FYI - All 170 rudders are not the same
Posted: Tue Aug 11, 2009 4:56 pm
Short story...I am rebuilding a 54 B model that got flipped on its back. Of course the rudder was damaged. It was fixable but I decided to try to find a decent used one. Found one on Ebay for a reasonable price and even though it looked okay, I decided to reskin it since the rest of the rebuild was going to look really nice. I ordered skins from our good friend up in Bow, WA and made it look brand new. I even bolted it on the rebuilt vertical to admire my work while I reskinned the elevators. I finished the elevators yesterday and again was so excited that I decided to bolt them on the airplane as well. I couldn't get the right one installed because the cutout in the rudder for the elevator torque tube is too small. After inspecting my buddy's A model, It turns out that the A model elevator hinge line is closer to the stabilizer spar by about 1" which makes sense because the cutout in the rudder is about 1" smaller on the A model rudder.
I spent a long time researching this before I bought it and searched "rudder" all the way back to 2005 on this site and never tripped on this info so I thought I'd share it to save someone else some headache. My quick reading of the 2 parts manuals tells me that the only difference is the small stub spar located just aft of the cutout. Figure 15 item 26 in my A model book, and Figure 15 item 11 in my b model book, both downloaded from micro-tools. The skins have a different suffix but are shaped the same as far as I can tell. I am debating on whether to just buy the different spar and install it in my rudder or try to find a B model rudder and reskin it. I have been moderately fanatical about my repairs looking original and this one would require about 4 extra rivets per side which 99.9% of the people who saw it would never know.
After reading how that sounds I've decided to just fix mine, I don't need to be that anal.
A few final thoughts...
-My conclusion at this point is that a B model rudder will fit an A model but not the other way around. Their balance specs are identical as well.
-Every 170 rudder I have looked at has cracks around the middle hinge point at the lower end of the spar. I believe this to be due to misalignment of the hinges but that is just my theory. I ran a string through the hinges on the vertical and they were perfect, but the rudder was way out and I had to add almost 1/4" of shim to the upper hinge point under the pillow block to get them straight. Check your rudders because I'm sure there are a few cracked ones out there.
I spent a long time researching this before I bought it and searched "rudder" all the way back to 2005 on this site and never tripped on this info so I thought I'd share it to save someone else some headache. My quick reading of the 2 parts manuals tells me that the only difference is the small stub spar located just aft of the cutout. Figure 15 item 26 in my A model book, and Figure 15 item 11 in my b model book, both downloaded from micro-tools. The skins have a different suffix but are shaped the same as far as I can tell. I am debating on whether to just buy the different spar and install it in my rudder or try to find a B model rudder and reskin it. I have been moderately fanatical about my repairs looking original and this one would require about 4 extra rivets per side which 99.9% of the people who saw it would never know.
After reading how that sounds I've decided to just fix mine, I don't need to be that anal.
A few final thoughts...
-My conclusion at this point is that a B model rudder will fit an A model but not the other way around. Their balance specs are identical as well.
-Every 170 rudder I have looked at has cracks around the middle hinge point at the lower end of the spar. I believe this to be due to misalignment of the hinges but that is just my theory. I ran a string through the hinges on the vertical and they were perfect, but the rudder was way out and I had to add almost 1/4" of shim to the upper hinge point under the pillow block to get them straight. Check your rudders because I'm sure there are a few cracked ones out there.