Airboat with Horizontal Fan

True, helicopters use rotating wings to get lift, but the net result is a change in momentum of a stream of air, just like a propeller.

The losses from turning flow through an angle twice (the inflow will also get turned as soon as the craft is moving) have also been mentioned before in this thread, when the poor efficiency of such a scheme was briefly mentioned.

If you turn all the flow through 90 degrees at the outlet side, to get a propulsive force in one direction, then you don't also get a lift force in the upwards direction. The direction of momentum of the flow has been redirected from providing lift to providing thrust. This is just Newtonian mechanics at work.

 

well said Jeremy, and no I hadn't read the thread before I jumped in but I have been ocationaly stopping buy to see if the game is still on.

My theory is it won't work for beans but I'm always up for listening to crazy ideas, lord knows I have my own share of them.

Cheers and points for a patient and well constructed explanation
B

eek
won't let me
not sure whats up with that but I tried

 

I am not convinced that there is a two fold change of direction particularly the incoming air flow, a helocopter is not affected by forward motion even at high speed.a boat would be slow in comparison. Cyclones do not colapse and move quite fast. Why have so many craft been designed with horizontal fans, they move forward at speed.
There are many designs being explored ( and computer models) which would direct the air flow against the supporting water and create forward thrust as well.The design shown is not set in concrete.

When air is displaced it is replaced by atmospheric pressure rushing in from all directions. The vaccum and syphon principles are not well understood even after hundreds of years from the proven experements.

Effeciency would come from from good power to weight ratios.

 

Helicopters are most definately effected by forward motion. This is called translational lift. Google it... What happens is that the lifting capability of a helicopter rotor substantially increases once you get some forward speed. The effect is most pronounced at low speed, from 12-16 knots.

You will get a very small amount of lift that is induced by the air near the inlet accelerating. This is often described as the "inlet lip effect". What is happening is the increased velocity of the air near the lip, creates some lift. It isn't much but it is probably what you are seeing in your measurements.

If you think about it you need a lot more vertical lift than you do horizontal thrust. You are trying to lift the boat to near the surface so it's going to take a considerable amount of force. That means either a large area (at a low pressure) or a smaller area at a higher pressure is required. If you research VSTOL aircraft you will find that the larger the rotor (or fan area) the more efficient the lifting is. Air cushion vehicles "cheat" by creating a large suface under the hull and then sealing it. This reduces the amount of pumping work necessary to that equal to amount of leakage out from under the skirt. If there was no leakage it would require no power to perform the lifting.

 

I can`t disagree with what you are saying.the intention is to "take advantage" of as much lift that may be available, not expecting to lift the boat. I appreciate that a large as possible "cone of lift" is desirable, but ducted fans increase efficiency and a big diameter roto is not practical. I understand that a helicopter lift comes from airflow coming down through the top of the roto,not up from underneath as in an autogiro. I don`t think I should use too much of helicopter technology and "transnitional lift" in respect of this design as forward speed in not essential but thrust is. I appreciate you comments and I would love to get the involved airflow clear. there seems to be a lot of contrary opinions.

 

Ducted fans *can* improve efficiency, but only over a relatively small range of working conditions. Open props are generally more efficient over a broad range of working conditions, which is why most commercial hovercraft, for example, use open props for propulsion. Ducted fans, particularly axial flow types, are extremely sensitive to small changes in pressure ratio. Talk to any small leisure hovercraft owner, or better still, go along to a hovercraft race, and you will see and hear first hand the fans stalling, loading up and generally running sub-optimally for much of the time. The small hovercraft people live with this inefficiency and just add more hp to overcome it. They pretty much have do this to because safety and the high pressure ratios they need to run pretty much dictate that they use ducted fans, at least for lift, plus many of the popular race craft use integrated lift/thrust systems. Centrifugal fans are more tolerant of pressure ratio changes, but tend to have lower peak efficiency and are big and heavy when compared to an axial flow fan of the same mass flow rate.

In your application the pressure ratio changes will result from varying inflow and outflow conditions. For example, as forward speed increases there will be a pressure drop at the inlet, due to flow across it, unless you point the inlet forward (I gather this wasn't your intention). Wind will have the same effect. Even modest forward speeds and wind speeds will be comparable to the inflow velocity at a short distance (say around the duct diameter) from the intake, BTW. Adding rudders in the outflow for steering will similarly change the outlet pressure, increasing it as the rudders are turned. You can hear this effect, even on an airboat, as the pressure ratio across the prop changes from the added resistance presented by the rudder as it changes the direction of momentum of the flow.

Helicopter lift comes from the rotors acting pretty much like aircraft wings. Autogyro lift also comes from the rotors acting pretty much like aircraft wings. Both helicopters and autogyros get their lift the same way. The difference between the two is that autogyros always operate in autorotation, with flow through the disc providing the energy to keep the rotors spinning, whereas helicopters can choose to either use power to spin the rotor and get lift or can autorotate just like an autogyro. Forward propulsion in an autogyro comes from a propeller usually, forward propulsion in a helicopter comes from tilting the disc so that the net lift vector is tilted forward slightly.

Whatever way you choose to look at this you will lose energy as you change the direction of the airflow. Even cyclones tip over as they go along, BTW.

If you want to test this idea and find out first hand how these things affect performance, why not make a small model? Model aircraft electric motors and props are fairly cheap and easy to get hold of. A hull should be easy enough to knock up from light ply or balsa. A few model experiments would quickly show whether or not your ideas have merit, particularly if you were to do a comparison test with the same motor driving the hull via a horizontal axis prop.

 

I agree with everything you have nicely pointed out. I have made models using model aircraft motors etc., a long time back even a lawnmower motor version on a surf board and because they worked well considering the difficulties, I am now back at experimenting. Because displaced air is replaced at close to the speed of sound I can not see to much airflow problems with the slow speed I am working with. I want to "see" the airflow patterns using what cheap and simple equipment is available to me.I do have access to very sophisticated commercial gear but it is all expensive and wasteful of a lot of time. I can watch the propagation of radio waves so why not airflow in U/V and other spectrum.

 

Unfortunately this isn't at all correct. Inflow velocity depends on mass flow rate and can be very low because it's pulling from a hemisphere,whereas the outflow will be roughly a tubular jet the diameter of the fan (in fact the flow jet will narrow slightly downstream of the outlet). If you want proof, then hold your hand either side of an ordinary desk top fan. You'll find that you can easily feel the outflow, but barely feel the inflow. The fan or propeller imparts energy to the air flowing through it and this manifests itself as a greater velocity at the outlet than the inlet. Generally speaking, fans and propellers can't create flow velocities greater than low transonic, due to compressibility effects, so it is clear that there is no way that inflow velocity can get anywhere near these speeds. Because fans can't operate above the transonic region supersonic turbojet aircraft need to use inlet ramps/cones at their intakes to slow the supersonic inlet flow down to subsonic speeds to allow the engine compressor fan blades to work.

There are clever photographic and ultrasound doppler flow measurement and visualisation techniques, but they aren't needed to do what you want. You can visualise low speed airflow in a number of cheap and easy ways. In the low speed wind tunnels we used to use atomised vegetable oil (ordinary cooking oil) sprayed from small nozzles (aerosol can type size) to create a very fine mist. This mist is easily visible with good lighting and shows up well on photos, as the oil droplets reflect flash light well. It's also pretty harmless to the environment, so can be used in open flow visualisation systems. For very low speed work (up to maybe 30kts) we would stand inside the tunnel and use a long wand with the nozzle on the end to investigate fine detail areas on a model, a technique that would work OK for your model tests.

What would be useful is a hypodermic pitot probe though, to investigate dynamic pressure distribution. A simple manometer probe, using a tilted water filled U tube hooked up by a thin flexible tube to a large bore hyperdermic needle (we used to use the big bore canula needles used by vets on cattle and horses), with the tip cut square and mounted at the end of a thin stick, would be fine for comparative assessment. One leg of the U tube manometer needs to be tilted over to about 30 deg, because the pressure changes are tiny. Angling the tube allows small vertical displacements to appear as larger horizontal displacements along the tube that are easier to see and measure.

What you'll find pretty quickly is that the flow does pretty much what we've been saying. Inflow velocity will be generally low, outflow velocity will be moderately high and there will be flow velocity losses as the flow turns around any bend. There's no substitute for actually doing this and seeing the results when it comes to really understanding what goes on when air is made to move in particular ways.

 

I should have said that inflow velocities "can" reach speeds close to the speed of sound because I have experimented a lot with engine induction and ram tubes where inflow is virtualy an "implosion". Actually watching air flow through transparent induction tubes is very facinating and trying to measure the ram effect of moving air.Inflow as I understand is not "pulled"in but "pushed" in by atmospheric pressure and that has limitations,whatever the method of acceleration of air particles is. Your suggestions are very good and I am aware of most of them or have tried them for some reason or other and I don`t disagree with most of what you have sensible conveyed.Perhaps I am getting carried away and wanting to see or now to much about these technolagies for such a project.

 

Nope, inflow velocities to a fan or prop don't even get close to supersonic, nowhere near.

What you're looking at on ram tubes is a very high mass flow being forced through a narrow venturi, to the point where the velocity increases massively. This doesn't happen with any fan, under any conditions.

Even with the sort of pulsating flow seen in internal combustion engine intakes the flow rates don't get anywhere near supersonic, although the action of valves opening and closing does create acoustic waves that pass up and down the intake tract and creates some odd intake effects.

Fans and props are just rotating wings and work the same way. There are still endless arguments as to whether wings lift by suction (via Bernoulli effect on the upper surface), or by deflecting air downwards from the lower surface. Both give the same net result, a change in the direction of momentum of the net flow over the wing (or fan/propeller blade).

There are some easy to use simulation programs around that will estimate things like mass flow, velocity, thrust etc for props and fans. Often they need a fair bit of work to get good results from, but some can work well. I still prefer to do tests on large scale models to get confidence in any simulation, though.

 

Well Jeremy I will have to take your word for that and discount all the research of Jaguar,Chrysler and all the other leading research Scientist on carbuation and induction systems. It does not realy matter as the is multitude of horizontal fans performing what I want to do already researched.I will read your post more thoroughly to see where I am missing on the information, I contribute to these forums to learn and keep up to date.
I doubt if this Flying Kiwi will want to go into "Treasitional flight".

WWW.horsepowercalculators.net is a very interesting website.

 

A carburettor or induction ram tube isn't driven by a pressure limited device like a fan. Fans cannot produce very high velocities at their intakes because air flow separates from the upper surface of the blades and causes them to stall and stop working. This happens at a tiny fraction of one atmosphere of pressure.

If you draw air through a pipe with a piston this limitation isn't there, as a piston running in a cylinder is a positive displacement device. As a consequence you can theoretically get close to one atmosphere of pressure differential across an intake or ram pipe on an engine.

 

Thank`s for that info I am afraid I am starting to forget a lot of that information I will have to re-research for all the new studies. There was an interesting item on New Zealand TV news tonight regarding the Kiwi Jet Pack Flyer showing the Jet Pack rising and flying up to 5,000 Feet above the Canterbury Plains South island NZ.

It was flown remotely from a helocopter with a pilot (dummy) and it deployed a parachute and decended sustaining minor damage.

 

A high tech experiment showing a computer fan,(42.5 CFM Static pressure 0.23 inch-H20) pushing air into a plastic bag (airboat air tower) and air exiting out the side of the bag.
Fan and bag are on a springboard allowing measurement of lift created.

Lift shoud come from the fan trying to rise and air pushing venturi up similar to the action of a Kort nozzle on a boat.

 

A concept to make use of lift and thrust for a airboat with horizontal fan.