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Canopy Pressure Force

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jgnunn

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This is an off the wall question, but I figured someone may have an answer; does anyone know approximately what level of force is trying to pull a canopy off the plane in flight? I am in the middle of making a canopy mechanism for my Skybolt, and I stumbled across some light lever-type latches with a safety catch with a 150 pound rating...2 latches would be required.
 

orion

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Depends on the design, the level of curvature and of course the width. The short answer is "lots". I know that in the case of Glasair the force is significant enough to fatigue and crack the windshield bond if done improperly and newer design had to have mechanical fasteners installed.

If I recall right, Billski has a while ago had several posts discussing this issue. You might want to do a quick search - I don't recall which thread that was in.
 

wsimpso1

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I don't believe in doing engineering calculations involving risks for free, but we can discuss how you can do them for yourself.

I don't recall the thread, but I am sure that you can find my previous posts with a bit of searching.

For best guess, you will need some estimate of V/v over the canopy. First you have to think about the flow fields involved. In low wing monoplanes the low pressure available from the wing wraps over the top of the fuselage and spreads out a bit too. V/v at Vd and max +g there will look a lot like the V/v for the wing. You won't have exactly the V/v of the wing, but it will be fairly close. For a biplane, you will have this effect plus the effect of the top wing also acting on the flow fields, and probably trying to cancel some of the aero loads from the lower wing... Remember that at high AOA, V/v will be quite high forward and reduce towards the trailing edge, the pressure from the wing will spread out some on the fuselage, but the presence of the fuselage will also raise V/v somewhat as the air goes around the fuselage. There is also the effect of the prop making thrust adding to V/v...

Once you have an estimate of V outside the canopy, compute dynamic pressure for that velocity (Bournoulli's equation) and compare that to internal pressure. Multiply the projected area of the canopy by the pressure difference, and that is your total canopy load.

Where to get started on V/v? Check Appendix 1 of TOWS.

Short answer - I seriously doubt that two latches rated at 150 pounds will hold a Skybolt's canopy at Vd and 6 g's. The story is about that the RV6's canopy has literally a ton of force attempting to remove the canopy, and the Long EZ has even more. Need more references? Just ask...

Billski
 

Robby

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Pardon my ignorance but I must be missing something regarding force vectors.

If there is a TON of force trying to lift the canopy UPWARD then what in the name of blueberry muffins do I need WINGS for ????

That amount of force alone would lift the plane !!!!
 

GESchwarz

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A ton seems a little high. These canopies are held on my pretty light brackets, pins, and rollers.

I'm planning on being able to fly with my sliding canopy open, so I have questions about the amount of force encounter while open and at various speeds. I'm sure that there is a open canopy Vne and that some type of perforated wind break would be useful...I think I've seen something like that for open canopies in flight.
 

jgnunn

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Thanks guys. I do have an existing retaining mechanism that I will now keep; I thought it was a little too beefy so was looking for something lighter. I knew there was force acting upon the canopy, but I didn't realize it was so significant.
 

wsimpso1

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Let's remember a few things that are in play simultanously with stuff in the vicinity of the wing:

That "ton of lift" is with the airplane at max positive g and the airplane either at Va (stall AOA or close to it) or at Vd (which is a few percent above Vne);

The belly of the airplane in the vicinity of the canopy is coming up with a significant fraction of that lift in the opposite direction, cancelling quite a bit of it in terms of total lift but providing loads to be carried by all of the structures in between. The wing skins have the same thing going on too - the loads trying to separate the skins can be multiples of the lift made good by the wing.They are real and actually design things like rib spacing, rib stitching and other rib attachment methods, and skin thickness or core thickness in composites. We have talked about these forces elsewhere on the forum,

The wing is generally modelled as if the wing was continuous and acted upon by the aero forces as if the fuselage was not there because, it acts that way - the lift of the wing through the fuselage is applied and carried by the various parts of the fuselage. So, it takes putting the wing on the airplane and then making lift with it to make the these forces as large as they are.

If any of you don't believe it, go run some numbers. Start with an estimate of the wing V/v at Cl of 1.6 (most of the stuff in TOWS is at 0 and 0.21 or something close. To get to high Cl, you have to extrapolate. Then figure for a 300 ft/s airplane, and run that through Bernoulli's equation, and look at the pressures... Then, just use V/v of 1.3 or so for zero g flight at 400 ft/s (Vd). The outside of the airplane is at the numbers you compute, and the inside is at ambient pressure and zero velocity in the same equation.

Billski
 

orion

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The ton of load on the canopy would be at Vne, and it may include the windscreen, but I'm guessing on the latter. However, the magnitude is fairly close to what I've seen as a "limit" design requirement even for moderate performance airplanes.

Regarding operational limits, I've owned several Grummans, all of which have the ability to fly with the canopy open. The allowed opening is only about ten inches and the airspeed limit is somewhere around 115 mph (working from memory here). I have known folks who flew the plane with the canopy all the way back and I've done it once or twice myself but you can do so only with the airspeed limit and only in level flight. But full open there is not much track left engaged so it's not recommended.
 

autoreply

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Rotterdam, Netherlands
I've flown the Libelle sailplane. You can open the canopy by a lever, with a 3:1 ratio or so, it lifts the canopy an inch and a half upwards. The force is pretty low at thermalling speeds (40 kts), but I was astonished by the force required at interthermal speeds (70 kts). Almost impossible to close and we're talking only 80 kts...
 
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