Author Topic: Cowl hold down techniques (Read 3599 times)

Dennis Toth · « **on:** January 10, 2020, 11:11:15 AM »

Guys,
Working on the top block and cowl and need to decide on how to hold down the cowl. In the past I've used small bolts and blind nuts, bolt through the tank with alignment tabs, magnets with adjusting screws. The ship has an upright engine with 1 3/4 " spinner and muffler. I would like to be able to remove the engine and tank under one cowl. How have you guys be doing the hold downs?

Best, DennisT

Ken Culbertson · « **Reply #1 on:** January 10, 2020, 12:01:41 PM »

Quote from: Dennis Toth on January 10, 2020, 11:11:15 AM

How have you guys be doing the hold downs?
Best, DennisT

Since going electric I have found that access to everything at the field is a big plus. I make my access by making the bottom of the fuselage removable. I use 1/2 x 1/16 Bass the full length of the hatch so that it slips into the fuselage like a doubler. 4-40 x 3/8 wood bolts through the fuselage side into blind nuts in the hatch make it a structural part of the fuselage. I bush the holes with aluminum tubing and bevel the tubing to match the taper of the bolt making them flush.

Just one of a thousand ways to skin this cat.

Ken

Avaiojet · « **Reply #2 on:** January 10, 2020, 01:28:19 PM »

Magnets.

Howard Rush · « **Reply #3 on:** January 10, 2020, 01:38:03 PM »

I use two pins in front and a mason-jar-type latch in the back. It's quick.

Howard Rush · « **Reply #4 on:** January 10, 2020, 04:57:55 PM »

Quote from: Ty Marcucci on January 10, 2020, 02:15:46 PM

Once, in sheer desperation on a Barnstormer, I used two big rubber bands. Worked like a charm. No extra points though.

That's what I did after I tried to use magnets.

Ken Culbertson · « **Reply #5 on:** January 10, 2020, 06:08:43 PM »

Quote from: Howard Rush on January 10, 2020, 04:57:55 PM

That's what I did after I tried to use magnets.

bob whitney · « **Reply #6 on:** January 10, 2020, 06:12:58 PM »

last 4 ships 4 magnets and 2 locator pins. on my Chief the tank is flush with the top of the fuse so stuck the magnets to the tank. no problem so far

Air Ministry . · « **Reply #7 on:** January 10, 2020, 08:27:35 PM »

Bicycle Spoke & Nipple . Available in steel & aluminum . Also in tecnocolour . H.Duty = Motor Cycle in various gauges .

Drawback is usually on centerline . So R E or through the commutator might not work .

A ply plate inside , drilled for shank , is the usual thingo . Glued Securely .

Bicycle shops often have many wheels heading for the tip . But you end up with a hub & rim that way also .

Dennis Toth · « **Reply #8 on:** January 11, 2020, 10:03:52 AM »

Lots of good approaches here, thanks. I did see one of Windy's videos on the Big Job cowl and it seems to fit with what I am doing. I think I will go with two side tap hold downs and a rear pin/magnet. On the video Windy mentioned the button cap screws/bolts that are available from Micro Fastners (https://www.microfasteners.com/), these button caps have socket hex head and come in lots of thread sizes (I bought 2-56 & 4-40) available on there web site, they take Credit Cards or PayPal.

So to start I have two matching 1/32" plywood end caps shaped to the 95% shape for the end of the top block and the match at the cowl. I aligned and clamped them together and drilled a 1/32" pilot hole. Now I can get on with the top block final shaping and drill/install the mount eyelet in top block. After that I can rough shape the cowl block then with the end cap install the rear pin. Once that is in I can tack glue the cowl to the fuse and add the nose ring and final shape. Next will hollow and add side tabs and bolt blind nuts. Simple what could go wrong?...

Best, DennisT

Randy Powell · « **Reply #9 on:** January 11, 2020, 01:08:21 PM »

I've seen Howard's method. It's clever.

Jim Hoffman · « **Reply #10 on:** January 11, 2020, 01:26:38 PM »

I prefer screws, blind nuts and pins. I use both #2 steel cap screws and 4-40 nylon cap screws.

•   I bush of screw openings with alum or carbon tubes to prevent damage of surrounding balsa.
•   I use at least two screws to prevent a single point failure should a screw come out.
•   I use steel (music wire) pins and use metal (any metal) on the female tube receptacle.
•   I line the faying (mating) surfaces of the cowl and the airframe with 1/64th plywood, again to prevent wear or damage to the soft balsa.

The location of the screws and pins is dependent on the installation (the configuration of the front end). I have found it not wise to figure it out at the end. Plan ahead.

Avaiojet · « **Reply #11 on:** January 11, 2020, 03:29:16 PM »

Dennis,

If you use double magnets, that is, magnets in the hatch and in the fuselage, striking each other, be hard pressed to have that pull off.

I cart wheeled my P-40 broke some stuff but never lost the hatch.

Cleaner look also.

Charles

Trostle · « **Reply #12 on:** January 11, 2020, 05:02:40 PM »

I used magnets once. Inverted engine. The cowl had pins at the front behind cowl ring and two sets of magnets, one set on each side of the cowl at the rearmost section of the cowl. The thing came off on the first inside loop. Have not used them on a stunt ship since.

Keith

Air Ministry . · « **Reply #13 on:** January 19, 2020, 06:24:36 PM »

Seems the super magnets even , magnet to magnet is required . With the magnets securely glued .
took to one briefly with the disc grinder , to roughen surface. Seemed to be no harm electronically .
Or is that magnetically .

Curare · « **Reply #14 on:** January 20, 2020, 12:37:29 AM »

Quote from: Howard Rush on January 10, 2020, 01:38:03 PM

I use two pins in front and a mason-jar-type latch in the back. It's quick.

Can you elaborate on this? I can't picture this. Actually I can, it just looks a fuselage with one of these on it?

Avaiojet · « **Reply #15 on:** January 20, 2020, 06:07:02 AM »

Quote from: Howard Rush on January 10, 2020, 01:38:03 PM

I use two pins in front and a mason-jar-type latch in the back. It's quick.

Back in the 30's it was quite common to see a similar, "mason-jar-type latch" hold down application, on many of the Racers of that era. Possibly other aircraft as well.

HDWE was produced by a few suppliers just for the aircraft industry. Although these fasteners, pictured on this Gee Bee Z, are a tad larger than they should be for the model's scale, they are quite similar "in look" to what was offered then.

The fasteners on this Gee Bee Z were sold by a company who makes HDWE for the "small box" industry. I do know the graphics are perfect.

Dennis Saydak · « **Reply #16 on:** January 20, 2020, 10:54:10 AM »

Here's my Jaguar cowl. I basically used Steve Buso's method shown on the plan with two pins on the rear and 4/40 S.H. bolts into brass threaded inserts in front.

Don Jenkins · « **Reply #17 on:** January 23, 2020, 05:32:22 AM »

Quote from: Dennis Saydak on January 20, 2020, 10:54:10 AM

Here's my Jaguar cowl.

Meticulous workmanship Dennis! Would like to see it when complete!

Don

Phil Krankowski · « **Reply #18 on:** January 23, 2020, 08:20:05 AM »

Pins on one end, magnets and an internal eye hook for a safety rubber band. Have the band go over a peg to a hook a few inches away so you get good tension with lots of stretch.

Pins on one end and a screw I have seen snaps used, but replaced that with a screw on my super ringmaster.

Phil

Howard Rush · « **Reply #19 on:** February 01, 2020, 02:07:36 AM »

Quote from: Don Jenkins on January 23, 2020, 05:32:22 AM

Meticulous workmanship Dennis!

That's what I was thinking.

doug coursey · « **Reply #20 on:** November 16, 2021, 09:40:11 AM »

magnets dont work 100% of the time.....its the 1% that gets you

Serge_Krauss · « **Reply #21 on:** November 16, 2021, 03:53:59 PM »

This is Wes Dicks "Ares": pins and latch.

Ken Culbertson · « **Reply #22 on:** November 16, 2021, 04:00:19 PM »

Quote from: doug coursey on November 16, 2021, 09:40:11 AM

magnets dont work 100% of the time.....its the 1% that gets you

And the 2%, and 3%..... I will never use magnets on a top, bottom or outside hatch again...ever.

Ken

Steve Helmick · « **Reply #23 on:** November 17, 2021, 07:45:17 PM »

Quote from: Serge_Krauss on November 16, 2021, 03:53:59 PM

This is Wes Dicks "Ares": pins and latch.

Those latches are/were available from Fourmost Products and either Kavan or Graupner. Pretty slick, but a single 4-40 screw with locator pins is slicker. I've seen a few use 2-56 screw, which is plenty strong enough, but harder to find when one drops it in the grass or even on pavement, and much less likely you'll be able to mooch a 2-56 than a 4-40. On the nose is not a place where the weight difference will mean anything, anyway.

I recall Howard's fun with the magnets. Also recall an electrified Nobler shedding the battery through the cowl and pulverizing itself due to the CG shift. Velcro straps are not adequate for battery retention, plus they do wear out.

Steve

Mark wood · « **Reply #24 on:** November 18, 2021, 11:01:52 AM »

Quote from: Howard Rush on February 01, 2020, 02:07:36 AM

That's what I was thinking.

Make it three..

Mark wood · « **Reply #25 on:** November 18, 2021, 11:09:12 AM »

Quote from: Steve Helmick on November 17, 2021, 07:45:17 PM

Those latches are/were available from Fourmost Products and either Kavan or Graupner. Pretty slick, but a single 4-40 screw with locator pins is slicker. I've seen a few use 2-56 screw, which is plenty strong enough, but harder to find when one drops it in the grass or even on pavement, and much less likely you'll be able to mooch a 2-56 than a 4-40. On the nose is not a place where the weight difference will mean anything, anyway.

I recall Howard's fun with the magnets. Also recall an electrified Nobler shedding the battery through the cowl and pulverizing itself due to the CG shift. Velcro straps are not adequate for battery retention, plus they do wear out. Steve

I pretty much only do side load batteries for this exact reason. Not quite as purdy but shedding a battery or hatch is not usually a concern. Turning the hatch in to a cheek cowl helps on the appearance side and magnets work. I usually double up the fuse side magnet and single the hatch side so three magnets per stack. Two stacks on a large cowl. Done right all the magnet is doing is restraining the internal pressure from the cooling airflow. If the cowling is vented correctly the pressure won't be very high.

FLOYD CARTER · « **Reply #26 on:** November 18, 2021, 12:57:06 PM »

Magnets work fine as long as nothing inside the cowl is rattling around to push on the hatch cover during maneuvers. Look at it this way; the balsa hatch weighs maybe 3-4 grams. The holding power of 4 pair of magnets, each pair oriented N-S has to be several ounces! Where people have problems is stuffing a heavy battery in the fuselage with no means of restraining that weight, So, it's going to flop around and bang against the hatch cover.

Avaiojet · « **Reply #27 on:** November 18, 2021, 04:13:52 PM »

In an IC model, if you can find a way to bolt the muffler last, more prevention.

Brett Buck · « **Reply #28 on:** November 18, 2021, 05:21:15 PM »

Quote from: Avaiojet on November 18, 2021, 04:13:52 PM

In an IC model, if you can find a way to bolt the muffler last, more prevention.

That is very impractical. You need to be able to swap engines without disturbing anything on them, particularly the muffler or tuned pipe, and the needle valve. The solution to cowls falling off is to mount them properly as noted above.

Fortunately that is relatively easy to do, with some pre-planning.

Brett

Howard Rush · « **Reply #29 on:** November 18, 2021, 09:33:03 PM »

Quote from: Mark wood on November 18, 2021, 11:09:12 AM

If the cowling is vented correctly the pressure won't be very high.

Conversely (Conversely?) if you use magnets and the cowl blows off, you will know that it's not vented correctly.

Mark wood · « **Reply #30 on:** November 19, 2021, 07:21:50 AM »

Quote from: Howard Rush on November 18, 2021, 09:33:03 PM

Conversely (Conversely?) if you use magnets and the cowl blows off, you will know that it's not vented correctly.

Yup.

kevin king · « **Reply #31 on:** December 21, 2021, 01:00:14 AM »

I love the cowl set up on my Spitfire. 1 bolt and the cowl comes off. After disconnecting the fuel line and loosening the 4 motor mount bolts, I flip the plane over, give it a shake or 2 and the motor drops right into my hand. I can then pull the motor out with the rear exhaust muffler and deflector in one piece. 😁

Dennis Toth · « **Reply #32 on:** December 21, 2021, 11:46:55 AM »

I have used the Neodymium magnets (K&J Magnetics) on the hatch for my Stuka with good success. Two things are important: you need a way to adjust the location of what the magnet is hooking up to. You want the magnet to be flat so the piece needs to pull away from the magnet not slide. I used a flat head screw in the small corner block of basswood. I have the screw head set down into a drilled hole so it can adjust up/down to get a solid connection between the magnet and screw head. The magnet sits in the hole to hold position like a pin. After painting can do final adjustment.

Second, need to make sure that nothing is touching the hatch when it is in place that would keep it from setting on the screw head. The only time I had the hatch blow off is when I didn't check that the battery straps were clear of the hatch, and they caused a gap, this kept the magnet from seating properly.

Best, DennisT