# Hydraulics for dummies.

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#### joe nelson

##### Well-Known Member
I want to drive my aft rotor rpm with a hydraulic motor mounted on the front rotor. The front rotor is driven by the air flowing through it and I want to maintain approximately the same rpm on the rear rotor. This is not to sync them just to keep the rpm approx. the same. Can this idea work?

#### highspeed

##### Well-Known Member
I would think the losses involved with mounting a hydraulic pump to the front rotor and a motor to the rear rotor would make it impractical. Why not link them with drive shafts? Possibly a lighter, simpler solution.

#### orion

##### R.I.P.
Two points - hydraulics are very heavy and inefficient. That inefficiency appears as excess heat that you somehow have to deal with. If the hydraulic systems get too hot, the efficiency drops further. That means extra weight in providing the system with sufficient heat exchangers and ducting.

Then, don't you have to sync your rotors to keep them from coming in contact? A geared/shaft system would be more efficient and might end up lighter.

#### joe nelson

##### Well-Known Member
Highspeed and Orion,

This is my tandem gyroplane project and I was going to use a hydraulic rotor prerotater anyway. Then I thought that with some extra piping I could keep the rpm closer together so I can use a slower approach speed (15 to 30mph). With a gyro of this design the two rotors are separated by 5 feet in heigth so syncho. is not neccessary. This configuration doesn't require gearboxes or all the heavy control linkages needed as in a tandem helicopter. It also has gimballed heads for control not the heavier swash plates making gearboxes impossible to use.

Some single rotor gyros use hydraulic partial power systems that uses a 5 to 10 hp engine to keep some power on the rotor at all times. It vastly improves their performance but it's never been use in this type of installation.

#### orion

##### R.I.P.
Interesting. Have you checked the blade deflection for conditions such as a hard landing? I know several helicopters had to increase mast size in order to clear the tail arm because the blades deflected much more than predicted.

#### joe nelson

##### Well-Known Member
Orion,

The prototype blades are 13 feet long with a 9" cord. The airfoil is a 8H12 and are extremely ridgid. With two three bladed rotors I should have 58.5 sq ft of blade area and loaded to 35# psf (industry norm) yeilds 2047.5 lbs of lift.

The front mast is 8 feet tall and the aft is 13. The front rotor head tilts 15 degrees to the rear, 4 degrees forward and 9 degrees left and right. The aft rotor is fixed at 4 degrees but can be adjusted for trim and it also moves laterally 9 degrees left and right. There is a 4 foot overlap of the two rotors with 15 feet between the two masts if my memory is correct.

#### antero

##### Well-Known Member
I am trying to understand what you are saying..

Because your rear rotor's angle of attack is fixed; when you slow down (increase the front blade's AOA) the front blade absorbs more energy and begins to speed up faster than the rear blade? And what you want to do is take some of that energy and apply it to the rear blade to keep them of similar RPM? Or am i way off?

What are the negative consequences of a delta in rotor RPMs?

If you start harvesting energy from a rotating blade and applying it to another, dont you now need to contend with torque like a helicopter does?

#### joe nelson

##### Well-Known Member
nsheryka,

During my testing of a 10% model I noticed that at high aoa the rear rotor lost rpm. This is the opposite of what I expected. After some research I came to think that this behavior was one of two things. The first was airframe interference where airflow is blocked from flowing up into the aft rotor or wash generated by the front rotor.

As a fix the aft rotor pylon was raised to reflect a 1954 NACA study on this problem. The result was improved rotor rpm at high aoa and in normal flight as well.

Now, when the front rotor pitches up and the rpm increases the aft rotor rpm lags behind but does come in time.

The hydraulics were to do two things ...fix the lag in rpm and prerotation for take off.

The torque question is yes but the rotors turn in opposite directions.

#### wsimpso1

##### Super Moderator
Staff member
Log Member
Generally, the world uses shafts and gearboxes for this sort of thing, despite having to deal with all of the torsional harmonics and resonance issues.

Remember this about rotors in general. The torque that a rotor is carrying (making and reacting back out in autogyros) is in proportion to the rotor speed squared. If one drops to 90% of the other, that one is running at 81% of the torque. To get them in synch will require transferring about half of the difference in their effective torques from one to the other. In our example case, it would have to be in the 10% of total. Not trivial.

Billski

#### joe nelson

##### Well-Known Member
Billski,

That's the nice part of gyros ...no gearboxes! I think is model would fly just fine with no hydraulics at all. I would make the aft rotor head controllable but I still need a pre-rotater. There's not much of a choice, electric, mechanical or hydraulic. With the mechanical I would need three gear boxes plus shafts, universals and couplings. Not excluding the change of rotor heads from gimballed to swash plates. I'm really trying to keep it simple. What would you recommend?

The only advantage in having this hydraulic system is the ability to approach at speeds of 15 to 30 mph instead of 45 to 60 depending on the weight.

#### wsimpso1

##### Super Moderator
Staff member
Log Member
If all it needs the hydraulics for is prerotation, who cares if its efficiency is less than optimal. Hydraulic motors and pumps can pretty light. Check out how much torque, rpm, and thus horsepower you will need to spin up the rotor, and then start checking pump/motor websites for suitable pumps and motors. Being as low weight comes when you run low torque and high speeds, but your rotors are probably only running a couple hundred rpm, you are still probably looking at some kind of reduction unit, perhaps belts and pulleys. You will probably want to de-clutch the hydraulics once flying...

Billski

#### joe nelson

##### Well-Known Member
Billski,

My original thought was to add some extra piping and make it partial power gyro. I was hoping for improved performance but it sounds too complex and heavy. I've have just gotten off two websites that deal in hydraulic; Northern and National Hydaulics requesting a catalogue from each. I'm just wanting to see what is available that might fill the bill.

One thing of interest was a three way valve on one webpage. I guess that's open, by-pass and closed postions...just what I would need if the pumps are engine driven.

#### antero

##### Well-Known Member
What is the gross weight of your design?

When you start dealing with the hydraulic parts you are talking about things get pretty complicated and expensive real fast.

#### joe nelson

##### Well-Known Member
The prototype has an empty weight of 1500# with a gross of 2000#. When the bugs are worked out, a full size gyro will be built with a gross of 4000# and an useful load of 1200 pounds.

At this point, I'm still working with a 10% scale model and will be for at least 3 more months. All looked promising on today's tests until the model was struck by a hard wind gust. Next phase is to repair the front rotor an start mounting the servos for free flight tests.

#### antero

##### Well-Known Member
With an airframe of that size, i think that once you look into it you will find that the kind of hydraulic system you are talking about will be way to much complexity and cost for your mission.

There really aren't any standard "off the shelf" components like pumps, reservoirs, and the like out there on the market and even when you track some down you will probably find out that even a simple engine driven pump will be in the $5-20K price range for aircraft components. You could use industrial hydraulic parts, but then you have to worry about weight and reliability. IMHO, a shaft drive is probably a better solution. #### joe nelson ##### Well-Known Member Most of the prerotaters out in the gyro world are from industrial applications. They turn up rotor up to 32 feet with little effort. The rotor diameter on the final project is 26 feet times two. If a hydraulic failure happens on a gyro, it means nothing more than an extended take off run. With a prerotater that turns enough rpm you may have a take off roll of 50' and without 300'. So it's not a big issue. Today, I was looking on the Northern Hydraulics' website and there are motors for as little as$200.00 each that look like they'd do the job.

I've looked that the shaft drive and it's really heavy. I found two marine gearboxes that were rated at 150hp and they were cheap too. Stripped down each weighed 60 pounds each not including mounts. Then you would need couplings, universials and 18 feet of drive shaft including the associated pillar block bearings. With all of this gear you would still need some way to spin it up!

#### antero

##### Well-Known Member
One more thing i am still missing.. if you are taking torque from the front rotor, and then applying the same torque to the rear rotor (minus losses), will this not introduce a delta torque across the airframe and cause it to yaw? How will you control the airborne vehicle in yaw under that condition?

#### joe nelson

##### Well-Known Member
The aft rotor is fixed in pitch only but it tilts side to side for yaw. I'm seriously thinking about changing the pitch from fixed to adjustable for trimming. This will be done through a trim wheel or maybe a collective type lever. If I go this route the hydraulics will only be needed for prerotation. This could be done by a hand pump and accumulator removing the need for an electric or engine driven pump. Maybe something like the chinook's hydraulic starter for it's APU.

#### antero

##### Well-Known Member
Got it.. interesting design!

#### joe nelson

##### Well-Known Member
I like to keep things simple! Simple = light and inexpensive, lol. Or maybe I'm simple but I did stay at a Holiday Inn Express last night, lol.