Hi

this is cut in my flips new carbon cowl. is this hole in the front big enough to cool my engine sufficiently? and is the hole at the back big enough to let all the hot air out?

Thanks
Ian[8D]

Yes and No. The areas look fine but you may need to make a small baffle in the cowl to direct the air flow up towards the engine. As you have it the air flow is almost straight through and may not cool the engine enough. I have put 2 pics on to show what I mean, pic 1: the RED area I have added whould be the rough size of the baffle. pic 2: A line drawing (very rough) with a side elivation showing you what I mean.

cheers, what do you recon id make it from, just a bit of covered balso or maby some bit of carbon fiber?

Edible, measure the exit hole. Hypothetically, if it is 1 1/2" X 1 1/2" it is 3 square inches. For proper flow the entry hole must be 3 square inches. An alternative to a baffle would be if the entry hole is now bigger than the exit hole, then the increased air flow from the larger entrance now has a harder time escaping and will roll around inside the cowl reaching all areas. Joe

NO !

for optimum cooling the exit area needs to be 3 to 4 times the inlet area, this is something to do with the hot air within the cowl expanding and maybe surface friction within the cowl

The air must flow over the parts of the engine you want to cool, so the baffle is required

Nice looking cowl [8D]

I would say that you will not have suffecient cooling. You must have airflow over the cooling fins. The cut out is too low. Now that it is there, you may have to add a baffle to the cowl to direct the airflow.

Why not be safe and cut out the hole more (up) so as not to take the chance to ruin your motor.

Ultimate, I must dissagree. I'm not saying that a baffle is not a viable alternative, but leaving airplanes out of the equasion for a minute and look at this in a physical manner. The opening dimensions and airflow, are for right now, hypothetical for lack of specifications. An amount of airflow (CFM) dictated by the OPENING size, will flow freely with a matching dimensional EXIT hole. A larger EXIT hole will not increase the volume of airflow due to the ENTRY only allowing so much CFM to pass. By increasing the ENTRY hole to a larger dimension than the EXIT hole it is now a form of baffle where the increased CFM now takes longer to escape and is forced to circulate inside the cowl. Here is a simple analogy: On a large bus traveling down the road. Open one window on the front and open one window on the back. The amount of air passing thru is entering and exiting in an equall flow. Now open two windows on the back and the same amount of air is still passing thru the window on the front. the air now just has an alternate form of exit. The amount coming in is still dictated by the dimensions of the opening. Joe

Youre forgetting something. Your large bus doesnt have a bloody great engine inside heating up all the air that goes through
When air is heated, it expands by a certain amount.
Lets say for example, all air flowing the cowl expands by 25% as a result of absorbing heat away from the engine.
So you effectively have 25% more air that should be coming out the back than the front.........
So unless your exit hole is 25% larger in area than your inlet hole, not all the hot air is going to escape.
follow?

guys c'mon now! no fighting. Actually its a combination divergent duct and bernouli's principle. For the divergent duct this is what you have: Your entrance hole that is smaller then your exit hole. The air passing is through essentially a DIVERGENT duct. In a divergent duct, velocity of a fluid decreases, but pressure increase. so now you have a little "high" pressure in the cowl (actually its not much but enough) This coupled with the pressure drop from the curve of the cowl speeding up the air causes a suction that pulls air through the cowl while in flight.

BTW: This is the way fullsize aircraft do it too! Ask an A&P tech (me)

sean

Cos i this is the correct cut out according to the bits on the original cowl!

It looks bloody awesome! done by the incredible Dr. Jason So light, that and the canopy save over 100g over the original GRP and clear cowl and canopy.

Ill go and make a baffle (if thats wot that bit of balsa is?!?!) and glue it in with a bit of epoxy then ill get some photos of it in the air tommorow if the wethers ok.

Ian

Sticking a piece of balsa would detract from the COOL look of this cowl. If you have a spare bit of fibre I would use that (just for asthetics ).

o well, too late. Done it and it covered red lol. Never mind. Tho im really pi$$ed off cos i mucked up the exaust cut out and its got a big hole there now[:@]

Luke, air will only expand to measurable volumes when it is trapped in a chamber. When flowing it will not have the same effect. It will warm but not expand by a noticable volume. As we all know, air is made up of gasses. Nitrogen, oxygen, argon, neon, helium, krypton, hydrogen, xenon, etc. Gases will increase in volume by roughly 1 percent for every 6 degrees farenheit. So if it is 70 degrees outside and these engines (nitro) reach about 95 degrees then the increase in volume is 4.166. That is trapped air. Moving air expands less by constantly being replenished. Joe

BTW, there is a big engine in the bus and if air expands like you said it will, he better pull over because the hood must be ready to blow off.

I thought the engine is seperate to the passenger compartment?

It is, I'm just pullin your leg. Joe

"By increasing the ENTRY hole to a larger dimension than the EXIT hole it is now a form of baffle where the increased CFM now takes longer to escape and is forced to circulate inside the cowl."




I agree. However, this is undesireable in our planes. The longer the air stays inside the cowl, the hotter it will get. The exit hole should be at least 3 times the size of the entrance hole. This will ensure that the air does NOT dam up and circulate inside the cowl.

Gentlemen, when you increase the exit area, the speed of the inlet air increases. What happens is that when the entrance is the same as the exit, you will reach a particular speed when the pressure inside will increase. This increased resistance keeps the inlet speed down to the exit speed. Increasing the exit area, will decrease the pressure and increase the inlet speed.

I stand corrected. Joe

all the builders around here use 2-3 times the size of entry. we figure it create's a kind of "Venturi" effect. which is an increase of airspeed though the cowl.

Increasing the speed of the air will increase the temp of the air, this application requires cool/cold air to keep the engine within working tolerences. A 1 to 1or1.5 entry to exit ratio will keep your engine within those tolerences set by the manufacturer. In edible_engine's case it is more to do with the directing of the air flow rather than increaseing the air flow.

plain and simple guys 1 in 3 out and thats it - best

So how did it go? Flip fly alright?

yes, all went brilliantly.Apart from the exaust cept comming loose cos i couldent get the nyelock nut onto the exauset bolt so i had to use a standar one which kept on falling off making the exaust come loose and loosing most power, not brilliant in the hover, this eventually lead to a crash, minor but irrotating. Got some good video but my stupid sony ccd-trv27e video com wont let me upload ont my computer[:@]

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