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Author Topic: Propeller Clearance  (Read 22566 times)

Tom Swearengen

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Re: Propeller Clearance
« Reply #15 on: November 29, 2012, 06:16:20 PM »

That it would seem, would be alot of water on the reef before the props would touch the water.
Many be slightly more clearance on the right side due to elevation change in the slope towards the beach?
Tom Swearengen TIGHAR # 3297
 
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Greg Daspit

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Re: Propeller Clearance
« Reply #16 on: November 29, 2012, 06:53:50 PM »

For now I think trying to get a baseline on flat ground is a good starting point but have some questions for later.
I don't think you can ever tie down exact location, maybe a general area but I thought it might be worth looking for remnants of a stake in one of the cracks in the reef in case they tried to keep it from being washed away.
At what point does the tail start to float and change the pitch and therefore reduce clearance?
Is the analysis of the Bevington photo, possibly showing one strut with blown tire and possibly showing a 2nd tire floating, going to effect the calculations?

3971R
 
« Last Edit: November 29, 2012, 07:01:57 PM by Gregory Lee Daspit »
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Alan Harris

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Re: Propeller Clearance
« Reply #17 on: November 29, 2012, 06:56:09 PM »

A side view where the drawings was scaled to get the cowl to 53.5” then the prop drawn in in at 9’. A line was projected from the wheel and a prop clearance obtained of 3’-0 1/16”. I found it difficult to find where the top of the cowl was and used my best guess.
FWIW, I have a point of convergence with you on this one.  With the admittedly imperfect scale sketches I've played with, the pitch angle of the prop shafts above horizontal was looking like it should be 13 degrees or a bit more.  On your upper version, it measures just a hair over 14 degrees.  That's pretty close, and so I can readily accept what you've done here.  To get a clearance as low as 24" requires a significantly shallower pitch angle than 13 degrees, which very quickly begins not to look like the side-view photographs.

Quote
The head-on picture was re scaled so the prop was 9’-0”. AE ended up being about 5’-7  1/16” tall doing this. The clearance obtained was 2’-3 7/16”. The side view was used  to determine the angle so I could get the prop ellipse. Note, if AE was aligned with the prop at the angle shown, the prop would be in front of her at its lowest point and the dimensions obtained would be different (basically moving the line under AE's feet closer to the bottom of the photo and getting a bigger clearance number.
On first look I think the correction I've "bolded" in your quote above could be significant, and possibly enough to raise your second result much closer to your first.  When I get time I will look into that a little further.

--All disclaimers as to complete accuracy/precision of these photo methods, as previously expressed by me and others, continue to apply--
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John Ousterhout

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Re: Propeller Clearance
« Reply #18 on: November 29, 2012, 07:06:18 PM »

Greg - nice work.  It makes me regret not having CAD at home (and I'm not allowed to do this sort of playing around at work, sorry to say).  I can do the work with pencil and paper, but frankly it will be more accurate to con, er, ask you to do it digitally. Would you be willing to re-do some of the work on the head-on view and calculating the tip clearance for each prop?  Since the starboard engine is a bit closer to the camera than Amelia, and the port engine a bit futher away, their average should be exactly the same scale as Amelia.
Alternatively, but a bit more cumbersome, would be to relocate the line under her feet so that it is in the same vertical plane as the prop hubs and AE.  You show the line under her feet as not quite parallel to a vertical plane passing through the prop hubs (note the apparent distance from each tire to the line that passes under her feet), which as you mentioned under-represents the dimension on the starboard side, and over-represents it on the port side.  You also mentioned that moving the line toward the camera would increase the clearance number, but rotating the line about her feet, rather than simply moving it, would also correct the measurements on both sides and bring it into the same plane as AE and the other prop hub.  Rotating the line will provide a more accurate vertical reference from each prop tip to ground.  The vertical scale will be slightly different under each prop, but luckily the props themselves provide handy references.

LTM
Cheers,
JohnO
 
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Greg Daspit

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Re: Propeller Clearance
« Reply #19 on: November 29, 2012, 07:26:07 PM »

John, I was thinking about doing exactly what you said but was not sure how to explain why as good as you just did. Also trying to determine how to adjust the line at AE's feet so it aligned with the rotated plane was going to be somewhat of a guess on my part and abilities. The first pdf I did, I think was left in another thread, had both props calculated but the line at AE's feet still horizontal. It also had the props wrongly drawn as circles instead of ellipses. I can and will do a calculation with all the methods you mention as best I can.

Alan, you are correct, the angle was a hair over 14 on the computer. I have angle units defaulted to the nearest degree on my settings. I found determining precise edges and angles on these small pictures difficult. I agree 100% with your disclaimer. I will be interested to see the method used to determine how far to move the line so it aligns with the prop at its lowest point in the head-on picture

edit:
I was also going to "eyeball" the center of hub best I can instead of selecting the midpoint of the line between tips. However after thinking about it, this kind of stuff is best left to people like Mr Glickman who have the software and skill and I will not pursue this any more. I also put hyperlinks of where I got the 9'-0" propeller dimension from in my previous post.
3971R
 
« Last Edit: November 30, 2012, 07:58:03 AM by Gregory Lee Daspit »
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pilotart

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Re: Propeller Clearance
« Reply #20 on: November 29, 2012, 10:56:44 PM »

Another factor which must be considered for Propeller Clearance is Tailwheel elevation.

Were they to have broken it off, like the Croydon it would considerably increase prop clearance (as you can see in the "Better than Average Luck" photo on the reef).

Even with an unbroken tailwheel, it would be more stable with the two main wheels 'chocked' after parking it with the tailwheel in some sort of a hole or depression with the nose or tail facing the surf action.
Art Johnson
 
« Last Edit: November 29, 2012, 11:01:47 PM by pilotart »
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william patterson

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Re: Propeller Clearance
« Reply #21 on: November 30, 2012, 01:19:18 PM »

Maybe I am missing something, but if you don't know if one tire is stuck in a hole, or if a tire is flat, what is the purpose of this excercise in mathematics? Personally on the concrete, it looks  to be as high as the bottom of Ms.Earhart's Buttocks. Figure out how long her legs were, and there is your clearance!  ;) That's my scientific analysis.
« Last Edit: November 30, 2012, 03:43:32 PM by william patterson »
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