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  #1  
Old 10-13-2016, 01:51 PM
AX-O's Avatar
AX-O AX-O is offline
 
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Default Warning: Suction cup mounts may damage your canopy during prolonged uses

Perhaps a moderator can consider making this a sticky

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All,
Please be careful with your canopy and video camera suction mounts. Or any other suction mounts. Prolonged use without removing the suction mount may warp your canopy.

My aircraft spent about a week outside in 80 degree temps during the Reno races. Our crew was very diligent about only removing the canopy cover (Bruce custom) during races and recovering the canopy (Todd clear canopy) once I got out. The canopy was uncovered probably less than 2 hours total the whole week.

My guess is that the heat over the week in combination with the fact that we never removed the camera mounts allowed the canopy to become somewhat malleable. The suction cup on the mount ended up sucking the portion of the canopy where the mount was attached. I did not notice it until I got home. I now have 2 suction cup size indentions on my canopy.
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  #2  
Old 10-13-2016, 02:43 PM
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rv7boy rv7boy is offline
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Default

Wow, your suction cups must really "suck!"

An RV friend suggested to me several years ago not to leave my GPS suction cup on my 172 windshield for extended periods. I routinely remove it and hang it close by.

Sorry you bulged your canopy...and a big thanks for sharing the experience with us. In my work, we would call this a "Lesson Learned."
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  #3  
Old 10-13-2016, 03:22 PM
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erich weaver erich weaver is offline
 
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Default

That's a real bummer.

What about trying to reverse the process by installing the suction cups on the other side on a warm day? Not sure there is much to lose.

erich
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  #4  
Old 10-13-2016, 03:45 PM
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Default

Or maybe gently coaxing it with suction and a heat gun?
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  #5  
Old 10-13-2016, 03:47 PM
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Ironflight Ironflight is offline
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Default

Sorry to hear that Axel - canopies are not fun to replace.

However, on the plus side, I'd guess that proportional to the size of the canopy, the dimples would be about the size of a dimple to a golf ball? Don't the dimples on a ball reduce drag? Something to measure.....
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  #6  
Old 10-13-2016, 04:33 PM
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Default Golf Ball Dimples...?

I suggest continuing the process by moving the suction cup a couple inches every day, until you have dimples everywhere, just like a golf ball. My guess is you could gain at least 4 knots!

Thanks again for sharing your Reno experience with everyone.
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  #7  
Old 10-13-2016, 04:51 PM
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Weasel Weasel is offline
 
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Default The Old Golf Ball Theory

My theory is the increased drag as a result of making the flow turbulent and making it stay attached farther around the ball, or airfoil is the same as the amount of drag reduced by the attached flow.

If the golf ball dent theory really did reduce drag you would see wing skins like this all over Reno.

The dents keep the flow attached in a more uniform manner and make the ball fly strait.
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  #8  
Old 10-13-2016, 05:21 PM
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Quote:
Originally Posted by Weasel View Post
The dents keep the flow attached in a more uniform manner and make the ball fly strait.
Not too many of my shots support this theory.
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  #9  
Old 10-13-2016, 07:57 PM
larosta larosta is offline
 
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Default Not Just the Suction Cups

I think that it was probably the mass cantilevered off the suctions cups along with the G's Axel was pulling out there around the pylons put too much stress on the plexiglass already softened by aero thermal heating.

On a more serious note:

I looked up the Tg for Poly(methyl methacrylate) (AKA Plexiglass). It is the temperature at which the material starts to transition from the "glass/hard" state to the "soft/malleable/plastic" state.


The following is excerpted for a Wikipedia article on Poly(methyl methacrylate), https://en.wikipedia.org/wiki/Poly(methyl_methacrylate).

"The glass transition temperature (Tg) of atactic PMMA is 105 °C (221 °F). The Tg values of commercial grades of PMMA range from 85 to 165 °C (185 to 329 °F); the range is so wide because of the vast number of commercial compositions which are copolymers with co-monomers other than methyl methacrylate. PMMA is thus an organic glass at room temperature; i.e., it is below its Tg. The forming temperature starts at the glass transition temperature and goes up from there."

I am not a materials expert but I have spent a lot of time in investigations as to why things like this happen and working with some very smart scientists and engineers trying to find the cause and corrections for problems like this.

Here are some things to keep in mind:
- There are aircraft parked all over the southwest with and without covers during the summer months and we do not see the canopy material reforming because of the stresses from the suction cup mounts or the stress areas from the canopy mounting structures.
- Temperatures inside a cockpit on a aircraft with a sealed/non-vented cockpit with a canopy similar those on RV's has been measured at over +160 deg F in direct sunlight on a mid July day in the Mojave desert. This data was taken from an extensive study on cockpit heating several years ago. No canopy material deformation was observed.
- I was in the pit area during the race week and I doubt that the temperatures ever got high enough to soften the material to the point where it would flow into a deformed state. Not saying that it did not deform under the conditions on the ramp there at Reno but I would be surprised if this was the root cause of the deformation.

With those observations in mind some other things to think about:
- Could your canopy material be something other than plexiglass.
- I have noticed that some of the suction cups I have used have material that is loaded with plasticizers that appear to be used to keep the suction soft and compliant. Could this chemical compound have migrated into the canopy material itself causing it to soften, deform, or change its properties?
- Could the location/geometry of the suction cup have caused the sunlight to be focused, concentrated, or integrated over time causing a hot spot that was higher than the Tg?

Just some random things to consider. At a minimum I would give any suction cup a good bath in soap and water to at least remove most of the chemicals that may be on the surface before mounting them.

See you next year in Reno.

- larosta

Last edited by larosta : 10-13-2016 at 09:01 PM. Reason: Added content
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  #10  
Old 10-14-2016, 04:01 PM
SampsonSurgical SampsonSurgical is offline
 
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Default

Quote:
Originally Posted by AX-O View Post
Perhaps a moderator can consider making this a sticky

Break

All,
Please be careful with your canopy and video camera suction mounts. Or any other suction mounts. Prolonged use without removing the suction mount may warp your canopy.

My aircraft spent about a week outside in 80 degree temps during the Reno races. Our crew was very diligent about only removing the canopy cover (Bruce custom) during races and recovering the canopy (Todd clear canopy) once I got out. The canopy was uncovered probably less than 2 hours total the whole week.

My guess is that the heat over the week in combination with the fact that we never removed the camera mounts allowed the canopy to become somewhat malleable. The suction cup on the mount ended up sucking the portion of the canopy where the mount was attached. I did not notice it until I got home. I now have 2 suction cup size indentions on my canopy.
I have run into this in the past with a car window believe it or not. In that case I think the problem was precipitated by a black cup absorbing too much radiant heat, and the polymer layer in the windshield glass melted. Was your suction cup black?
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