Hydraulics for dummies!

joe nelson

Senior Member
Joined
Jan 20, 2007
Messages
1,297
Location
cincinnati,ohio
Aircraft
I have owned an AA-7B, Quad City Challenger, WAR P-47 and have flown several other type
Total Flight Time
12000hrs approx
I'm thinking about driving my prerotater by hydaulics and a small engine. Would partial power be worth the effort.:noidea:
 
If you're talking about using a small goped engine I would advise against it, hydraulic prerotators are only about 50% efficient and you would be wasting half of the engine power. An all mechanical drive system is the best way to go with a small engine prerotator, why cheat yourself out of those extra rotor rpms.

Weight wise an engine/hydraulic setup is going to likely be more than an engine/telescoping shaft drive.

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Thanks Alan,

I was wondering if this type of set-up would work. I was thinking that the hydraulic system would be easier to install but you a right about the efficency. I was doing some breakfast napkin engineering yesterday and wanted some oppinions...Thanks!
 
Hydraulics for Dummies

Hydraulics for Dummies

Hi there Joe,

To get any horse power out of a weed wacker motor, it needs to be at around 8000 - 10,000 RPM. At this RPM they develop abt 3 HP.
The old Mac 101 Go-Kart engine produced around 15HP at 10,000RPM hence the large ratio chain drive.

A miss conception with hydraulics is that you need pressure as the main component. This is wrong. You need FLOW.
Once you know how much oil you need to move around, you then need to know how much power will be required to get the job done.

You will also have to choose a high pressure or low pressure. Low = 1500psi
High = 2500psi. There are other pressure ranges, but for a fixed system, like the one we are talking about, these will do.

So, to find the power required to push the oil around AT the FLOW we need, we use this formula:
hp = gpm x psi x .000583
As you can see in this relationship that an increase in either pressure or FLOW will increase the horsepower.

The GPM (gallons per minute) will be decided by the driven MOTOR, at the rotor head. Then you need a PUMP to provide the FLOW required to turn your MOTOR at the speed you want.

There are many combinations of MOTOR and PUMP available. The biggest stumbling block I see with a GoPed motor is the RPM it has to turn at to develop the house power required.
On a shaft drive system this can be accommodated with a chain drive reduction and connected to the shaft etc.
The same woud apply to a hydraulic set up. A small redrive would be needed to reduce the output RPM to about 1000RPM, like around 10:1.

This ends section 1 of this primer.
 
Bless you Jim,

That was spot on...just what I needed. Many thanks!
 
Any part of a prerotator sysetm that heats up shows a power loss, power which could be delivered to the rotor. In my experience with mechanically belted and geared goped prerotators the parts just don't heat up showing low friction loss.

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You may want to have a look at Mike G's comments at this thread:

http://www.rotaryforum.com/forum/showthread.php?t=21841&page=47


Also, the goped engine of choice for a prerotator (at least the mechanical ones) is the GP460:
Engine Stats: Displacement: 45.7cc. Power: 4.2+HP @11,000 RPM, 2.35 lb/ft torque @8,000 RPM. Fuel: 91 octane or higher mixed 25:1 with 2-stroke oil.

http://www.davesmotors.com/s.nl/c.885035/n.1/it.A/id.4506/.f

Run on a tuned pipe you can get around 5HP.

Note the low torque value and what it would be if you were trying to drive a hydraulic pump at it's required efficiency speed.

Anyhow, just some info to ponder.
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Alan,

Your help is much appreciated! This is for a small project that I'm playing with for a gyroglider to train and to test rotor blades. My tandem is taking longer than I had anticipated and I have the erge to aviate before I die, lol.
 
Hydraulics for dummies

Hydraulics for dummies

Joe,

If you are doing it yourself, I would suggest using a belt rather than a chain. At the high RPM you are looking at the chain will wear quickly without lots of lubrication and the associated mess, unless you enclose it.

I started with a bicycle chain and it worked fine as long as I lubed it every time. That got messy, so I switched to a direct drive with a big hyd motor. Lots of torque but heavy, about 15 lbs. However on my machine it's easier than a shaft and more powerful than most of the electric units.

I have used a hydraulic system on my craft for several years. If you are planning on using it repeatedly for practice on short fields you should consider an oil cooler. J C Whitney has several for less than $100. If you fly for more than 15-20 minutes between uses you should be OK without the cooler.

Larry
 
Thanks Larry,

This is exactly the information that I need. I'm just drawing it up right now and I need plenty of input!

What type of belt? Vee or cogged, single or double sheve?
 
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Belt

Belt

Joe,
It depends on your preferences and local supply house stock. If you have an auto parts store nearby that you are happy with, I would use an automotive type micro v belt. These are the belts you see on most newer cars, about 1/4" thick with a various number of mini grooves.

They are very flexible and can go around a small pulley allowing you to get a wide range of ratios. I don't know how much your rotors weigh but I would think that a 3 or 4 groove would be more than you need for any prerotator. I use a 4 groove to drive the hydraulic pump and alternator. My rotor blades weigh about 70 lbs.

If you don't have a trusted auto supply house nearby, there are industrial versions of the belt. The industrial versions are different profiles and are not interchangeable with the automotive profile. Automotive are "K" profile, while the industrial profiles are "J", "L" & "M". The L & M profiles are too heavy and are not suitable for high rpm use.

If you want to use a traditional v-belt, I would think probably a B or maybe a C profile would be suitable. The best choice would be dependent on the HP and rpm you are dealing with.

Larry
 
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Thanks Larry,

I have had very limited experience with hydraulics. In my life I've worked around stuff that was hydraulic but never took the time to investigate it. Just in the last few days, with the help from you and others on the forum, I am finding it very interesting.

Have you heard of National Hydraulics? I was looking at one of their old catalogs and they seem to have much of the things I'll need.
 
National hydraulics

National hydraulics

Joe,

I haven't had any dealings with them. Two that I have found most helpful with reasonable prices and wide selection online are Surplus Center (Lincoln, NE) and Applied (nationwide branches).

Old catalogs from twenty years ago have a wealth of information in them.


Larry
 
Thanks Larry, I'll look them up on the web to maybe get a catalog or two.
 
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