• Jon

How to 3D Print an Aeroplane - Day 11 - Staying Cool

I have built the cooling scoop for the aft engine and also made some extrude cuts in the fwd firewall to allow some cooling air into the fuselage. First, a digression...



In a real piston engine aircraft cooling is of upmost importance, in fact the Cessna Skymaster is infamous due to cooling issues with the rear engine. The stories go that the rear engine would fail on taxi out but since the front engine was still purring along nicely unsuspecting pilots would take off single engine. This was such a problem that Cessna ended up designing a procedure whereby, on takeoff the pilot would advance the aft engine first to check for some thrust followed by the front engine. (Personally I would of thought - Cleared for take-off, Set power, fuel flow: check - would have done the job...)


Piston engine aircraft are generally air cooled unlike their water cooled counterparts on land. The key word in aviation engines is reliability and air cooled engines have little to go wrong with the cooling system itself. They do need careful handling though, especially bigger more powerful engines. Often you will notice cowl flaps. These are doors that open, usually behind the pistons to allow air to flow in from infront of the engine and then out the back. You have them open for ground ops and the climb out, basically high power, low airspeed configurations, and then close them for the cruise when you have good airspeed and want less drag. In the early days of my flying career a flew PA-31 Chieftains with cowl flaps, the difference between open and closed in the cruise was probably 10-12kts.



Cowl flap operation schematic


Open cowl flaps on a B17 radial.


Overheating wasn't the only problem with these big piston engines, shock cooling was just as big of an issue. Imagine the scenario. You have been sitting in the cruise at 10,000ft for two hours. The engines are thermally stabilised, nice and hot, and we have just noticed we are 7 minutes out from our landing field! You chop the power, stuff the nose down and aim for 1500ft/min descent rate, it won't be comfortable for the passengers ears but we'll make it! Unfortunately the temperature of cylinders 1 & 2 at the front of the engine has just gone from running at 450F at 80% power to idle with 160kts of cold air rushing in. CRACK! You have just shock cooled your cylinder heads and landed yourself with a big bill. With these types of engines you needed to anticipate your descent point and slowly start reducing power, if I remember correctly the rule of thumb was 1in/hg of manifold pressure per minute, that's slowly reducing power 10 minutes before starting your descent for a turbocharged engine.


Back to our model. We don't need anything like the cooling of a piston engine but it is worth thinking about, especially if you are building a model that has tightly cowled electrics. The Skymaster has a positively cavernous fuselage so in terms of cooling I don't think we are going to have any problems. I have added some extrude cuts through the fwd firewall, this will allow some air into the cabin but if you want more cooling flow then you need to add an exhaust - the equivalent of having the cowl flaps open. I have made this by making another extrude cut in the rear firewall.


The rear engine air scoop doesn't really serve any purpose on our model except looking cool. I have though about making a cut at the back so air can flow through it, this would reduce the drag it causes but it potentially raises more structural and printing headaches than it is worth. I have also split it into sections for ease of printing, the forward halves attached to the wing joins and the rear part to the firewall.





 
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