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Alternative canopies?
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Author:  cluttonfred [ Mon Mar 21, 2016 1:31 am ]
Post subject:  Alternative canopies?
The T-18 canopy design seems to be pretty universally accepted by builders, but does anyone know of any alternative designs that have been tried either to 1) increase headroom and elbow room and/or save weight over the stock designs? Perhaps retaining the original high-back turtledeck and using a one-piece blown bubble on a frame? Cheers, Matthew
Author:  jrevens [ Mon Mar 21, 2016 2:35 am ]
Post subject:  Re: Alternative canopies?
Matthew- just a couple of comments: The existing 2 piece windshield/canopy design would be pretty hard to beat for a number of reasons. There is no extra weight in a frame for the windshield - it is a roll bar structure that most people would agree is a pretty important part. There is no other "frame" for the windshield - it attaches directly to the fuselage skin. Secondly, the canopy portion already has a very light aluminum frame structure. Additionally, the canopy shape was very carefully designed by John Thorp, and not only is it good looking (several other homebuilt designs have used it), but it also adds considerable aerodynamic lift. It is extremely doubtful, IMHO, that any minor weight savings in a frame for a blown bubble would even come close to the loss of lift. Some have raised the canopy to increase headroom. I've never considered it a problem and I'm 6'3", but someone with a longer torso than I might appreciate that.
Author:  cluttonfred [ Mon Mar 21, 2016 5:15 am ]
Post subject:  Re: Alternative canopies?
Thanks, John, that's very helpful, I hadn't thought of the canopy providing significant lift. Since I am interested in building light and simple, I am more drawn to the original T-18 and was thinking that a slightly bulged, one-piece bubble (but still with a plain roll bar of the stock height and position) might be lighter and add a little elbow and head room over the stock arrangement, But if you have no trouble at 6' 3" then since I am about 5' 10" and my son 6' I don't think headroom will be an issue.
Author:  jrevens [ Tue Mar 22, 2016 1:38 pm ]
Post subject:  Re: Alternative canopies?
Matthew & other readers,

It has been brought to my attention, by a friend, that the Thorp canopy does not actually create much (if any) lift, as evidenced by the fact that the canopy tries hard to close at anything other than idle. To quote: "As designed, there should be little lift generated as John balanced, or better, coordinated, the pressure distribution curves.

The problem with lifting bodies, as explained by John, is that they can generate some really high cooefficients of lift, (above 3.0) but that comes at a much, much greater (than proportional to the) increase in drag."

I was under the impression that the closing tendency in the 3 point attitude, while taxiing, was due to the lift created. In flight, the canopy also tries to remain closed at a nose down attitude of course. I guess that I had never really thought about this issue much, or analyzed it properly. I'm hoping that maybe this will generate some interesting conversation, and I apologize for opening my mouth before engaging my brain. :-X
Author:  lance38dt [ Tue Mar 22, 2016 5:31 pm ]
Post subject:  Re: Alternative canopies?
All good info John

Lance
Author:  mattst18 [ Tue Mar 22, 2016 8:03 pm ]
Post subject:  Re: Alternative canopies?
I always assumed the positive pressure at the rear of the canopy is what pushes the canopy forward. It sure makes for great airflow through the rear vent. :)
Author:  flyingfool [ Wed Mar 23, 2016 12:30 pm ]
Post subject:  Re: Alternative canopies?
Seems to me that NOT having the canopy produce lift also eliminates in the need for massive strong locking and hold down mechanisms. And if those mechanism's fail, then the canopy departs. Along with the lift it "was" producing.

On the other hand, if a person had to bail out of the plane via parachute, having the canopy to easily depart would be a good thing.

I have original believed that the pressure or force that keeps the canopy closed was due to poor airflow. However Lee sent me a very old picture with tuffs of yarn over the entire aircraft including the canopy. And the picture pretty clearly shows that the airflow seems to remain perfectly attached and in line along the full length of the canopy top. So a picture is worth 1,000 words.

So the canopy closing phenomena is clearly a pressure thing, not an airflow thing. And that the canopy as designed is very clean aerodynamically.

The theory I have is that the positive pressure MAY result from the narrowing of the fuselage both in height (from the max height of the canopy) down to the turtle deck and along the canopy and fuselage sides. This results due to the airflow converging at a point directly behind the canopy since all the air is remaining attached (more or less) to the fuselage/canopy sides and top. Therefore resulting in a lot of air molecules trying to fill a smaller volume of space. More molecules in a smaller space is almost the textbook definition of what would increase pressure (relatively).

My only complaint if there is one about the canopy is that I wish it would open wider. But it appears the modifications to make that happen are not worth the effort.
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