Making a fiberglass air scoop

[skippydiesel]

Fiberglassing work, is possibly my least favourite task to do with aircraft modification. I am in the process of modifying my engine cowling to accommodate a relocation of my radiator (Rotax 912) in the hope it will improve engine cooling.

 

The  mod is a large scoop that, with styling/streamlining ,takes up about 2/3 of the original  cowls underside.

 

I have made a male mould in situ, created the bulk of the scoop but am at a loss as to how to create a professional looking leading edge-  helpful suggestions ???

 

Edited September 20, 2023 by skippydiesel

[kasper]

Depends on what you want as the lip edge. 

Simplest given your choice of carbon for the scoop is a sharp edge without return - > simple die grinder on the set edge and be VERY safety aware with the carbon fibre dust etc.

 

If you want a rounded return then given what you have already done its a patch and repair the patch you have already done to get your outcome.
You have, from my experience, three options.  Each vary in weight and complexity but each will give you pretty much what you want on a return lip

1. Trim scoop and then lay in and shape foam to the inside of the scoop going beyond the trimmed edge and feather it back to the inside of the scoop.  Then lay light weight CGM over the edge enclosing the foam followed by lightweight glass cloth  then fill and sand the surface for paint prep.

2. Same as 1 for trim buy lay a jute/hessian cord as the lip - apply with cryo/super glue then resin the cord.  User filler on the cord to then sand to your rounded lip and glass over similar to 1. above ... but given you have filled the cord you go straight to the glass cloth.

3. Trim your edge then build a temp form over the gap.  Use plasticine and radius balls to crate a temp mold for the radius lip you want then glass from within the scoop with gel coat/surface cloth and glass cloth.  Take your mould away then finish the surface with filler/sanding.

 

Each has their costs/benefits and all will take a couple of hours to complete.

 

Lots of fun ... and the reason people hate it when I give a real quote for a build and supply of a plug and mold for any aircraft parts 😀

[skippydiesel]

Kasper you are a gentleman.

 

Few questions;

 

I aspire to the rounded edge/return however as with all matters "little aircraft" weight is of concern - what would you do?

 

lightweight CGM over the edge enclosing the foam followed by lightweight glass cloth - please expand using every day terminology (if possible) as I will have to purchase these items and like to have some awareness  that it is the right material for the job.

 

cryo/super glue - This sounds to be a way of achieving a quick fix (🙃) - would a glue gun (which I have) be as good?

 

plasticine -  OK with vinyl ester?

 

radius balls - having no balls (worn out 🤥) would guess that any spherical object, of the correct /acceptable dimension would do?

 

As for a couple of hours to complete - be advised that the male mold, for the body of the scoop, took me about 10 days to make and this was with the aid of a jig/form  (home made) to try & acheive symmetry and correct size.

[440032]

I have a professional looking leading edge on my scoop, without having anything on or to the leading edge.

My air scoop is about 3mm thick. Balsa form made on the cowl, covered in packing tape, glassed over it. Pop the part off, cut trim sand instal.

I just sanded the sharp edges off the front. Simples. (insert that sound the Meerkats make)

 

[kasper]

16 hours ago, skippydiesel said:

Kasper you are a gentleman.

 

Few questions;

 

I aspire to the rounded edge/return however as with all matters "little aircraft" weight is of concern - what would you do?

 

lightweight CGM over the edge enclosing the foam followed by lightweight glass cloth - please expand using every day terminology (if possible) as I will have to purchase these items and like to have some awareness  that it is the right material for the job.

 

cryo/super glue - This sounds to be a way of achieving a quick fix (🙃) - would a glue gun (which I have) be as good?

 

plasticine -  OK with vinyl ester?

 

radius balls - having no balls (worn out 🤥) would guess that any spherical object, of the correct /acceptable dimension would do?

 

As for a couple of hours to complete - be advised that the male mold, for the body of the scoop, took me about 10 days to make and this was with the aid of a jig/form  (home made) to try & acheive symmetry and correct size.

Well if you want the simplest then 440032 has provided it - just glass rounded.  The reason this not quite as easy for you is you have gone with carbon fibre whcih means you have far less than 3mm to round over ... but if trim the scoop leading edge to your shape and then lay some glass inside to build up and around the trim edge you can get the same result as 440032 whcih will be light and simple.

 

IF you were after any of the built up over foam/chord items then:

1. polyester and vinyl ester resins will eat polystyrene foams - they use styrene in the resin so will melt the foam - only plan on using epoxy on styrofoam

2. eurathane foam is fine for all resin systems

3. hot glue gun not recommended for fixing parts that are then to be glassed - cyano is compatible with all resins

4. the other very simple way is to use a flox filler to build up the edge to then sand back to size - if you are looking for a very small radius (eg 3mm) that wins on time, weight and simplicity

5. the plastacine mold way is ideal if you wanted a hollow large radius return eg a 3-5mm radius - and plastacine is compatible with all resins

 

For people without experience or indeed an actual interest in doing composite works Id say 440032 has done the simplest and most practical scoop

[440032]

Laziest person finds quickest way of doing something.

This scoop was a bit easier than the instrument panel I made for a guys '56? Ford F100 hotrod a rear window brake light for his 56 Customline.

[Markdun]

Try some vinyl hose ( say 10mn od), cut a longitudinal slit in it and fit over edge, hold it down how you want the radius to be like with fine safety wire or hot glue…. Coat in pva release agent.  Another method is to supa-glue light braided polyethylene ‘rope’ to the edge and just wrap the fibreglass around it, leaving the ‘rope’ in the layup.  Remember also that in laying woven fibreglass cloth, it will go around small radii if you lay it ‘on the bias’, ie the warp and weft go +45 and -45 degrees to the edge.  It’s a real bugger if you have any fibres aligned 90 degrees to the edge.

 

A ‘scoop’ is going to be draggy & inefficient. Why not have a pressure recovery chamber?  Is space an issue?  Not that I’m fussed about this as my LCH J200 has the radiator full frontal between the prop and oil cooler inlet.

 

 Mark

[skippydiesel]

Thanks Mark,

 

First attempt was to use  polystyrene solid tube draft excluder, wrapped in "cling film" to prevent melting - might have worked but for the next comment.

 

Found out, the hard way , about the glassing of small radius. Starting again, only three day lost this time - getting used to the frustration.

 

Pressure recovery chamber?  - do you mean NACA duct?

 

In my limited experience - space is always an issue. 

 

I assume the LCH J200 with radiator is Rotax powered?

[Geoff_H]

I bought some very light and fine close weave.  You don't roll it you just crumple  itto a ball.  So fine that it doesn't stay crumpled.  It will cover very tight edges.  You can have the small quantity that I have if you wish.

[skippydiesel]

1 hour ago, Geoff_H said:

I bought some very light and fine close weave.  You don't roll it you just crumple  itto a ball.  So fine that it doesn't stay crumpled.  It will cover very tight edges.  You can have the small quantity that I have if you wish.

Your a true gentleman Geoff - I think my Son has some "fine" stuff. Its more like a soft fabric, than a fiberglass woven matt. If its not "the goods" I will be in touch. Thanks again.

[Markdun]

Skip, maybe some semantics. Stay away from ‘mat’, like ‘chopped strand’ mat unless you are building an anchor. You want cloth fabric with warp and weft around 200gsm or less. For larger surfaces go the woven rovings but I’d say 600gsm is way too much. Your goal is that fully wetted fibreglass/ carbon/Kevlar with epoxy has less than 50% epoxy resin.  If you want more stiffness in the layup do one or all the following: increase compound curve, introduce a layer of foam, even 2mm between two layers of cloth glass (Jab do this on their cowls) & epoxy, or use carbon cloth or lay on some ‘stringers’ (strips of poly rope with a layer of cloth on the bias laid over it).

 

My LCH J200 has a Gen3 Jab3300 in it.  ‘Pressure recovery chamber’ is where you have an expansion chamber after the air inlet to reduce the kinetic velocity of the ram air which is converted to pressure as per Bernouli to increase the air pressure differential across the radiator (which is the beginning and end of liquid cooling). The RV racing guys are the full bottle on this for their engine cooling plenums.  It allows you to reduce cooling drag to a significant extent if you are going fast. A simple scoop (or a radiator facing plain ram air like mine) is like a dragging a flat plate through the air like a parachute.

[skippydiesel]

Thanks Mark,

 

By accident, my scoop is much like what you describe as a "pressure recovery chamber" having a relativly small opening, that widens into a lager chamber, with the radiator being the only (I hope) exit for the air .

 

[Markdun]

That’s good. There are some articles on this in either Kitplanes or EAA (RV focused).  Seems the critical thing is not having the inlet too big, having a smooth transition as the inlet expands so the airflow stays attached, & having a big expansion chamber. The Jab standard cooling plenums apparently fails on all three.  It’s also a factor for the cooling air exit; but there it is a matter of accelerating the now slow warm air up to your airspeed. Theoretically you can gain thrust with an exhaust augmentor, but I think the practical goal is to reduce drag by providing for the exit air to accelerate (as opposed to putting in a large ramp/skirt angled down fwd of the air exit).  Some racers put streamlining on their bottom engine mount tubes and ensure all fuel/oil tubing is neatly tucked away so as not to disturb the exit air smooth flow departure. We are talking about a 100-200kias aircraft aren’t we?

 

 Another point to keep in mind is that even though ‘the radiator is the only exit for the air’, it will only work IF there is a pressure difference. If the air pressure behind the radiator is close to ram pressure because you have two large cheek inlets filling the engine cowl with air at ram pressure, then it’s doomed to failure.  When I did the LCH conversion on the J200 I had to reinstall the jab fibreglass cooling plenums, because without these the pressure behind the radiator was too high (as well it left the cylinders not well cooled).  When I find a round tooit that I’ve lost I’m gunna halve the cheek inlets’ aperture and construct lighter cooling plenums from those cheek inlets directed to cooling the barrels. I might also then halve the exit air outlets & chop 15cm off the exhaust so the exhaust can help accelerate & extract cooling air.

[skippydiesel]

I inherited , with an almost complete homebuilt,  a forward oil cooler and an aft radiator. 

 

The radiator was beautifully installed, so that there would be no air leaks. The cowling also had few air leaks. Result - On Ground, high coolant/head temperatures, fair oil. In Air, good coolant, high oil temps.

 

After trying all sorts of fixes - realised that the radiator itself, being the only exit for the air,  does not allow for sufficient air to pass through cowling/heads/oil cooler ie pressurising cowl to the point of restricting adequate air flow through oil cooler. .

 

Have returned aircraft to workshop - Radiator relocated from aft position, leaving large exit hole, to position under crankcase (fwd). Oil cooler, now the Rotax Extra Large (formally Large), relocated to aft position but not blocking exit air. Sounds a little odd I know but this follows (sort of) my last aircrafts successful cooling lay out.

 

One of the original, partially successful, fixes, was the installation of a pilot controlled cowl flap. I am hoping to use this to control exit air - that is to encourage more flow on the ground/during TO & CO and reduce flow (higher velocity) in cruise.

[Markdun]

Sounds good. Having radiators in series like you describe means you need twice the pressure differential (entry minus exit pressures) for them to work which would be hard to do, particularly at low airspeeds. A friend’s Tecnam has the oil and head cooling radiators facing the airstream (like my J200) which gives you plain ram air pressure at the entry and engine cowl pressure behind on both radiators.  Lots of air leaks, and with two small cheek vents and a small exit, it’s not surprising he has high OT two up in the summer.  Its 24 registered so no playing with it.

[facthunter]

The adjustable cowl flap is what I agree with. The cooling need varies a lot with changes of ambient temp.  Nev