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machine countersink stab skin?

J

James R WilliamsA&P/pilot

Member
HI, I have a question about dimple vrs machine countersink on the stab.I see in my plans and in mech manual 65. it would be o.k to machine countersink two .032 sheets and vans manual say the aircraft is designed to be OK with this ,but then they say dimpleing it makes for stronger rivet line" great."I would like to know has anybody seen this done "machine countersink only" on the -3 stab rather then the dimple and since I am assembling bare bones 0235 v.f.r. no major stunts -3 would it be OK to not dimple and machine countersink only on the stab?
 
I machine counter sunk the empennage skins on my -6. I would not do it again. It's a real pain to get them all looking good. Dimples are easier, better looking, and slightly stronger.
 
Agree with Mel but if you dimple the skins you will have to dimple the spar flange and that wil curve it. Ask me how I know.
Tom
RV3 x 2
 
H.S. skin/rivet countersink

Thanks for the reply this is great ! After seeing reply I thought how about dimple skin and machine countersink flange so I do not get the cruve in the spar flange as stated in post reply. Our just pick the standard method of dimple counter sink both skin and flange?
 
Dimple both the skin and the spar. It will straighten out as you cleco.
 
It's easier the way...

Mel said:
I machine counter sunk the empennage skins on my -6. I would not do it again. It's a real pain to get them all looking good. Dimples are easier, better looking, and slightly stronger.
Click to expand...

...Mel says, but the RVs built by Art Chard all used machine countersunk rivets in 0.032 skins, IIRC. The skins all looked great and very smooth.
I personally think that perfect countersinking looks better, but is hard for us mortals to achieve.
Check out a Blanik sailplane some time.

It can be done, but dimpling is easier and stronger for the rest of us...:)
 
Everyone I talked to that machine countersunk their -4 or -6 skins (and many did) said they wouldn't do it again because after a while, the rivets have a tendency to loosen slightly and start smoking.
 
Dimple, but counter sink a few spots.

It will make your life a lot easier to machine countersink the one spot where the inboard ribs overlap the front spar. It is also easier to counter sink the skin where the hinge brackets make it difficult to get a dimple die and squeezer in.
I have machine countersinked skins with mixed results. I agree with Gil, you have to be super human, like Art, to make them consistently good looking across a broad area.
 
osxuser said:
Everyone I talked to that machine countersunk their -4 or -6 skins (and many did) said they wouldn't do it again because after a while, the rivets have a tendency to loosen slightly and start smoking.
Click to expand...

None of my countersunk rivets loosened after 16 years. They all look great. It was just a pain to get them right initially.
 
In spite of Mels advice I would wait until a few builders wiegh in who have dimpled the skins and spar. As I said with dimpleing the spar flange you will curve the spar and a lot of pressure may be needed to bring your holes into alighnment for riveting. You are preloading the skin and spar and thats not the proper way to do it. Parts should fit passively before riveting.
ymmv
Tom
RV3 x 2
 
RV3 Pilot said:
In spite of Mel's advice I would wait until a few builders weigh in who have dimpled the skins and spar. As I said with dimpling the spar flange you will curve the spar and a lot of pressure may be needed to bring your holes into alignment for riveting. You are pre-loading the skin and spar and that's not the proper way to do it. Parts should fit passively before riveting.
ymmv
Tom
RV3 x 2
Click to expand...
I have done this on quite a few airplanes with no problems. The spar will straighten with very little pressure.
Even though I machine countersunk the skins on my horizontal and vertical stabilizer, I used the dimpling method on the rudder, elevator, aileron, flap and the rear wing spars. So far, 16 years later it's still flying.
Of course you should never do anything you are not comfortable with.

I just went back and checked my RV-6 plans. They state that; "Although the .032" spars may technically be machine countersunk, we strongly recommended that they be dimpled."
 
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osxuser said:
Everyone I talked to that machine countersunk their -4 or -6 skins (and many did) said they wouldn't do it again because after a while, the rivets have a tendency to loosen slightly and start smoking.
Click to expand...

The only RV I ever looked at with machine countersunk HS rivets belonged to Bill Knight. Almost all of the rivets along the rear spar started to smoke within a few hundred hours. Mel says that his are fine. I believe him. However, I suspect his skill level is above the average first time builder.
Charlie Kuss
 
chaskuss said:
The only RV I ever looked at with machine countersunk HS rivets belonged to Bill Knight. Almost all of the rivets along the rear spar started to smoke within a few hundred hours. Mel says that his are fine. I believe him. However, I suspect his skill level is above the average first time builder.
Charlie Kuss
Click to expand...

I machine countersunk mine, 2000+ hours no smoking. average builder at best.
Tom
RV3 x 2
 
Mel said:
I have done this on quite a few airplanes with no problems. The spar will straighten with very little pressure.
Even though I machine countersunk the skins on my horizontal and vertical stabilizer, I used the dimpling method on the rudder, elevator, aileron, flap and the rear wing spar. So far, 16 years later it's still flying.Of course you should never do anything you are not comfortable with.

I just went back and checked my RV-6 plans. They state that; "Although the .032" spars may technically be machine countersunk, we strongly recommended that they be dimpled."
Click to expand...

On the RV3 those skins are all less than .032 so dimpling is required.
Tom
 
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I dimpled mine without any problems. My first HS came out twisted because my jig moved but I never had any trouble aligning the holes. It would make for a nice looking surface though.
 
Dimpled with no issues....

RV3 Pilot said:
In spite of Mels advice I would wait until a few builders wiegh in who have dimpled the skins and spar. As I said with dimpleing the spar flange you will curve the spar and a lot of pressure may be needed to bring your holes into alighnment for riveting. You are preloading the skin and spar and thats not the proper way to do it. Parts should fit passively before riveting.
ymmv
Tom
RV3 x 2
Click to expand...

Locate your holes, within edge distance tolerances, toward the rear edge. If you drill your holes on the center line, you will scrape the spar doubler on the rear spar and pucker the spar web on the front spar unless you use a ground down die. This may cause the curving?, I dont know.
I had no issues with the spar curving and needed no pressure to align the skin on my 6 or newly complete 3 empenage. All dimpled except a few spots due to hinge interference as previously mentioned. Your results may vary.
 
Mel said:
I thought we were talking primarily about whether or not to dimple the spars.
Click to expand...

Then why did you use those as an example?

No dog in this fight, I've already built mine, just trying to offer another opinion. Like I said let others who have dimpled thiers offer thier observations. The fact that someone does it one way and its still good after x amount of years does not make it the preferred method. Lets let him make up his own mind.
Cheers
Tom
RV3 x 2
 
RV3 Pilot said:
Then why did you use those as an example?
Click to expand...

My examples were rudder, elevator, aileron, flap and rear wing SPARS!

I also quoted from Van's instructions that dimpling the spars IS the preferred method.

Please go back and read my post.
 
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If you are considering machine countersinking the empennage spars, picture this; If you countersink .032" material for a 3/32" rivet, the bottom side of the hole ends up almost knife edge. This is where the shop head of the rivet will be. Draw a picture of this cross section and see how much material is holding that shop head.
I, like Van, strongly recommend dimpling these spars.
 
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