Porterfield Airplane Club

Keep the Skinny Birds Flying Safely

Question about Headknockers - aileron balance weights

I have a 1940 CP50 that I am restoring from the ground up.  I  built new ailerons and transferred the lead weight from the old aileron to the leading edge spar of the new aileron.  While in Osh Kosh I saw a LP65 with headknockers for balance weights. My CP50 was not equipped with headknockers. I searched for AD's but could not find any for the CP50. Does anyone know if the headknockers was an STC or some other add-on? My ailerons are quite heavy at the trailing edge. I have always been told that ailerons must be balanced to prevent flutter. Comments please?

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Comment by Andy Gelston on January 7, 2018 at 11:02pm

Did you two ever receive the drawings I emailed you a few weeks ago?  If not, check your junk mail folders.


Comment by Andy Gelston on December 29, 2017 at 3:44pm

I fabricated new wing struts for 41VT and had to substitute 4130 for the original 1020 streamline tubes.  It required a field approval and so I had to create a table comparing the specs for 1020 in the Metallic Materials Properties Development and Standardization (MMPDS) (formerly known as MIL-HDBK-5) with the data from the heat certs from the mill that extruded the 4130 tube (AS&S can provide this for an additional $15)...or you can use DOM 1020/1025 steel tube from Shapiro Supply in St. Louis.  You'll still need a 337 for the "owner manufactured" headknockers and aileron mods, but it won't require a field approval.  Your AI just signs off the work on the 337 and in the logbook, and sends a copy of the 337 to OK City.

Comment by David G Reichard on December 26, 2017 at 5:44am
Comment by Joseph B. Stanistreet on December 26, 2017 at 5:39am
I am in process of removing wing fabric, have head knockers and associated drawings and specs. I will dig the out. Andy, i am very interested in seeing the seaplane mods PDF for the wings and the fuselage.
Comment by Victor Briley on December 25, 2017 at 8:04pm

The 1949 AD specifies .049 -  1025 steel. Is 1025 an old spec that has been replaced with 4130? Aircraft Spruce does not have 1025 steel in their catalog.

Comment by Victor Briley on December 25, 2017 at 6:56pm

Thank you David and Andy for your input. David, I spoke with you at Osh Kosh last July and it was your plane that I noticed the headknockers on. Andy, I would appreciate it very much if you would email the PDF's to me at vicbriley@frontier.com . I still have the original A50 engine freshly overhauled and ready for installation. I have the left wing nearly complete but not covered yet. This would be the ideal time to perform the mods before I get too far into it. I can work on the right wing while making the mods for both wings. I am working with a local A&P/IA but he has never worked on a Porterfield. Any suggestions you might have would be greatly appreciated.

Comment by Andy Gelston on December 25, 2017 at 9:00am

Thanks, David!  This is the only AD for the skinny birds.  It's a shame the FAA just scanned an old document instead of updating it, but maybe that's Brad's responsibility as TC holder.  Anyway, as an A&P/IA, I would recommend performing the mods on the ailerons and installing the bath towel dryers (that's what I use them for when camping) and build and install the nose ribs, too.  Actually, I highly recommend, if you're rebuilding/restoring your wings, to make all the seaplane mods to them, as this makes them strong enough to safely up the gross weight to 1325 pounds.  I have PDF's of the drawings for the mods that I can email to those who are in the middle of wing work.  If you have your fuselage uncovered, I recommend performing the seaplane mods to it as well, and at the very least, strengthen the landing gear Vee and gear attach points, and update the forward pulley and brake pulley locations.

As for whether or not this applies to the CP-50 or just the -65's: if you still have the A-50 engine installed, I would say it's optional, but if your little girl has an A-65, you should fulfill the AD, as you could possibly encounter aileron flutter at the speeds those extra 15 HP will give you.

Comment by David G Reichard on December 22, 2017 at 4:02pm


Shows the headknockers (and I have knocked my head - don't) but is titled '44-04-03; Title: Joe Rankin Model CP-65 Airplanes' so I don't know that it applies, but the airframe is the same as far as I know. Always consult you A&P/IA on matters like this.

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