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PA-12 outperformed by identical PA-12 on floats

squirrelhunter44

Registered User
Prince George
So my boss has 2 identical PA-12's on floats. They have the exact same mods (VG's, crosswind STOL, Flaps etc), both have Edo 2000 floats, and both have 160HP Lycomings. However, the one is a real dog compared to the other, you can have identical loads on and the one will get up and go no problem while the other won't even get up on the step. Any ideas why this could be?
 
Use a smart level to check the angle of incidence float top to wing bottom. (there are differences in lengths of rear N-struts that can make a big difference in ability to get them out of the water). Any difference? How about the bottom of the horizontal stabilizer to the top of the floats? (saggy tail?)

What do the bottoms look like?

Is one leaking water, not pumping water out?

Look at the lift strut forks- does one plane have them cranked in or out on only one side?

Check how strait the tail is by measuring from the outside of the aileron bay to the outside of the stabilizer on each side, (that is often a speed changer though).

How about the props? Measure the length, and the actual angles of each blade at different stations- how do they compare.

Engines? One making more power than the other? (check exhaust for blockage per A/D?)

Is one running a short mount, the other not?

Just some places to start looking! Have fun:lol:

One more thing to check- float angles- top of each float vs. float on other side, distance variation front of one float to tail of other. Are they parallel and square.

Very small change makes a big difference in them draggy beasts. I ran out of time once so splashed the cub back to the water with the tail of the plane vs tail of floats 1" off to the side. The plane flew 5 mph faster than I ever had!

Last summer they did a part change on wire pulls on the 185, said it was good to go. I brought it back after a couple of landings and asked them to re-set all the rigging on the floats. After a discussion, (pilot vs. mechanic knowledge and linage type:oops:) they decided to at least look at it.

It flew better and faster the next day.

1/2" difference in the square will make a difference. Most planes have been tweaked, floats also, so you might need to experiment to find what works best for that combination.:-? Takes time but worth it in the end.
 
..... while the other won't even get up on the step. Any ideas why this could be?
What Tango says and if it won't even get on the step then the bottoms of the floats could be distorted generating water drag and/or the engine is not producing as much power. Are there repairs on the bottoms of the doggy floats? Hydro booster missing?

Pull them both out of the water and compare the bottoms of the floats. If they are off floats for the winter swap the floats in the spring to see what happens.

Exchange the props to see if that makes a difference.

Do the throttles on both carbs go all the way open?
 
I agree with the above as well as checking rgging and actually the angles of fuselage and wings with a smart level. One could have been wrecked and fuselage not repaired correctly.
 
That is exactly why I suggest that he swap the props between the two airplanes.

Easier and faster than measuring them at all the stations, but, what if it is a multiple problem issue- little less power in one engine, little less prop thrust, little rigging?

Just switching props might tell lots, just not the entire story. Measuring will.
 
I don't think there is a power issue, still worth looking at though, as the machine is pretty amazing once on skis or wheels. The props may or may not have been switched already, I am not positive, I know they have talked about it 8). The short or long mount I have no idea about, very possible one is long and the other short. Is it obvious when they are sitting on the ground if it is long or short?

Wing incidence has been played with but washout hasn't.

The floats are pretty tight, only one compartment each side that really has any water in it ever, and its never significant. They are pretty low time too, apparently they are the last set of Edo 2000's made in America :p. That doesn't mean there is no damage on them though, I'll look into that.

Rigging seems like the most likely option to me right now. The plane was wrecked at least once before he bought it and then it was looped a couple years ago. Both times required fuselage repairs and/or tail repairs. I have been told these Cubs never fly the same after a wreck.

Also, I just remembered that the doggy cub actually will cruise a few MPH faster than the other one.

I wish the lakes weren't all frozen now, I'm curious to try some of these things out. ;-)
 
Quickest and easiest to verify if they're on the bank but still on floats is to check the rigging on the floats of the doggy airplane.

Tape a hook of some sort to the en of a very long tape measure. Hook the tape to the tie down ring on the right wing, then measure the distance from there to a fixed point on the toe of the right float.....some folks use a specific rivet, or the front of the forward cleat. Write that measurement down.

Move to the other side and measure the same space on the left float, using the same points. Write that down.

Now, measure from each tie down ring to a fixed point on the very aft end of each float, and write each of those down.

Compare the front distances with each other, and the aft distances with each other. They should be VERY close to each other, side to side. I like them within 1/4 inch, side to side, but 1/8 is doable and better.

Finally, measure the length of the two diagonal wires that run between the spreader bars.

If any of those measurements are off, you'll need to adjust the rigging to make them so.

That is the most likely culprit if the planes perform equally on wheels but not floats. I've seen a lot of floats that were sloppily rigged, and that's not acceptable to me. It's not that hard to rig them right. It's also easy to get them out of rig when removing and re-installing them. Check the rig closely each time the plane goes back on floats.

Finally, not likely, but I've seen float rigging that was sold by a company with the wrong length aft struts for the aircraft. I've also seen one set of rigging (new from manufacturer) where the aft struts were different lengths, and the mechanic installed them that way. Didn't fly for ****.

So, measure all four aft struts from the two sets of floats and verify they're all the same.

Lastly you said that wing incidence had been played with. Exactly how was that accomplished? To the best of my knowledge, wing incidence (the relationship between the wing and fuselage) is fixed on these planes, and cutting and welding would be required to "play with" that.

But from the sounds of it, you have a float rigging issue.

MTV
 
Or maybe it's the pilot. Mike (electricsnail) and I fly the same seaplane, and he always get off the water quicker than I can.

Someone had to say it......

Ken Cornell
 
What Mike says is correct. Here is something else. You said that these floats were the last ones built in America which tells me that they were assembled not long ago. Some people don't read the installation drawings correctly when assembling these floats. I have found the spreaders attached to the floats incorrectly. The special bolts which go into the floats to which the spreaders attach have a chamfer on one side. (Two of these bolts at each end of each spreader. Eight bolts in total.) The front ones have the chamfer on top and the rear ones are installed with the chamfer on the bottom. When these bolts are installed incorrectly, the floats do not track correctly nor are they rigged to the fuselage correctly. The spreader drawing clearly has a note to this effect. If need be I could rifle through my drawing pile to come up with the drawing number. Even when these bolts are incorrect you can get the proper measurements as Mike has described.

One note here is that if the plane will not get on the step with a load and the other plane will, it has nothing to do with the airplane's rigging. It's either power or floats. There is a possible CG situation, but you said that they were loaded the same.
 
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The faster cruise speed but won't get out of the water is an important tell!

Now you are getting into the angle of incidence of the wing to the float. Could be the floats not square, but I would guess that the angle of the floats to the angle of the wing is flat.

Rear spreader bars a little bit longer can destroy performance off water.

Again, being square is the best usually, but my experience is that you need to test the floats on both sides of perfect square (as MTV describes) to find the best. It could be a square issue, but good speed and bad water make me think you have an N strut length issue. Measure all of the N strut lengths, see if there is a difference from the top of the floats to the longeron, and if the angles might be different.
 
The faster cruise speed but won't get out of the water is an important tell!

Now you are getting into the angle of incidence of the wing to the float. Could be the floats not square, but I would guess that the angle of the floats to the angle of the wing is flat.

Rear spreader bars a little bit longer can destroy performance off water.
This is exactly what happens when the chamfered bolts are incorrect as I have described above. The relationship of the floats to the fuselage is different as those bolts have an offset hole where the strut fitting attaches.
 
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Or maybe it's the pilot. Mike (electricsnail) and I fly the same seaplane, and he always get off the water quicker than I can.

Someone had to say it......

Ken Cornell

HaHa unfortunately I don't think it is. My boss who has about 14,000 hrs of experience, pretty much all in floatplanes, flies both planes and its clearly an issue with the airplane.
 
Thanks a lot everyone for the help! I have a place to start investigating now. I really hope we can get it figured out and have another performer in the fleet!
 
Be sure to check back with the results of your investigation. It could be a help to others here to know the specific finding.
 
This is exactly what happens when the champhered bolts are incorrect as I have described above. The relationship of the floats to the fuselage is different as those bolts have an offset hole where the strut fitting attaches.

Very nice to have a theoretical idea, and have someone give a mechanical reason to support it!

This is a nice thing to know, I will watch that on 2000's in the future
 
Out of ignorance and curiosity I have to ask...what bolts have the special chamfer and where do they go? Maybe a pic from the prints? Old guys have to know about these things to teach the young.

GAP
 
Out of ignorance and curiosity I have to ask...what bolts have the special chamfer and where do they go? Maybe a pic from the prints? Old guys have to know about these things to teach the young.

GAP
Start with this: http://www.kenmoreairharbor.com/upl...per_pa-18_series_aircraft_extruded_struts.pdf
Look in the lower right circle of figure 1 at item 4-7. On page 2 item 4-7 is 88-C-706 eyebolt.
EDO drawing 89-C-703 "installation mod 89 spreader assy" This drawing is included with the installation package for ALL 1650, 2000 and 2130 float installations. That means ALL installations of these models.
On this drawing there is a note: "Note - shown for front spreader-reverse eyebolt with shank on these lines for rear spreader, with chamfer facing down."
 
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Do a pull test on both aircraft, then you will know if it`s a power problem or rigging problem, easy to do.

Claude
 
Out of the hole performance is almost certainly a thrust issue. In flight differences would be rigging. If one plane has a healthy 8241 and the other a well worn toothpick or a courser pitch the flat prop will spank it out of the hole every time while flight performance won't be much different. A less powerful engine would have reduced performance in every phase.
 
Now I'll have something to do next summer when walking by 2K's on Cubs...checking on the chamfers. Wonder if the hole's offset from the C/L of the eyebolt that attaches the spreader fitting to the barrel? Must be something like that.

GAP
 
Now I'll have something to do next summer when walking by 2K's on Cubs...checking on the chamfers. Wonder if the hole's offset from the C/L of the eyebolt that attaches the spreader fitting to the barrel? Must be something like that.

GAP
That's exactly what it is. It is not exclusive to Cubs, it's all 1650, 2000, and 2130s.
 
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