Light Aircraft Association

Can anyone advise me on suitable dimensions (or maybe provide a pattern) to make the air-scoops that fit over the barrells and heads of a 1600 VW? Ones that I've seen seem to range in volume of air that is being scooped and forced past the rear barrels. Too small = inadequate cooling, too big = lots of drag and uglyness!

Maybe someone could measure the distance between the forward upper head studs and the upper edge of the scoop for me?

Hi Phil

Next time I'm up the airfield I'll measure mine for you.

Cheers

Gary

Phil,

The formula I used for the opening area of the cooling inlets on my Jabiru installation in the BGP1 Biplane was based upon the requirement that the cooling air would rise in temperature by 30 degrees celcius on its passage through the engine cooling fins. Seems like an arbitary figure, but this was based upon the survey of a large number of aircraft using flat four engines, and it has been proven to work for my engine installation and lots of others as well.

The first bit of information you will need is the power output of your engine. If it is a 1600cc VW, I would expect that it will produce about 55 hp. This is equivalent to 41 kW (kW = hp x 0.746).

For a normal internal combustion engine the heat output from the cooling system will be roughly the same as the power output from the crankshaft. Therefore the heat to be disposed of to the cooling air will also be approximately 41 kW.

The next bit of information you will need is the formula kW=V x Cp x TR.
Where kW is the heat output from the engine, V is the volume flow rate in cubic metres per second, Cp is the specific heat of air in kJ per metre cubed, and TD is the temperature rise in the airflow through the engine.

kW is 41kW as noted above.
Cp is 1.21 kJ/metre cubed
TR is 30 degrees celsius, as noted above.

This will allow you to calculate the volume of air required to pass through the engine in cubic metres per second, to limit the air temperature rise to 30 degrees celcius.

The area of the openings in the front of your cowling can now be calculated, assuming that 41 kW will need to be discarded at CLIMB airspeed. Remember the air cannot enter the cowling openings at a velocity above the speed at which you are moving through the air. The volume flow of air entering your cowlings will be Area x Velocity, where the area is in metres squared and velocity is in metres per second. (To convert from MPH to metres per second multiply mph x 0.447, or knots to metres per second multiply knots x 0.514).

So to calculate the air volume flow at your climb speed of say 50 kts (50 x 0.514 = 25.7 m/s) your cooling air volume flow will be given by 41kW= V x 1.21 x 30. Therefore V= 1.13 metres cubed per second.

The next formula you will need is Volume flow = Velocity x Area of intake.

The intake area will therefore be, 1.13 metres cubed per second, divided by 25.7 metres per second, = 0.044 metres squared = 68 square inches. This is equivalent to two openings each 5.8 inches x 5.8 inches or roughly 6 inches by 5.5 inches.

Voila !

In order for this to be effective you will need to ensure that the OUTLET area from the base of the cowling is AT LEAST 1.5 times the intake area. Also make sure that the engine is well provided with cooling air baffles to ensure that no air is allowed to by pass the cooling fins wasting its cooling potential.

Hope this helps.

If you need more please send a PM.

Kind Regards

Barry Plumb

Once again this site shows me just how much I don't know.
Sometimes it's more like I don't know how much I don't know.

Never knew there were calculations for this sort of thing, although with hindsight it sems obvious that there ought to be.

Good point, well made, about the outlets being of sufficient size. I had to explain to my partner in crime about that one to stop him blocking up the gaps in the back of the lower cowlings! In the absence of mathematics on my side I had to settle for the explanation that you can only keep shoving air into the front of the cowlings if there's somewhere for it to go out the back!

Thanks Barry.

Barry,
Do the calculations take into account the amount of heat that is dissipated via the exhaust syatem. A fair bit of that 41kw must get fired out the pipes.

Bill,

Generally speaking most internal combustion engines will convert around one third of the energy released by combustion of the fuel to mechanical work, (or useful power output), about one third comes out as heat through the cooling system and one third comes out as red hot exhaust gas.

The calculation I showed simply uses the power output as a direct measure of the rate that heat is rejected to the cooling air supply. Since each is one third of the heat released from the fuel, and we know what the rated power output is, then we also know what the heat output to the air will be.

Of course the hot exhaust pipework inside the cowling will also transfer some heat to the air inside the cowling, but actually this does not matter since most installations have the exhaust pipes below the engine, and the cooling air has already done its work to cool the engine before it gets to the hot exhaust pipes.

The 30 degree Celcuis temperature rise mentioned in the calculation is through the engine cooling fins. If the air is further heated by the exhaust inside the cowling before exit then it will not actually affect the cooling of the engine its self.

One area that is worth looking at though is the location of the gascollator. If this is in an area where air passing over it could easily be up to 70 Deg C in summer then this could cause fuel vapour locks. A separately cooled compartment for the gascollator, or an air blast tube from the air intake plenum may be required to prevent problems.

Kind Regards, Barry Plumb

I've never seen a cracked VW head that wasn't cracked in the fin area outboard by the exhaust port (ie. where the big head bolt sits)
IIRC, the best bet is to get air to go through the vertical finned bits right up close to the rocker cover.
http://s82.photobucket.com/albums/j279/ ... olda-1.jpg
[plan "A" to warm the manifold, but it also shows large plenum to slow air and duct it round the barrels. The top sheet of black alloy can be bent down and re-rivetted to reduce the cooling air intake size]
VW has a nice set of "super-cooling tin" that goes between the barrels and above the rocker tubes (ie. before tubes go in) to duct the air round the barrels for best cooling. Careful use of a plate can create a low pressure area behind the front lower section to draw air out from below the cylinders, thus helping airflow.
Not sure if this helps much!!

Thanks for all the info, I've completed my airscoops by a bit of a lower tech method! I had a look at lots of photo's of various VW instalations that I have accumulated over the past few years and made them sort of the same!

Heres a picture : http://s21.photobucket.com/albums/b278/ ... C00270.jpg

The opening at the front stands about 3 1/2" tall and is 5 3/4" wide which equals about 20 square inches each side. The rear portion forms around the back of the barrells to within about 3/8" clearance. I can only assume this will force most of the air past the fins rather than letting a boundary layer establish in contact and then the rest of the air slipping past on top without doing any usefull work. My inspector's not seen them yet so I'm not sure if they will be up to the job...

I planning to have a sort of radial type cowling with a large spinner, with the cylenders poking out of the sides. A bit like a Sea Fury or Chris Lodge's Taylor Monoplane G-AYSH...

Rob,

You are absolutely correct about the VW tinware, it is very good to promote maximum use of the available air flow. You are also correct that the cooling duct should be taken right up to the top of the heads. VW put all of those cooling fins there for a purpose so dont waste them.

Phil. Your ducts do not go to the top of the cylinder heads and this wastes all of the cooling surface around the valves etc. Also I know that is is common to see the ducts reduce in height towards the rear, but actually this is bad practice. It is desirable to allow the air to slow down as it enters the plenum chamber. Remember the Bernoulis theorem that enables your wing to produce lift. By slowing the air down its static pressure will increase, giving it a greater potential to push down through the cooling fins, so ideally a straight or expanding plenum is better than a narrowing one. I realise that you have to avoid the intake manifold but keep the volume of the duct as large as possible to the rear.
A lot will depend upon the climb speed of your aircraft and how hard you need to work the engine in flight. With slow draggy aircraft like my biplane or a Fred the engine cooling needs all the help it can get, but with a faster aircraft like your Taylor Mono you will get away with a few more compromises.

Kind Regards

Barry Plumb

http://www.jbugs.com/store/merchant.mvc ... y_Code=136

Shows a good source of ready-made panels that you can adapt. Very cheap, too. Single-port head is easier to cowl with homemade top tinware.

I'll try to get a better shot of mine during the week.

Bugger. They say that you've not built an aircraft untill you've built it twice, looks like I might have to rethink the airscoops then! In my defence, my old monoplane had a fully enclosed cowling but not a typical pressure type like you find on most factory aircraft. It used a similar set up to the one I've just made, but hidden inside the cowl. If I remember correctly, the head temperatures were never an issue but the oil temp would be the first to get on the high side on a hot day. Typical climb speed would be 50kts and cruise would be about 75kts at 3000 rpm.

The top of the heads in my set up are not covered by the scoops but they are hanging out in the breeze, maybe this kind of cooling by ozmosis might just work for me like it does for everyone else!

Welshman said "...the area of the opening for the cooling air ducts can be decreased quite dramatically if the edges are curved inward".

VW found that out on their air-cooled estate car (Variant) back in the sixties - they changed the leading edge of the air-intake louvre slot to an inward curve, and found the airflow was inwards even without the engine fan operating.

I guess the airflow tries to follow the surface, like an aerofoil.

There was a photograph and comment about the formed cooling inlets, to prevent disturbed flow, in a past issue of the PFA magazine.

When looking at Beetle etc bits don't lose sight of the fact that the engine is mounted backwards on the car relative to its proper use in aircraft.

I'm sure Great Plains used to do cooling tinwork, but I can't seem to find any mention of it now. I'll check again at home.