A difficulty encountered in trying to arrange for effective ventilation of the cockpit, hot and cold, is finding a suitable place for the air outlet. This must be at a point where the air pressure is reliably lower than that in the cockpit, otherwise reverse flow is possible under some circumstances and poor flow is likely much of the time.
Ferg Kyle describes a solution to this. Essentially it is a device to generate a low pressure area, an airfoil-shaped surface attached to the aircraft skin, with a row of holes drilled in the surface along the line of lowest pressure, through which cockpit air can be extracted. It is similar to one called Extractor and described on a web page which has unfortunately vanished. The author there, apparently from American Affordable Aircraft, makers of the Vision plans-built aeroplane, said of it :-
It didn't take more than a few seconds in the air to notice, not only the flow through the eyeball vent but the added flow coming in from leaks in the canopy frame. The airfoil "extractor" was truly drawing air through the cockpit with some force and the unit was directly upstream from the rudder and caused no noticeable turbulence over the surface.I intend to play with these a lot more in the future including experiments in engine cooling outlets, exhaust augmentation, etc. While there is nothing earth shattering here, these do work.
Ferg writes (the diagrams follow the text),
DEVELOPMENT:
This is not a new device - (however the term "Extractor" may be protected). The purpose is to provide a low-pressure point for the extraction of cockpit air, without creating excessive drag. The Clark Y airfoil was chosen for simplicity and familiarity. The device was developed as follows:
1- Draw a line from A to C on 1/4inch grid paper. Build the profile on this line (which represents zero degrees alpha). The squares are on a grid, the numbers horizontally between the arrows are the horizontal axis; the numbers within the profile give the vertical heights to form the profile. Join points smoothly. This represents the airfoil.
2- Raise the front of A-C mentally by lowering the base line to B-C. This represents an increased alpha angle for greater lift coefficient. B-C now represents the aircraft skin line.
3- Rationalise the form of the device by smoothing out the curve at B and at C. Then ramp the ends down smoothly to form a square.
4 - Because we are interrupting the skin structure, loss of integrity may be reduced by opening up the skin only to the extent of of the size of ducting to it. This is designed for 2 inch diameter SCEET. This latter is used in order to duct cockpit exhaust air past the flap holes.
5- In order that the cockpit be slightly pressurised above ambient air (to discourage engine exhaust incursion, smoke, etc.) there should be an adjustable outlet to 'choke' the exhaust slightly. Maximum flow is only possible when the exhaust is able to 'choke' the total airflow (hot plus cold at maximum). This device (approx 5 x 5 inches) will accomodate about 8 or 10 3/8inch holes so will not cover all exhaust flow. (Duct = about 3 sq. in., = about 27 3/8inch holes, see 'F' above, but ten 3/8inch holes are only a third of the equivalent area.) At D-E in the diagram is the site of a 2inch diam. hole in the skin to match the ducting. Another way might be to drill ten or more 3/8inch holes to retain skin integrity. However accomplished, one should be able to reduce the outflow at D-E to pressurise when reduced inflow is desired (cold day, etc.). I presume to do this by means of a spring-loaded sliding disc.
6- With this (these) in place, one would select the source of cockpit air desired (MIN/MAX, HOT/COLD). Then one would adjust the outflow to just prevent the inflow through the pedestal slots or elsewhere to avoid exhaust fumes.
7- There are inflow sources to avoid engine exhaust flows and fuel vent flows, so nothing sucks inlet air on the bottom. The high pressure ambient air is probably at the bottom of the windscreen and low pressure air is at the top and curve of the forward cockpit roof (this latter will not give a flow to rear of the cockpit of course) so one will have to select a site further behind. I will use Christmas tree "icicles" pinned near each orifice to detect inflow until I have the system adjusted. My machine will have a Carbon Monoxide detector as last resort.
P.S. Don't forget to screen against any little critters ....
Any suggestions or advice gratefully received.
Happy Landings Ferg Kyle #A064
This is a small rendition, to give the idea. To download a full-resolution version (2312 x 904 pixels, 39KB) right-click here
To widen the context a bit, Ferg also wrote
I detect some missunderstanding in regard to pressure distribution outside and inside - the source of air movement unless one puts in a fan. Just like the wing, the cockpit roof produces low pressure where it speeds up the air (top of windscreen), and high pressure where it slows the outside air - bottom of windscreen middle. There are other sources of course, but the former are fairly general. So we should take air from the lower windshield area, and exhaust it from the roof - in general.There is another caveat which has my belief: In order to provide safe ventilation, the exhaust capability should equal the total of cold and warm air volume. As well, the cockpit pressure should exceed the ambient pressure (to prevent infusion of bad air and clearing capability if smoke arises. Admittedly this does not seem to give the same characteristics, but if the exhaust outlet is controlled, then both can be achieved. If you want hot air, close off cold air and open warm, then adjust exhaust for positive pressure. For cold air reverse the sources as before. If you want windshield deforst/demist, blow clearing air up from bottom of windshield and exhaust at roof nearby. If you want fresh air, close roof and open rear exhaust (I will be using an aerodynamic form at back of fuselage).If smoke threatens, open up all inputs, and adjust exhaust for best flow.
and
What I was trying to convey was that there should be two outlets - one to divert air overboard to (a) clean a misty windshield and (b) divert smoke before it reaches the driver's nostrils, and the other to invoke a through-cabin airflow for those who wish to breathe.
Ferg has provided a plot of an NACA duct in a suitable size for the cabin air inlet..
Again, for a full resolution version of this diagram (2304 x 3112 pixels, 208Kb), right-click here. (For more background see, e.g. p.187 of 'The Sportplane Builder' by Tony Bingelis) [Note: The image was scanned at 300dpi to preserve the detail. In order to get an accurately-sized print-out it might be necessary to use a graphics manipulation package to crop the image a little and possibly resize it, so that it prints from your word-processor (or whatever) at the correct scale.]
