View Full Version : Adjustable splitter

nickyd - n/a
31-May-06, 07:55 AM

I have a question about adjustable splitters, hopefully I can explain the question so it can be understood......

If I want to modify my existing splitter to make it adjustable is it OK to literally chop it along the back and hinge it so that to close the lift gap the front of the plate drops down from the horizontal? Or should I lower the whole splitter so that the front has to come up from the horizontal to increase lift?

What I am concerned about is the thrust air when in low lift mode. If I do it the first way then the thrust air has to travel up the splitter before it can exit the duct. The other way, the thrust air can travel straight over the splitter plate and out of the duct, and there is effectively a bigger duct area with no restriction.

Have attached a pic to try and explain this a bit better. Hope someone can understand it and give me some advice!




srn4 - n/a
31-May-06, 09:26 AM
Mechanically the simplest solution is the one on the right...That is pretty much what I have done on my craft and it seems to work ok.

Use a cable from below to "hold it down" rather than a cable above trying to push it down!


tonybroad - n/a
31-May-06, 09:42 AM
another issue is your splitter should 'fail safe' so if the cable or whatever is controlling it snaps, the splitter goes to it's highest position for safety

i'd go for the design on the right


RHudson - n/a
31-May-06, 09:42 AM
You will find that most racing craft have the version on the left which means that whenever the splitter is lowered the air has a much smoother flow out of the duct, thus increasing thrust whilst creating the 'turbo' effect that the splitter produces ! http://hovercraft.org.uk/images/icons/smiley_icons/icon_smile.gif


Ian Brooks - n/a
31-May-06, 06:44 PM

The answer to this question comes from the aerodynamics.

Version on right:

This is a divergeant duct - the flow has to slow down as it moves down the duct. This is hard - if the angle of the splitter exceeds about 8 or 10 degrees, the flow will break away from the surface and resulting in turbulence losses. This is bad!

If the angle is kept below 8 degrees, this duct will give better thrust at low speed.

Version on left:

The duct is converging and therefore the flow is accelerating - this is easy. You will get away with a 25 or 30 degree angle without introducing too much loss.

In addition, since the exit flow is faster, this duct will give greater thrust at high speed. (but this will be a marginal effect unless you are going really fast!)

Fail safe:

Both designs will be subject to the plenum pressure - which is the cushion pressure plus a bit of duct loss. In many craft this will be in the region 15 lbf/ft^2, multiply by the number of square feet and this is the upforce, and this will be acting to increase the splitter size.

With a little ingenuity and a spring, this force could be used to create an automatically varying splitter - at low cushion flow the splitter would raise to increase lift; at high flow the opposite would happen. I could provide a sketch if anyone fancies having a go!



Paul Fitz - n/a
31-May-06, 10:38 PM
Nick, while agreeing with Ian's answer I think it is worth considering the following observations.

RH Version.

Although this *is* effectively a divergent duct, the angle involved cannot be determined by the normal criteria because the splitter plane inevitably induces turbulence regardless of whether it moves or not. It is probably more relevant to consider that in most cases if the splitter is raised to improve lift volume, lower thrust is required anyway. But see the note below.

LH version

Converging the air in this manner will increase the loading on the fan and effectively lower its efficiency, although the net result will be a higher thrust and potentially a slightly higher maximum potential speed.


Although the plenum pressure will tend to lift the splitter, with a long (deep) splitter plate as shown in the diagrams, this effect WILL be overcome by the pressure of thrust air impinging on the upper surface. The result will be, that you require considerable force to lift the splitter at full thrust. The same is true for downward movement at large angles of splitter movement.

Note: Because of the above it will be beneficial to estimate what the required 'normal position' will be and minimise the movement needed in both directions for typical 'in flight' adjustment. An additional upward movement can be accommodated if higher volumes are required for manoeuvring at low speeds. This approach also minimises the change in fan/splitter gap. You will need to design some mechanical advantage into the control to overcome the thrust air pressure even with these measures.

Ian Brooks - n/a
1-Jun-06, 04:41 PM

Although the plenum pressure will tend to lift the splitter, with a long (deep) splitter plate as shown in the diagrams, this effect WILL be overcome by the pressure of thrust air impinging on the upper surface.

Not sure I entirely agree with this - taking my Osprey-503 as an example:

At the design lift condition, the craft plenum pressure is about 18 lbf/ft^2 (Cushion is about 12 lbf/ft^2).

The total pressure of the fan flow above the splitter plate is about 12 lbf/ft^2 - so not enough to cause a downforce, even if fully recovered. To generate more pressure than this, there would have to be a significant restriction to the fan flow above the splitter - so I would suggest that for a 25 or 30 degree splitter angle the net force will indeed be upwards.

However, I do agree that there will be an angle at which the splitter force does change sign - but I suggest it will be rather more acute than 30 degrees.