Hydraulic Hijinx

What is the prime mover that will power the hydraulic pump? Is there a reason that this prime mover could not drive the fan directly?

Thanks for your interest, Tornado. The prime mover is a light weight, internal combustion engine of automotive origin displacing 2 liters, developing approximately 100 brake horsepower at 5,000 rpm and about 80 lb./ft. of torque in the 3000-5000 rpm range. Alternative methods do exist for powering the fan directly from the prime mover; the "direct power" method is, in fact, more commonly used than hydraulics in single-engine applications.

Although the direct power method is certainly highly efficient from a power transfer perspective, I believe its use in the h-cart application is more a function of the dearth of knowledge about hydraulics among non-engineer constructors rather than the superiority of direct power in this particular application. Optimal design and balance considerations require that propulsion and lift components, which do not spin in the same plane, be placed at opposite ends of a vehicle which, of necessity, is at least 12 ft long and must be very light in weight, subsequently lacking somewhat in torsional rigidity. It is also important to maintain the lift rpm fairly constant while at the same time varying the prime mover rpm so as to vary propulsion.

These challenging parameters have caused the vast majority of hobbyist builders to incorporate two prime movers into the design, despite the attendant weight, drag and balance penalties of placing an additional 80lb engine at the very nose of the vehicle. The only direct power method which has seen common acceptance in the relatively few single-engine vehicles has been the use of a v-e-r-y long v-belt stretching from the very back of the vehicle to the very front, guided so as to make the transitions necessary to power the front fan, which spins in a plane about 45-60 deg. back from the vertical plane at which the propulsion fan spins. A variable-diameter pulley driven by the prime mover is used to vary the rpm of the front fan.

The tortuous path that this belt must take, and the constant tensioning and fiddling required to prevent slippage and jumped belts, have convinced me to seek the hydraulic alternative. Any advice is appreciated, especially as relates to selection of a light weight, variable displacement pump. Thanks all for listening.

Good points about the nonparallel axes, and the need for one to be (near) constant speed while the other varies.

The proportions of power going to lift versus propulsion will vary according to vehicle speed. I am curious as to what they are under the various conditions.

Variable displacement pumps can be of either piston (e.g., Sperry/Vickers) or rotary vane design (e.g., Continental Hydraulics), and maybe others. Those are just examples. Your selection criteria would include rpm limitations and weight, and the possibility of "stacking" two on the same shaft. Or would the engine drive one load directly, with just one hydraulic pump for the other?

Thanks for that information about the variable displacement pumps, Tornado. I am not very familiar with the rotary vane type other than an eccentric housing air pump I had a problem with once. I'm a little more so with the piston type and swash plates, but will try to learn more about the rotary. Are they perhaps lighter, cheaper, more flexible or efficient?

I'm also familiar with piggy-backing pumps, but as you have correctly discerned, I plan to mechanically drive the propulsion off the back of the crankshaft, even perhaps employing the readily available automotive clutch (depending on flywheel weight penalty) while belt driving the pump off the crankshaft snout as one would an air conditioning pump (except without the weight penalty associated with an unnecessary electromagnetic clutch).

And so, I have a good deal of flexibility in picking the belt pulley ratio so as to keep the pump in its efficiency sweet spot at cruise rpm while not over-speeding it at fan or motor redline. As a starting point, I'm considering 3000 cruise rpm for the prime mover, which puts it well into its torque sweet spot and meets propulsion fan operating requirements; drive ratio for propulsion being 1:1. Redline is contemplated to be 5000 rpm where hp peaks, assuming that prop redline doesn't occur first and that the prime mover has the "suds" to spin it that fast. If these assumptions hold true, and assuming a pump redline of 3000 rpm, my thinking is to under drive the pump with a pulley ratio of 5:3. On the other hand, maybe the pump will have an intermittent over-speed capability and allow more flexibility.

As you can see, there are a lot of assumptions and best guesses here despite my referring to available prime mover and fan power curves and pump efficiency maps. This likely will require a "dial in," which is the reason I was hoping to spend money only once on the more expensive variable-displacement pump component while gambling more on a cheap gear motor. As far as pump weight goes, 20-30 lbs would likely be do-able, although less is always more in this application. Any ideas as to a suitable candidate?

As you also note, the power split between lift and propulsion will vary according to vehicle speed. I am no aerodynamicist, but from what I've learned regarding the h-cart design I expect to use, is that the lift power requirement will fall off as forward speed increases. This conclusion relies only on anecdotal evidence from the seat-of-the-pants experience of builders as nobody is trying to actually measure power draws, but I consider it reasonably reliable based on the small frontal area (pointy nose) and short skirt height (yikes- not off topic!), and the angle of attack of the lift fan duct (closer to vertical than horizontal) thus benefitting from a "ram air" effect. My guess as to power split between thrust/lift is 80/20 static and 90/10 at speed.

The variable-volume constant-pressure rotary type has the vaned rotor in an oval cylinder. A pressure feedback line or other control device moves the axis of the rotor either toward or away from the center of the oval. At center, zero volume is pumped. With the rotor at one end of the oval, volume is maximum. I don't know for sure, but I suspect the rotary vane type is lighter than the piston type; fewer (and smaller) moving parts.

If the power distribution is close to your estimates, the idea of direct drive to the propulsion fan, with a relatively small PTO to the pump, makes a lot of sense.

How do plan to brake the forward motion? Variable-pitch prop maybe? Some air-boat ideas might be helpful.

The variable displacement rotary pump sounds promising; will investigate further. Anyone out there know more about this type hydraulic pump?

Thanks for your assessment, Tornado. Given that relatively low power is directed up front, and power demands up front are further reduced with forward speed, it seems like a relatively small, lightweight hydraulic motor will work.

In my research, I just ran across an example (the first one I've ever seen!) of a very simple application involving a small vehicle which either runs "all out" or not at all (a racer). Given the full speed operating characteristics, they do not use a variable displacement pump, but just a simple gear pump and gear motor sized to go all out. Clearly, the efficiency of such a system, in my part-throttle cruising operation mode, would drop precipitously with a lot of unnecessary heat created. But it is interesting from a sizing standpoint.

The gear pump displaces 0.488ci and gear motor displaces 0.366ci. Both are rated to operate at 3500rpm / 3930psi maximum, continuously, and 4000rpm / 4230psi peak, intermittently. Operation is said to draw / develop 8-10 hp. I will attempt to attach a schematic, and welcome all thoughts on the circuit design, sizing, etc.

I have tried saving or exporting AutoCAD drawing files in various ways, but haven't yet found the combination that lets me import the image into these postings. Your schematic is very clear; maybe you can tell me what I have been missing.

The check valves accommodate expansion/contraction of the fluid without needing a tank. Neat idea.

Yeah, I also was impressed by the design these folks used; very simple and apparently used successfully in 2 demanding racing craft applications. It's super light weight too; the pump and motor don't weigh much more than about 5-6 lbs each.

I was hoping to use a similar closed-loop system for simplicity and for doing without the weight penalty of a reservoir and the added oil. I also understand that a variable pump in a closed-loop system can incorporate feedback devices between pump and motor, and thus respond automatically to prime mover rpm variations and variable load conditions at the hydraulic motor end. Man, this would be the holy grail(!) but right now I'm just trying to get to the point where I can manually control a reliable variable lift system.

Tornado, thanks for the kudos on the schematic, although I must confess that I'm probably way behind your abilities re use of AutoCAD and similar drawing programs. I really don't have that kind of software, not even the free trial versions. My low tech approach involves using the Windows accessory program "Paint" and I keep BMP (bitmap) files with schematic symbols I have imported from the excellent reference books online. (Gotta admit though that even Paint kicks the heck out of the old way of doing it; remember the old T-square?)

Cutting to the chase, I drew the schematic in Paint's BMP format and when I finished it, I then saved it as a JPEG file. Then, after writing my reply to you, the menu allowed me to browse my files to find the JPEG file and attach it, and easy as pie it just popped up below the text. I think JPEG may be the answer because it is so universally accepted. Don't know if you have the option of saving a proprietary Autocad formatted file as a JPEG. Take a look at your "save as" options. Hope this helps.

I love hydraulics, but they are expensive as hell. For a h-craft, look at the configuration for the vtol jets. The motor and fan are horizontal, and ducts do the rest. It's easy to turn air 90 degrees with a duct, and simple to direct it elsewhere.

This is an interesting variation. Which of the two (lift, propulsion) would rely more on the static pressure capabilities of the fan, versus sheer volume? With hydraulics, there will be the weight of pump, motor, tank, fluid, hoses, and maybe a heat exchanger. But louvers for directing air flow might well be lighter. They would probably need to be symmetrical, lest the fan alternate between different pressures and self-destruct from vibration. Annular region maybe? (I'm really speculating here.)

Don't forget the torque the rotating fan will create. If there is no balancing force, your platform will rotate in the opposite direction as the fan (known as yaw). Using vanes to redirect the air, allowing the fan axis to be horizontal will eliminate this. If it counter-rotates relative to the propulsion fan, it will improve roll stability also.

here is a good one

http://www.youtube.com/watch?v=4vnpTqEZOWQ&feature=related

!!! Lots of fun, but wear earphones. A 30-knot side wind might affect the maneuverability, especially if gusting. Not so bad on open water (or open land), but rather dicey on a highway.

A 30-knot side wind might affect the maneuverability, especially if gusting. Not so bad on open water (or open land), but rather dicey on a highway.

That's what killed Curtis-Wright's attempt with the Aerocar back in the late 1950's. I saw their unit on display in the Fall of 1959 when I was a freshman at Notre Dame in South Bend.