Here is a list of all the postings Tim Flyer has made in our forums. Click on a thread name to jump to the thread.
|Thread: Cowl modification|
Edited By Tim Flyer on 10/03/2020 09:14:58
Edited By Tim Flyer on 10/03/2020 09:24:01
It’s moved from the small area in the front to big open area in the rear...high pressure into a low pressure zone. I guess on some models the rear cowl is tight so the air velocity won’t change much. There are loads of variables but I’m 100% certain that this set up will work. I was first shown how to use extractor venting on fast IC boats in the 70s and it certainly works 😉.
Edited By Tim Flyer on 10/03/2020 09:04:06
Edited By Tim Flyer on 10/03/2020 09:08:15
Edited By Tim Flyer on 10/03/2020 09:08:34
The airflow in the rear of the cowl is much slower
Outside is much much better! The whole idea is the fast flowing unrestricted external flowing air over the spoiler creates a low pressure area behind the rear lip. That’s why a rear facing spoiler can be called an extractor. A front facing one is a scoop. If I put it inside it would be pointless as the already restricted (by baffles) outward air flow is much slower and does not need directing to the exit. A hole in the bottom of a cowl with no spoiler at all is also not so great as depending on shape and cowl geometry air can actually be forced in or very poorly vented. The idea of rear facing extractors is to point exiting air in line with the air stream then it’s actually “sucked out “. I also use a cowl extractor on my little Acrowot on the bottom of the cowl . It works very well.
Edited By Tim Flyer on 10/03/2020 08:43:50
Edited By Tim Flyer on 10/03/2020 08:47:52
Edited By Tim Flyer on 10/03/2020 08:55:41
Not sure about aerodynamic efficiency as when I’ve looked inside model jets there is often no intake ducting to the turbine....I’m Still not sure why?
Edited By Tim Flyer on 10/03/2020 07:24:36
Thank you Ash . Whilst balsa is a great material for the flat bits the mesh allows very easy production of compound curves etc . It’s easy to snip with tin snips too. I just tacked it in with a few blobs I’d glue ( plasticine or blue tack could be used to help) and then covered it with matting and filed it with my perma-grit file afterwards to tidy it up . Smooth curves and shapes can be made by light tapping with a mallet too before putting the mesh in . I always use aluminium plate for spoilers or scoops as it’s so easy to shape and also cuts so simply with tin snips😊
This is the mesh I used it’s very fine and light. . Obviously it can be bought cheaply too in larger rolls.
Edited By Tim Flyer on 09/03/2020 12:09:06
Thanks Jon . I was initially planning to cut out a small vent in the bottom at the rear of the cowl and put a small spoiler on it to act as an extractor rather like in my Chipmunk. . I’m not so sure now as the rearward facing side air exits already built into the cowl will provide good suction. ....However a little exit on the cowl floor would also ensure good airflow past the carbs.....hmmm maybe I will it only takes a few minutes and I can do it while the canopy glue dries 😊 it’s easier now to max out airflow than having to fiddle around later and need to degrease etc before fixing .
Cheers Cymaz thank you. I know there are quite a few amazing builders on this site but I thought my simple mods might spring a few ideas for others . The “clamshell” tabs fit really well and are very easy to do . The two piece cowl makes cutting the holes much easier! Other than repairs and model boats, this is the first time I have used fine aluminium mesh as a glassing former on a plane and I certainly think it makes baffles and air channels easy to make. Its quite a versatile product.
I can’t wait to fire the engine up. I’m hoping it will give the “DLE boys” at my club something to think about 😉😉.
Here is a side view of finished job. One thing some eagle eyed people may note is that my clunk tank has only 3 feeds. Two of them are carb fuel feeds and the third is a vent . What I have done to allow easy filling and draining is connect a small brass pipe which feeds one carburettor to the wooden engine mount spacer . This allows easy disconnection an re connection for filling and draining. Access is via removing the 4 plastic bolts holding the top engine cover . This cover can easily be removed without removing the prop/spinner or bottom cowl half. I always use plastic cowl screws as they are less prone to vibration loosening and cause less damage in mishaps!
Here are the exhausts mane from bendy pipe with silver soldered brass end inserts and fittings. I use k&S brass thin wall tubes that fit in concentric sizes so they are easily soldered for a good gas tight fit .
Here are the finished top baffles after glassing they angle the air down onto the cylinders. It’s quite easy to make a complex shape with wire mesh and glass. It’s also super light 😉. There is a triangular balsa baffle between the two cylinders below.
Cheers Ron . It’s such bad weather I thought I would get on with it 😊.
Just thought I would post up my modification job on a Wots Wort XL cowl I’m adapting for an old laser 200v . I decided to split the cowl into a top and bottom section . It fits together with threaded inserts and plastic bolts in glass tabs moulded along the full section of the joint. These joining tabs are easy to make simply by taping the cowl together after cutting and adding cling film in the upper half and glassing over with wing tape and epoxy to form a tab . At that stage I also add the wooden plugs to later take the 3mm threaded inserts.
|Thread: Closed loop...|
Sounds a good plan👍
The rear has a virtual pivot point 10mm back from actual pivot point . This means to offset that a.virtual pivot point at the front needs to move forward not rearward. The current set it will mean that the wire being released will be released at a slower rate than the wire being pulled. The curved servo arms as in photo can offset this. If the curve is too much or arm too long it just means released line goes very slack so no problem 😊. Sorry but have to do bedtime duties now! Good luck John
Edited By Tim Flyer on 23/02/2020 20:53:50
Edited By Tim Flyer on 23/02/2020 20:57:15
Edited By Tim Flyer on 23/02/2020 20:57:35
In practice you will be using a wider arm than the tail so the released wire will be slacker . With a big arm you won’t need to move the servo pivot point the full 10mm forward... just a bit
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