cowling screw attacment

[Tomkat]

What is the general consensus on cowling screw backing? what I am asking is what type of screws ? and arethey backed with tinermans or nut plates?
I looked in the plans and there was little information. and in the material list it said due to differances in building there was nothing listed.

 I am thinking of bonding nut plates to the cowl or lip as needed .
please let me know what others have done
Thanks
Tom

[Dan Patch]

Tom,

For the VariEze, cowl installation is covered in detail in Section II A, Step 15, and that's the method that I used.  Note that there are 'screwless approaches' like Bill James, Klaus and others have used; and there is the camlock approach discussed by Joe under the earlier "Engine cowlings screws..." topic.  There are several good ways to skin this cat.  

So, the plans recommendation is for non-structural 10x32 machine screws, the old AN 507, used in conjunction with an appropriate anchor nut, like the K1000-3.  The current screw numbers are: MS24693-S273 for the longer screws (5/8th inch total length) that I use where the spar & firewall flange is thick, and the shorter MS24693-S272 (1/2") everywhere else.  The anchor nuts are pop riveted in place using the appropriate, small size that the plans give as Avex 1604-0412 or Cherry MSC 43.  These are flush head and you want to countersink them in the glass.  I don't know anything about bonding nutplates?  Just be sure that there is no chance that a nutplate could spin on you or you'd be stuck drilling out the screw (if it didn't spin too).  

The stronger AN 509 structural machine screws are not needed in this application, and I would have a concern (but no real data) that they would be prone to wearing out self-locking anchor nuts.  I've also heard that stainless screws are prone to galling and screwing up the nutplates.  You don't want to have to change out nutplates if you don't need too!  I've had my cowl off hundreds of times using the 507s and the nutplates are still in excellent shape - none replaced!  

A drop of oil on a new screw probably helps as my self-locking nut plates are still pretty stiff for a new screw.  I probably get 6-8 shots out of a screw before I toss it.  These days I throw away sooner rather than later, stripping out the head is no fun to deal with and screws are essentially free.  

I use Tinerman washers under all leading edges of the cowl (spar flange and firewall flange).  I've had good luck just going with just screws everywhere else, but you have to be sure that the cowl has plenty of beef where the screws seat, because cowl security is a serious flight safety issue, to be sure.  Carefully monitor the condition of the cowl, screws and nutplates to be double sure that everything is, and stays, OK.

[tomkat01]

Thanks Dan

   I hope tha did not seem like to dumb of a question. I looked in both sets of plans last night and could not find it. I of coarse found it this morning after you said it was in the engine install plans.
  I am going to stick wiith screws also.It  seems simple and one of the
best ways not to have fasteners going through the prop

Tom Rodgers

[Joe Dubner]

What is the general consensus on cowling screw backing? what I am asking is what type of screws ? and arethey backed with tinermans or nut plates?

Tom,

Like Dan Patch, my aircraft uses riveted nutplates and Tinnerman washers.   But I've changed from countersunk Phillips head screws to Torx-type fasteners for ease of "hooking" with my cordless electric screwdriver.

Spend $16 for a bag of 100 10-32 x 3/4 T20 100-degree countersunk SS screws from Microfasteners.com (http://www.microfasteners.com/catalog/products/SSCFCMXS.cfm) and you'll have enough for two cowls.

[Dan Patch]

Agree with Joe.  Whatever you use, and Torx seems like a reasonable thing to do athough I haven't tried them, buy 'em by the 100 and toss when the anchor begins to lose its grip (with AN 507s the screw wears, not the anchor) or if the head isn't in perfect condition.  No need to tighten the be-jeebers out of them, just snugged down firm is plenty.  More just munches the cowl and tears up the head.  Never had one come loose and go through the prop.  Not a bad idea, however, to double check after installing the cowl to verify that they are all in and screwed down (not just started).  Also agree that an electric screwdriver or drill with a slip clutch is the way to go.

[Snappy]

I also use the 507's and after a ding in my Silver Bullet prop, I toss all screws with any sign of wear.  You can't properly tighten a screw with a bad head.  Also, I have found there is a difference in screws!  Aircraft Spruce sells good screws BUT I got some from Chief Aircraft that are not machined properly and cause heating and thread realignment upon insertion.
The bad ones look exactly the same, but will begin to bind after the 1st few threads when you start to tighten them.  Throw these away too!  They will force you to put way too much pressure on the screw driver to keep the head/bit seated and you can dislocate the anchor causing it to relase and spin.  You DON"T want this to happen 500 miles from home.  The spun anchor always comes loose when you are removing the fastener and you will be sitting there with your cowling hanging by one screw that you can't get out!

One more thing, get a power screw driver with an adjustable torque setting.  Then throw away the cheap bits and get yourself a couple of GOOD BITS.  They will last and won't strip out the screw head so quickly.  I bought a Craftsman cordless driver for about $19 and keep it in my flight bag.  I can remove or install both cowlings in about 10 minutes

Good Luck!

[pushyhog]

check out screws on nose of cessna citation.
it would be a fun project to install but problems would be gone forever.