# TIGHAR

## Project Midnight Ghost => General Discussion => Topic started by: Ric Gillespie on March 24, 2022, 01:22:44 PM

Title: Breaking Point
Post by: Ric Gillespie on March 24, 2022, 01:22:44 PM
I'm trying to reconstruct the accident based on the available evidence and I need some help with some engineering calculations.
•  We know the explosion occurred on, or close enough to, the island for significant debris to be left on the island which was collected up by locals over the years.
•  The most likely cause of the explosion was the rupture of the fuel tanks.  Collision with the rocks on or around the island was the most likely cause of the rupture - so the plane probably hit the island.
•  The tanks were protected by the surrounding hull structure which was (roughly) half-inch poplar and birch plywood glued together and crossed at the knots.
•  If we can figure out how fast the plane had to be going or the hull structure to fail, we'll know how fast it was going when it hit the island.
•  According to NACA Circular #50 "Levasseur Transatlantic Airplane", "During static tests the breaking point has been fixed at a coefficient of 12.2 with a load of 6, 790 lb. on the tail skid."  I don't know what that means.  Does it help us estimate what kind of impact it would take for the plywood hull to fracture?
Title: Re: Breaking Point
Post by: Don Yee on March 24, 2022, 03:27:56 PM
I'm trying to reconstruct the accident based on the available evidence and I need some help with some engineering calculations.
•  We know the explosion occurred on, or close enough to, the island for significant debris to be left on the island which was collected up by locals over the years.

Ric,
Is there any evidence that any of the debris was washed up onto the island at a point after the explosion? My recollection was that most of the debris found on the island was small. Could it have floated there based on wind + rise in lake levels? Could the explosion have occurred father from the island?
Don...
Title: Re: Breaking Point
Post by: Ric Gillespie on March 24, 2022, 04:05:31 PM
Is there any evidence that any of the debris was washed up onto the island at a point after the explosion?

No

My recollection was that most of the debris found on the island was small.

Some pieces were said to be small,  but Anthony McGrath told me he recovered a piece 3 feet tall.

Could it have floated there based on wind + rise in lake levels?

All of the pieces were metal.  The piece I found in 1992 is metal.  Metal doesn't float.

Could the explosion have occurred farther from the island?

Anything is possible, but I can't construct a scenario that makes sense.
Title: Re: Breaking Point
Post by: Jeff Christmas on April 16, 2022, 09:56:11 PM
Ric,

A couple of weeks ago I downloaded a copy of NACA Circular #50 on the NASA Technical Report Server (NTRS) to see for myself what it said about the “coefficient of 12.2”, and I discovered that it said exactly what you said it said.  It is a cryptic sentence.

I then decided to track down the source material in hopes that it would bring clarity.  The title page of NACA Circular #50 states “From "L'Aeronautique," June, 1927 and "L'Aérophile," June, 1927”.  I found and translated those articles and discovered that neither contained refences to load testing.

This evening I decided to pick up the trail again.  I looked further back into earlier issues of L’Aeronautique.  Much of the Jan 1926 edition is dedicated to marine aircraft.  In it there was the following page about the Levasseur observation plane on which the White Bird is based:

https://gallica.bnf.fr/ark:/12148/bpt6k6555711j/f89.item

The article / advertisement is in three languages.  The language in NACA Circular #50 seems to be taken directly from the English blurb in this ad.  My interpretation of both the French and the Spanish blurbs (with the help of Google Translate) is that 12.2 is something like a factor of safety.  I think we may be dealing with a poor English translation.  Perhaps French and Spanish speakers on the Forum can look this advertisement over and comment on what was meant.

Jeff Christmas
Title: Re: Breaking Point
Post by: Ric Gillespie on April 17, 2022, 08:32:03 AM
Thanks Jeff.  Good research. The illustration alone is worth the price of admission.  That thing sat low in the water.
Title: Re: Breaking Point
Post by: Simon Ellwood on April 17, 2022, 12:24:10 PM
Reading it literally, it sounds like they established the load safety factor of the rear fuselage at 12.2 times its 1G normal load, with it breaking at a load of 3080kg applied to the tail skid to perhaps simulate tailplane loads on the rear fuz. If this is correct then it would follow that the normal 1G download on the tailplane would be 3080/12.2 = 252kg

A 12.2 factor overall sounds very high though for a wooden aircraft of that era (akin to modern jet fighters) - but we're talking here of a single test on the rear fuselage only, which is probably one of the strongest parts of the airframe - wings spars were usually the weak point on these wooden bi/triplanes.
Title: Re: Breaking Point
Post by: Ric Gillespie on April 17, 2022, 01:34:35 PM
A 12.2 factor overall sounds very high though for a wooden aircraft of that era (akin to modern jet fighters)

Consider this: The fuselage of a land plane only has to withstand a greater-than 1G load at the moment of touchdown and then only at the landing gear attach points which are beefier than the rest of the fuselage.  In a water landing, the entire hull has to be able to withstand hitting the water, and water at typical landing speeds is like concrete.  I remember landing on choppy water in my father's Seabee, and much later in a Lake amphibian, and being surprised at how loud it was and how the whole airplane shook with the impact.  A 12.2 factor might be credible.
Title: Re: Breaking Point
Post by: Don White on April 17, 2022, 05:13:17 PM
Looking at the picture in the ad, I had several thoughts in response.

I asked myself, why didn't they just make it a design that could take off from water too? Yes, it is sitting low in the water -- but that seems to give it greater stability in the water compared to a flying boat or conventional floatplane. Then I thought about how it was intended to be used, as a scout plane from a warship. Such planes, in use by many navies at the time, were launched from the ship, landed on water near the ship, and retrieved by a crane. In normal operation, they would not take off from water. There might be few instances when they would need to. One we know of is when one of the Colorado search planes landed in a lagoon to ask questions, then took off again to continue searching. That might be one of few times they ever took off from water (wondering how much training they even had in doing so). If the engineers designing this plane thought it would never need to take off from water, then this design makes sense (which it didn't to me before).

A prop plane designed for a water takeoff needs to have the engine high enough above the water that the propeller clears. That means either raising the entire plane high up on floats, or placing the engine or engines high up on the airplane. Either would be detrimental to stability and aerodynamic efficiency. This design lets the plane have a more conventional landplane shape with the benefits to performance and range that provides (and have no drag from fixed landing gear), yet still land on water and be recovered by crane. They forgo being able to take off from water but they may have reasoned that in normal operations it would never need to.

The choice of a W-configuration engine makes sense based on the picture, because the bottom of the engine isn't in the water as it would be with a radial, yet it allows for a powerful, multi-cylindered engine that is still relatively compact.

I also thought, were they hoping to sell more of these based on a successful ocean crossing that would demonstrate its superior range? Navies might care a lot about greater range for their scoutplanes.

LTM,
Don

Title: Re: Breaking Point
Post by: Bill Mangus on April 18, 2022, 10:31:12 AM
Scout planes were frequently used during WWII to rescue downed pilots, landing near a floating pilot, pulling them aboard and taking off again.  Read about Charlie Tanner, a Kingfisher scout plane pilot aboard USS Astoria in 1944-45, in Days of Steel Rain by Brent E. Jones.  He details the training Navy pilots received for landing and takeoff from the ocean.
Title: Re: Breaking Point
Post by: Don White on April 18, 2022, 03:49:27 PM
Thanks, Bill for that info which I hadn't known before. So it seems the French designers disregarded that possible mission need. The text in the ad actually refers to the PL4 having the performance of a landplane but the ability to land on water. Of course having no landing gear it couldn't land anywhere else. It was helpless without a boat to tow it back to the ship, as illustrated, as it couldn't taxi. Maybe it had oars for the crew to use in the absence of a towboat. So it's a pretty squirrelly design after all.

LTM,
Don
Title: Re: Breaking Point
Post by: Ric Gillespie on April 19, 2022, 08:45:21 AM
So it seems the French designers disregarded that possible mission need.

You guys are missing the point.  The water-landing capability was purely an emergency measure to avoid the loss of the aircraft in the event it couldn't return to the carrier, just like a parachute is an emergency measure to avoid the loss of the pilot in the event the aircraft cannot land. France did not even have an aircraft carrier when the PL-4 was designed.  The first French carrier, the Béarn (https://en.wikipedia.org/wiki/French_aircraft_carrier_Béarn), was under construction but it wasn't commissioned until May 1927 - the same month the PL 8 tried to fly the Atlantic.
I can't think of any fixed-wing aircraft of any navy at any time, that could operate from an aircraft carrier and also land and take off from the water.
Title: Re: Breaking Point
Post by: Don White on April 20, 2022, 03:48:54 PM
You're right -- I missed that water landing wasn't normal operation for the PL4. The ad certainly contributed to that impression, and that the PL8 was intended for only a water landing on its Paris-New York trip. I see that the PL4 was designed for the carrier Bearn (which was under construction -- conversion from unfinished battleship to carrier -- from 1923 to 1927, when it was completed and commissioned).

It is interesting sometimes when looking at the history of technology to see that for want of one good idea they needed a lot of work-around. The one good idea in this case was retractable landing gear, which were invented a few years later.

LTM,
Don
Title: Re: Breaking Point
Post by: Ric Gillespie on April 21, 2022, 07:36:06 AM
It is interesting sometimes when looking at the history of technology to see that for want of one good idea they needed a lot of work-around. The one good idea in this case was retractable landing gear, which were invented a few years later.

Retractable landing gear is a trade-off. It reduces drag, but the mechanism is heavy and can always fail.  Retractable gear on an airplane that cruises at 100 knots isn't worth the penalty, but drag increases with the square of velocity so as cruising speed increases retractable gear starts to make sense.  The first Lockheed Vegas were produced in 1928 and had a maximum speed of 135 mph and fixed landing gear.  As engines improved and speeds rose, retractable gear started to make sense and by 1930 Lockheed came out with the Altair, a retractable version of the fixed-gear Sirius.

Title: Re: Breaking Point
Post by: Don White on April 21, 2022, 08:51:17 AM
That retractable gear may not be worth the weight and complexity penalties on slower aircraft is why we still see it on small airplanes. I was thinking of the usefulness of retractable gear to make a true amphibious aircraft. However, amphibious capability (with its usual costs to performance) wasn't deemed useful enough for carrier-based aircraft. The PL4 seems to be the only aircraft intended for carrier operations to have water landing ability designed in, albeit only as an emergency measure.

So they took this idea and figured they could give the PL8 a performance advantage, not having the drag of the fixed gear, by dropping the fixed gear on takeoff and planning on a water landing.

Earhart crossover -- Thinking of what difference it might have made, had she taken the advice to use an amphibian instead of the Electra. I'm inclined to think that if she had done everything else the same, not much difference.

LTM,
Don
Title: Re: Breaking Point
Post by: Ric Gillespie on April 21, 2022, 09:09:37 AM
Earhart crossover -- Thinking of what difference it might have made, had she taken the advice to use an amphibian instead of the Electra. I'm inclined to think that if she had done everything else the same, not much difference.

interesting to ponder.  She still wouldn't have been able to find Howland and she would still end up at Niku.  She might have been tempted to land an amphib in the lagoon.  If she didn't hit a coral head and sink, she still would have been able to send distress calls and the plane would not have been washed into the ocean, so the Colorado pilots would have seen the Sikorsky floating in the lagoon and the story would have a happy ending.