Lockheed Electra 10E Special - NR16020

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The Lockheed Electra was designed to carry 10 passengers and 2 crew.

Joe Gurr: "The airplane, a Lockheed No. NR 16020, was really fascinating. It was obviously engineered and equipped for very long non-stop flights. The fuselage was solid gasoline tanks, two rows of them, from bulkhead to bulkhead, with just enough room between the tops of the tanks and the overhead for a person to be able to crawl from the stern to the cockpit on their tummy. Located in the stern, just opposite the main entrance door was a small table for the navigator's use. The radio transmitter was installed above this table. Jammed in what space they could find was a toilet."[1]

"Gurr's recollections do not correspond exactly to what is known about the aircraft: there was just one row of tanks, the transmitter was below the table, and the toilet was in a standard location."[2]
Lockheed Electra 10E Special at Burbank, California on May 21, 1937 just prior to the departure of the second world flight attempt. Noonan can be seen beside the open trunk of his Terraplane roadster. His wife, Mary Bea, stands near the front of the car. Amelia is talking with someone near the cabin door of the airplane.

Contents

Powerplants

"Lockheed had introduced the Model 10A Electra in 1934. Powered by two Pratt & Whitney Wasp Jr. SB engines of 450 h.p., the type enjoyed widespread success as a ten-passenger airliner. Deliveries of the 10E variant, featuring the more powerful 550 h.p. Wasp S3H1, began in January 1936. Earhart’s was the fifth airframe so equipped." [3]

"There's no such thing as a 'Wasp Senior.' The P&W R1340 was known as the 'Wasp.' The Model 10A Electra carried the smaller R985 which was dubbed the 'Wasp Junior' but nobody ever referred to the big engine as a 'Wasp Senior.' All Model 10Es were built with P&W R1340 S3H1 Wasp engines and NR16020 was wearing the same engines it was built with when it disappeared."[4]

Pratt and Whitney Data

From Birch Matthews:

Date Octane Takeoff Rpm MAP Normal Critical Alt.
11/34 80 550 Hp 2,200 34.5" 550 Hp 5,000 Ft
12/35 80 550 Hp 2,200 34.5" 550 Hp 5,000 Ft
12/39 80/87 610 Hp 2,250 560Hp 5,000 Ft
05/41 91 600 Hp 2,250 36.0" 600 Hp 5,000 Ft
11/42 91 600 Hp 2,250 36.0" 600 Hp 5,000 Ft
Data for Dec '35 and May '41 are from P&W power curves for the S3H1/R-1340-AN-1 engines. The remaining three dates represent information taken from P&W data sheets for the S3H1 engine. With the advent of higher octane fuel, Pratt & Whitney was able to increase rated power from 550 to 600 horsepower (with one temporary excursion to 610 horsepower).
Availability of higher octane fuels allowed engine manufacturers to increase manifold pressure through additional supercharging without encountering destructive detonation. Chemical energy of the various octane-rated fuels (about 19,000 Btu/Lb or 138,000 Btu/Gal) was essentially constant even though additives (primarily tetraeythl lead) decreased the detonation tendency at higher manifold pressures. Cruise fuel economy at a given power setting is essentially unchanged with higher octane fuels.
One way higher octane fuel can increase fuel economy is by changing the engine compression ratio. Efficiency increases somewhat by raising the compression ratio. This change was not employed on Amelia's engines, however. The ratio on her engines was 6:1 and remained at this level because she was using 80 octane fuel. As a matter of historical interest, Pratt & Whitney maintained this compression ratio during subsequent years even though high octane fuels became available.
Cruise economy is gained by keeping the manifold pressure up and the rpm low for a desired engine horsepower output. The piston engine is basically an air pump. For instance, Kelly Johnson recommended that Amelia run a fuel/air mixture ratio of 0.072 for all but one cruise power setting. At this ratio, 13.9 pounds of air were mixed with every pound of fuel consumed by each of Amelia's two Wasp engines.

The Bureau of Air Commerce "Aircraft Inspection Report," dated 19 May 19 1937, lists the engines as Pratt & Whitney Model S3H1 Spec. No. 143, Serial No. 6150 and 6149, 550 H.P.

Windows come and go

"As delivered in July 1936, the airplane had only two windows in the cabin. These were the aftmost standard airline windows and were directly opposite each other. Then in January of 1937 a window was installed in the cabin door on the port side and a larger-than-standard window was installed on the starboard side. This last window is the one that was later skinned over in Miami."[5]

Weight and Fuel Capacity

The License Authorization says "Weight empty as equipped 7265 lbs. (see equipment reverse side)."

On the reverse side it says, "Gas tanks: 6 in wing @ 394 gal. 6 in fuselage @ 754 gal." That's a total of 1148 gallons.

This was when the airplane was registered as R-16020 and it was sometime before 27 November 27 1936 when the total fuel capacity was increased to 1151.

For the complete story of the development of the fuel system, see "Research Bulletin: The Fuel System of NR16020."

At the bottom of the reverse side of the License Authorization it says:

The following values shall be considered when computing the gross load:

  • Gasoline at 6 lbs
  • fuel oil 7 lbs
  • Lubricating oil 7.5 lbs per gallon
  • Weight of equipment not listed above
  • Passengers and crew at 170 lbs each
  • Baggage and cargo at actual weight
Item Weight
Airframe 7265
1151 gallons of fuel @ 6 lbs per gallon 6906
Fuel oil @ 7 lbs per gallon
Lubricating oil @ 7.5 lbs per gallon
Other equipment
2 crew @ 170 lbs each 340
Total:
Baggage and cargo

Earhart said her plane's Gross Weight was about 15,000 pounds when fully loaded with 1150 US Gallons of fuel.

Battery System

Most of the airplane's electrical needs were met by 12-volt batteries. A dynamotor (a DC motor-generator or "dynamo" driven by 12-volt current) supplied high voltage to some of the transmitter's circuits.

Bob Brandenburg, Forum, 30 December 2010.

The batteries were Exide 6-FHM-13-1, 12-volt, lead-acid type, with 85 AH capacity.

Each battery could be switched onto or off the main electrical bus. We assume both batteries were fully charged on arrival at Niku, and that Earhart kept one on the bus, with the other off-bus as a safety standby.

The generator was an Eclipse type E-5, rated to provide 15 volts DC, mounted on the starboard (right) engine. A crucial question for our analysis was whether the E-5 would deliver 15 volts when the engine was idling at 900 RPM. Paul Mantz, Earhart's technical advisor, said in 1937 that the Pratt & Whitney Wasp S3H1 engine--the type used on the Electra--burned 6 gallons per hour at 900 RPM. TIGHAR verified the engine and generator performance in 2009, in an experiment using an S3H1 engine and an E-5 generator. This result established that 900 RPM was the lowest speed Earhart could use for battery charging, and that she would burn 6 gph while doing so.

The time required to recharge the battery after a transmission period, and the associated engine fuel burn, are important considerations in the post-loss signals.

Ideally, we would have the Electra battery charging specifications, but we have been unable to find that information. We could not use a modern 85 AH battery as a proxy because we could not be sure that the the internal resistance and charging efficiency of the 1937 battery were the same as in a modern battery. Therefore, it was necessary to derive the battery's performance parameters from first principles, using published empirical research data for aircraft lead-acid batteries of the period.

Aircraft lead-acid battery cells of the period had a specific gravity range of 1.300 at full charge to 1.110 at fully discharged, with corresponding voltage range of 12.86 volts to 11.81 volts.

The transmission system drew 65 amps at 12 volts, and thus could function at virtually full output even with the battery approaching zero charge. Hence, the combined available "end game" battery charge--after all engine fuel was exhausted--was 170 AH, if both batteries were fully charged at fuel exhaustion.

Low-voltage and high-voltage circuits
Source Comment
Michael Everette, NR16020 Radios: Technical Analysis The transmitter operated from the 12-volt DC electrical system aboard the aircraft. The tube filaments and the relays in the control circuitry were powered directly from 12 volts. High-voltage power for the tubes was provided by a dynamotor, a motor-generator unit which operated from the 12-volt system and produced 1050 volts DC at approximately 300 milliamperes.

Primary power requirements for the transmitter were approximately 11 amperes[6] on standby (tube filaments alone), and 65 amperes on transmit using voice (tube filaments, relays, and dynamotor). The tube filaments were energized continuously in standby mode; instant-heat tube technology had yet to be developed. The dynamotor operated, in voice mode, when the press-to-talk circuit was activated using the microphone button. When the transmitter was switched to C-W mode, the dynamotor ran continuously.

Chuck Varney, Forum, 15 July 2011 The transmitter was not powered by 1050 volts, per se. The transmitter received 1050 volts (at pins 13 and 16) from the dynamotor when it was activated for transmitting. This voltage was applied to the plates of the three tubes comprising the 1st and 2nd RF amplifiers. It was dropped across series resistors R10.1 and R10.2 to 380 V and applied to the plates of the oscillator and audio amplifier tubes (one each). That's all that 1050 volts was used for.

The transmitter received 12 volts from the battery for the tube filaments (pins 3 and 4 for +12 V and pins 1 and 2 for -12 V, or ground)--regardless of whether the transmitter was in standby (in which case it drew ~ 6 A), or in transmit (for which the average draw was 10.6 A). The transmitter received 12 volts from the battery on pin 5 for relays S4, S5, and S6, and for the oscillator crystal heaters if they were employed. Again, for both standby and transmit.

H. K. Morgan's Aircraft Radio and Electrical Equipment (1939), p. 144
13C and 13CB Western Electric Transmitters
  • Power supply voltage: 12 volts
  • Power consumption--standby: 6 amperes, approximately
  • Power consumption--modulated: 65 amperes, approximately

The credible post-loss signals occurred in clusters, or blocks. We assume that Earhart kept the engine running to operate the generator continuously during each block, rather than start and stop the engine for each transmission. We have developed a computer model that does the time line bookkeeping, giving the battery state of charge (SOC)--and fuel remaining--after each credible signal, and after a user-specified recharging period following each signal block. After we have identified all the credible post-loss signals, we'll plug them in to the model and see how battery charge and fuel consumption behaved over time.

Of course, the amount of fuel Earhart had on arrival at Niku depends on when she landed. We don't know exactly when that was , but we know the latest possible arrival time, which was constrained by tide depth on the reef. The maximum safe water depth for landing the Elctra was 6 inches (0.15 meter). Signal propagation analysis for the last Earhart signal heard by the Coast Guard cutter Itasca on July 2, 1937, gives us a bound on Earhart's distance from Niku then, and thus her earliest possible arrival time. These two limits bound the uncertainty of fuel remaining on arrival, and factor into our analysis.

The tide level also constrained when Earhart could run the engine for battery charging. The required propeller tip clearance was 24 inches (0.6 meter), so engine operation was impossible when the water level exceeded that limit.

Now, let's take a brief look at the dynamics of the electrical load and battery recharge.

The generator output current was regulator-limited to 50 amps.

We assume an ambient current load of 8 amps during each signal block: radio receiver on (1 amp), transmitter in standby (6 amps for vacuum tube filaments), and the cockpit instrument lights (2 amps).[7]

The transmitter drew 65 amps when transmitting, raising the total current load to 68 amps.

With the transmitter in standby, the generator could supply the 8 amp ambient load and have current to spare for charging the battery. But when the transmitter was keyed, the generator could supply only 50 of the required 68 amps. The battery would supply the remaining 18 amps, losing 18 ampere-minutes (0.3 AH) of charge for each minute of transmission time. About 13 minutes of charging time was needed to restore charge for each minute of transmission time, if the starting battery SOC was near the top of the exponential charging curve. At lower starting SOC, where the curve is steeper, the recharge time was on the order of 2 minutes for each minute of transmission.

Earhart's only way to monitor battery SOC was to watch the generator current output on the ammeter. The current would decrease toward the 8-amp ambient load as the battery approached full charge. But it could take a long time to get those last few ampere-hours into the battery if she wanted to recharge to 100%, and Earhart would have to be careful not to spend her precious fuel too lavishly. We don't know what strategy she used, but we can experimentally investigate the range of options and their effects using the computer model mentioned above.

Buoyancy of NR16020

"The opinon of supposed experts at the time was that, with all those empty fuel tanks, the Electra would float 'indefinitely.' We actually had some calculations run by Oceaneering International in 1991. There were 12 individual fuel tanks aboard NR16020 – three in each wing and six in the cabin. If all the tanks were empty and intact, the 7,000 lb (empty weight) airplane would be 1,200 pounds buoyant. Damage to one, or even all, of the tanks in one wing should not be sufficient to sink the airplane."[8]

N233PB

N233PB-ditching.jpg

Gallery

All drawings, images, and photographs ©TIGHAR 1985-2009. Not to be used, distributed, copied, bought, sold, or otherwise poached. For permission to use images, please email TIGHAR.

References

  1. Joe Gurr to Fred Goerner, 3 May 1982.
  2. Ric Gillespie, 2 March 2009 Forum.
  3. TIGHAR Tracks 1995 11:2.
  4. Ric Gillespie, Forum.
  5. Ric Gillespie, 31 May 2000 Forum.
  6. Brandenburg and others say that the transmitter only drew 6 amps on standby.
  7. These numbers don't add up. It seems that the 8, the 6, or the 2 must be in error. MXM, SJ.
  8. "Forum FAQs."

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