Hi everyone, I posted this in the RCV Manufacturing forum, but there doesn't seem to be a lot of activity in there so I'll post my question here. I apologize to the RCV folks ahead of time if they see this twice!

I purchased a VQ Macchi 205 and need an engine for it. It has a thin cowl and I'd like to not have to cut it. I natrually gravitated to the RCV engines, but I'm not sure which to put in it.
The Macchi has a 62" wingspan and will come out around 6.5-7 lbs when complete. They recommend a .46 2 stroke or a .70 4 stroke. Which RCV engine would fit the bill? I won't be doing aerobatics with this plane, but I would like power to swing a 3 blade prop.
Any help would be appreciated!
Cheers!
Mazer

You could probably go with the 91CD, 90SP or, at a push, the 60SP. My choice would probably be the 91CD.

My comparison between the series:

SP Series[ul][*] fits in tight cowls[*] cooling can be a challenge (air must be ducted through fins)[*] high torque, low prop revs so needs bigger, more expensive props[*] pulls bigger or multi-blade props[*] to use behind the prop starting you need to reverse your starter and remove spinner cone to install starter probe (or have a second starter)[*] to start conventionally using cone on spinner you need a more powerful starter due to gearing effect (you are spinning it at 2x the revs)
[/ul]

CD Series[ul][*] more conventional looks and size[*] 'normal' revs so normal props[*] cooling is less of a problem[*] conventional starting[*] no 120 size - yet![*] conventional cooling
[/ul]

I hope this helps. [link=http://www.flying-models.org/Videos/Spitfire%20Maiden.wmv]Here is a video of the 91CD pulling my 64 inch Sportsman Aviation Spitfire[/link] around.......

Richard

Thanks Richard!
I guess it will be the 90 size! I will measure the cowl and see if the CD will fit in.....otherwise it will be the SP. That is a great synopsis of the two engine styles!
Cheers!
Mazer

The SP has the smallest cross section and will swing a 3 blade prop... Adequate cooling can be an issue, but there are a couple of threads (a P-51 and Spitfire) where guys are looking at using mini fans for forced air cooling... pretty "cool", eh?

I think I saw one of the threads where someone put a computer fan behind the prop....but that didn't work out...too much drag on the engine. Other than that, I haven't seen any other threads on the subject...but i'll look for them! I hope this cowl has enough areas to cut out (there is an engine scoop for example) so that cooling won't be a big issue....I don't fly my planes at full throttle much so that may help as well.
Thanks for the input Jimmbbo!

Mazer,

Regardless of which engine you use you need to ensure that all the air flows through the cooling fins. The intake of my Spitfire is only about 1.5 sq.in but all the air is ducted over the fins and it is fine.

I built up a balsa frame within the cowl, narrowing down to about 60% the width of the cylinder fins then used stiff cardboard tight against the fins curved around to the widest point of the fins. I then removed it from the cowl, fibreglassed it, removed the plug and finally re-attached the fibreglass duct to the cowl before cutting the cooling intake. It took almost a full day to do but was well worth it.

You could also use a foam plug and the lost foam method of glassing - just don't be tempted to use metal for the duct - especially where it is close to the fins because if it touches you may get metal to metal interference.

Hope this helps.

Richard

Hi Mazer,
I am the one who is experimenting with using a ducted fan in my TF Spit for cooling. There are some photos in my gallery. So far, I am very happy with the way the SP is working out. Here are a couple of things to consider with the selection of the SP. I did have to cut a hole in the side of the cowl for providing clearance for the throttle arm. Also, because of the way the engine mounts, you will most likely have to make some adjustments to your firewall location. If you do, it's best to figure this out early on in your build.

Hi Richard,
I would love to see some photos of your ducting. I am not satisfied with what I currently have and would appreciate looking at solutions others have come up with.

Scott

Richard, very interesting post. I'd like to see pics too. Where is the 1 1/2 sq in hole located, and where and how big is the exit? Jim

Scott,

It's not an easy area to photgraph - the cowl is a uniform dark matt grey inside and photos do not come out well!

The inlet hole is just under the spinner. Exit hole is the enlarged area around the exhaust exit and is probably about 3 times the area of the inlet (it must be at least 2x). The scale exhausts are also opened up on mine.

Although not great photos, you can get an idea from these...





Richard

You betcha! I think the PC cooling fan idea has merit, especially for tightly cowled engines, since the trick in models is the same as that faced by FS airplane designers... baffling the cowl and engine to properly move the air from the inlet over the cooling fins to the exit...

Any cowled air cooling system still needs a "gozinta" and a "gozouta" and a pressure differential from one to the other. Historically, the exit area being larger than the inlet area ensures adequate flow. Using a fan in the system allows you to better control the airflow and ensure a positive pressure differential in the system, providing better air flow for a smaller duct system, if that makes sense....

Cheers!

Jim

I was doing some reading, the exit needs to be 3X larger than the inlet? I think the ducting of the air to the fins is the way to go.....at least....it would seem to be on first impression!

Beautiful Spitfire Richard!!!

Thanks, Mazer OK, I've managed to take some pictures. First off, I was a bit out on the inlet hole size - it is 35mm x 40mm approx. so about 2.25 sq. in and not the 1.5 sq in I said - sorry!

These photos should help visualise it......




You will see that the gap between the duct and head is very small and is 4-sided so the air can only go through the fins. Note the grey area in the first and last photos that is a part of the duct that stops the air escaping over the crankcase.

Hope this helps.

Richard