a pure mechanical phasing of the head with the links coming straight down to the swashplate. You really need to wrap your head around this as the electronic location of the servos rotated 120 degrees to the left. The rear elevator became the Pitch, the pitch became the Aileron, and the aileron became the Elevator. The swash driver still needed a bit of adjustment but we are talking a degree or three, not 20 to 30 degrees. The results were that when the main blade was placed over the boom and full elevator was applied via the transmitter, the blade did not move. However, the swash moved down and to the left at about 45 degrees!!.

This is old hat for a large number of long time scale multi-blade flyers but not for the average flyer that started in the last five years or so. The advent of the gyro for the cyclic (aileron and elevator axis to be precise) has rendered this mind boggling method of phasing the head, obsolete. In a growing number of modern full size helicopters, the phasing is now being done via computer just as our modern flybarless systems. The bug may have just been planted in my head to take advantage of the phasing option in the SK720 and set my links up straight in the 500MD TD and AS350 for an even more scale appearance. When I show the swash movement, it WILL turn heads and a few small question mark balloons will appear above them. I actually think this is very cool that the mechanical phasing is very simple to apply if the links on the full size are to be matched. I am looking forward to this and as soon as the weather breaks, I will be out testing in force. By the way, the 369/500 series of helicopters (OH-6 to the MD500N) all use mechanical phasing as described for the vertical link system, and it is still 100% pilot operated with no hydraulic augmentation. I might even have some plans for my Jet Ranger because of this experiment.

In the first picture you can see that the links are almost straight up and down as the geometry of the arms it not perfect for this application. Remember, the servos are not electronically the same as their positions have been changed on the receiver. The grip arms would need to be slightly shorter to let the links be vertical so a bit of correction with the driver is needed to let the blade remain motionless when placed over the boom with full collective added, either direction.

The second picture is the way it is setup to have a mechanical phasing done with the standard layout that most are familiar with now, elevator at the rear, aileron to the left, and pitch to the right. This is how the system looks in reality when you compare it to the drawings I made in the first post in the thread. This can be flown with or without a 3 axis gyro and the blade, when placed over the boom and full elevator is applied, remains motionless. The end result in flying is the same but the path to get there is completely different. Now, have I completely confused you? Take care.

Don

[pittsflyer111b]

How does head speed figure into the phasing? I found that when I was screwing around with the 3 blade head, that at different speeds it reacted differently. What head speed are you shooting for and how do you maintain it consistently?

Thanks
Bob

[Keyrigger]

Phasing is not connected to speed as it is a mechanical alignment of the blades so that when you input a command to the rotor, it goes in the direction you have chosen. If your head is of poor quality, then over-speeding the head may be the issue, not the phasing. Most 600 size heads on multi-blade setups are run around 1300 to 1500 rpm and not much more. The current best method is to use a governor and gear towards the optimum speed for your setup. It can be done without a governor but you absolutely need a tachometer to verify your head speed. You can get close by working out what speed you want, and then adjust your throttle curve to match but it is not the same.

Blade quality and design can be a problem, too. If the blade is prone to flexing, high head speeds will cause fluttering of the tips, leading to a destabilized flight. Blades too thick can cause excessive drag and slow things down a lot. Flat bottomed blades create a large amount of lift requiring less rpm to maintain flight but they are not the best when it comes to efficiency. I have semi-symmetrical blades on two of my helicopters and have a second set each of symmetrical blades to try out to see how they perform in comparison. Good quality modern blades, glass or CF, will work very well in a vast majority of situations but there is always the couple that want to march to a different tune.

I think this is a synopsis of recommended head speeds from memory. Starting with the largest:

2.5m - 750 - 850 rpm
2m - 850 to 950 rpm
1.8m - 950 to 1050 rpm
1.6m - 1050 to 1150 rpm
1.5m - 1150 to 1300 rpm
1.4m - 1300 to 1500 rpm
1.2m - 1450 to 1650 rpm
1m - 1600 to 1800 rpm

Some of those were from a guideline suggested by Vario Helicopters for their models and tends to match what guys actually set their helicopters up to do. I have set my 600 sizes up to 1350 and 1400, both being multi-blade, and my 550 is set to 1450. For smaller helicopters, what is normally done is to try and get a 450 down to 1900 to 2400, depending on the number of blades. 1900 would work great with a 4 or 5 blade head and 2400 would be more for a two blade, like a Jet Ranger. Hope this helps.

Don

[pittsflyer111b]

Thanks for the insight!

Bob

[bensidus197217]

Originally Posted by Keyrigger

Yesterday, I tried an experiment, since my AS350 is down for a repair to the 720, where I used a pure mechanical phasing of the head with the links coming straight down to the swashplate. You really need to wrap your head around this as the electronic location of the servos rotated 120 degrees to the left. The rear elevator became the Pitch, the pitch became the Aileron, and the aileron became the Elevator. The swash driver still needed a bit of adjustment but we are talking a degree or three, not 20 to 30 degrees. The results were that when the main blade was placed over the boom and full elevator was applied via the transmitter, the blade did not move. However, the swash moved down and to the left at about 45 degrees!!.

This is old hat for a large number of long time scale multi-blade flyers but not for the average flyer that started in the last five years or so. The advent of the gyro for the cyclic (aileron and elevator axis to be precise) has rendered this mind boggling method of phasing the head, obsolete. In a growing number of modern full size helicopters, the phasing is now being done via computer just as our modern flybarless systems. The bug may have just been planted in my head to take advantage of the phasing option in the SK720 and set my links up straight in the 500MD TD and AS350 for an even more scale appearance. When I show the swash movement, it WILL turn heads and a few small question mark balloons will appear above them. I actually think this is very cool that the mechanical phasing is very simple to apply if the links on the full size are to be matched. I am looking forward to this and as soon as the weather breaks, I will be out testing in force. By the way, the 369/500 series of helicopters (OH-6 to the MD500N) all use mechanical phasing as described for the vertical link system, and it is still 100% pilot operated with no hydraulic augmentation. I might even have some plans for my Jet Ranger because of this experiment.

In the first picture you can see that the links are almost straight up and down as the geometry of the arms it not perfect for this application. Remember, the servos are not electronically the same as their positions have been changed on the receiver. The grip arms would need to be slightly shorter to let the links be vertical so a bit of correction with the driver is needed to let the blade remain motionless when placed over the boom with full collective added, either direction.

The second picture is the way it is setup to have a mechanical phasing done with the standard layout that most are familiar with now, elevator at the rear, aileron to the left, and pitch to the right. This is how the system looks in reality when you compare it to the drawings I made in the first post in the thread. This can be flown with or without a 3 axis gyro and the blade, when placed over the boom and full elevator is applied, remains motionless. The end result in flying is the same but the path to get there is completely different. Now, have I completely confused you? Take care.

Don

I don't get it, how can the blades not move when you apply full collective in either direction? When you apply collective aren't you moving the swash plate up and down and if that's the case shouldn't the blade pitch angle change? I need help here Don I'm the kind of guy that gets lost on the way to the bathroom.

[Keyrigger]

That is why I did the first and second drawings so you can see how a simple mechanical phasing works. The problem is today, few want to actually dig into the full mechanical operation of a helicopter head so that they understand what is actually happening when they push the stick forward or sideways. What the majority wants is instant gratification so they can plop a 5 blade head on top of their T-Rex in a scale MD500E and fly like they know what they are doing. Now, they may know how to fly the model but have no idea on what is going on, therefore cannot understand what to do when something does get out of place. I have been on a very steep learning curve since the day I got back into helicopters and am making up for the 30 plus years that I have been away from helicopters.

Let's take the MD500 as it is the most popular RC helicopter scale body by a huge margin over the runner up. The actual control linkages are pure mechanical and have no assist for the pilot. What he inputs is mechanically advantaged so very little effort is needed to move the collective in normal flight. There is no gyro to help adjust what the helicopter does compared to the input from the pilot. An F-18 is computer and hydraulic assisted so once again, the pilot uses a joystick like our computer games to fly it. If he looses computer control, time to eject.

Back to the helicopter, you will notice on full size MD500s that the pitch links come straight down to the swash plate. This is where it goes a bit nutty. Like our models, the pitch link attaches to an arm on the blade grip that is out in front of the main body of the grip. It then goes straight down. Now, on our normal flybar helicopter, the swash tilts straight forward when we apply forward stick. That is because the timing for the blade is taken care off by the mechanical system that advances the input 90 degrees to the motion of the blade. Take your flybar heli, turn it on and put the blade straight over the boom. Push forward on the stick and nothing moves on the blade, however, the flybar is tilted to match the swash plate. When you move the blade forward to the position of the flybar, the tilt is still forward, the flybar is now level over the boom, and the blade is angled to match the swash. But, it is 90 degrees to the direction of motion.

So we now remove our flybar and add three more blades because we want to build that 500 with a scale head. What must be maintained is a 90 degree advance input to the blade so that when it is 90 degrees to the direction we are travelling, it is tilted in that direction with the advancing blades angled down as they hit 90 degrees to forward and the retreating blades are angled up at 90 degrees to the direction of travel. So, given that the arms cannot be very long because of the blade in front of it, the grips are short and the link is just in front of the grip and going down to the swash.

Over the boom, that means that the ball on the swash must be in the middle of its range of travel (as if it is a level swash) but we need to make sure that the blade will be fully angled down (amount depends on stick input) at 90 degrees to the direction, therefore, the swash much be angled earlier because of the short arm length. What that means is that the swash will be angled down about 45 degree from the direction of travel. You need to really wrap your head around it and look at the first photograph of my AS350 head. Because there are only three blades, it is easier to see how little the ball on the swash plate is advanced compared to the actual blade. The actual link comes slightly back (depending on point of view) to connect to the ball. I have not finished with this by any means as I have a video planned to show that head setup in operation. The manager of the LHS to me had his jaw on the ground when he saw the actual swash angle for a forward motion on that head. Oh, one more thing about that setup, and I did mention this in the post, the servos are moved in a circle to the left. Elevator is pitch, pitch is aileron, and aileron is elevator. You cannot get that system to work with the standard configuration of a modern model flybar/flybarless helicopter that you buy from Align, Blade, Compass, or SAB. If you tried, it would crash, period.

This stems from many new scale builders wanting to put a multi-blade head on a helicopter they are building but have no mechanical capability. I know you have some as you are building your own chassis so you should be able to sit down and take a look at the pictures, a sheet of paper, and see exactly what I have done. I can change the chassis back to the advanced link in the second photograph in a matter of minutes. That puts the elevator in its original position along with the other servos and then puts the linkage angle forward once again (the way I currently have the SK70 all set up for. I am going to make the links go up and down per full size when I get the gyro back and will have the swash setup per full size, too. Should be a real puzzler when guys notice what is happening when I push forward on the stick. Got to get back to putting the new motor in the 500MD TD, as I am now painting the new chassis cover. Take care.

Don

[bensidus197217]

You lost me at the first paragraph. I understand the 90 degree thing but are you saying the servos of a 450 need to be changed in the order they function? Don if you use too many words I lose track of what is first said, that age and memory thing.