AOE3104 Lecture Slides - Doolittles Raid

AOE 3104 Lecture 14:
Takeoff Performance Continued –
FAR Part 25 and Examples
E. D Crede*
Aerospace & Ocean Engineering Department
Virginia Polytechnic Institute & State University
*Special thanks to M. C. Cotting for preparing this presentation.
Lecture 14 Outline
• FAR, Part 25
• Runway Conventions
• T/O Data Standardization
• Doolittle’s Raid
Takeoff Performance
• Takeoff has two distinct phases:
1. Accelerate (on the ground) to a desired speed.
2. Climb to a minimum height, at a required
minimum speed, to clear potential obstacles:
• 35 ft height (civil transports)
• 50 ft height (general aviation and military)
• Required runway length: Total distance to point
where specified height is reached.
• In US airspace, takeoff performance is dictated by
FAR (Federal Aviation Regulation) Part 25.*
*Airworthiness for Transport Category Aircraft
Takeoff Diagram
Takeoff Speed Definitions
• Aircraft Pilot Manuals will list (at least):
V1, VR, and V2
• Values are mandated by FAR Part 25.
• Values are functions of takeoff weight, altitude,
temperature, wind, and runway slope.
• Other critical speeds are derived from these, to
ensure safe margins above Vstall and Vmc both in
free air and in ground effect.
• Stall Speed: The minimum speed at which
enough lift can be generated to maintain flight.
• Minimum Controllable Ground Speed: It is the
minimum speed on the ground for which a
sudden, single engine failure (with the remaining
engines at takeoff power) does not result in loss
of primary flight control.
• Decision Speed: The speed at which there is no
longer enough runway to stop:
– V < V1: An engine failure means stopping on
the runway.
– V > V1: Committed to take off, regardless of a
single engine failure (for multiengine aircraft).
• Rotation Speed: The proper speed to start the
rotation for liftoff.
Vmca :
Minimum controllable airspeed with gear retracted.
(Note: Multi-engine aircraft must remain controllable
in flight, even after loss of an engine.)
• Liftoff Speed: The speed at which the aircraft lifts
off the ground. (The aircraft does not leave the
ground immediately at rotation.)
Vmu :
Airspeed at and above which the airplane can safely lift off the
ground and continue the takeoff – not necessarily VR.
• Takeoff Safety Speed: The proper speed for
climb-out on takeoff (minimum speed at
specified obstacle height):
– 1.2 Vstall: Civil jets and two-engine civil turboprops
– 1.15 Vstall: Four-engine civil turboprops
– 1.10 ~ 1.05 Vstall: Military and general aviation
Runway Conventions
• Runways are marked so that pilots know how far down
the runway they are, and can therefore judge how much
runway has been used for takeoff.
Runway Centerline Markings
Balanced Field Length
• The FAR takeoff field length (or balanced field
length) accounts for engine failure situations.
• Should an engine fail during the takeoff roll at
the decision speed V1, the pilot may elect to:
– continue the takeoff on the remaining engines
– shut down all engines and apply brakes
"Airworthiness Standards: Transport Category Airplanes," FAR Pt. 25 (FAA, February 1, 1965)
Determining Field Length
FAR Part 25 T/O Distance
Lift-off distance:
The function f is depicted in a chart to follow.
FAR Part 25 adds 15% to the lift-off distance to
estimate total takeoff distance
Chart from FAR Part 25
Factors Affecting T/O Distance
Thrust Variations
Wing Loading
Density Altitude
Runway Slope
Runway Condition
Pilot Technique
– Most significant factor
– Toughest to control
Takeoff Data Standardization
• Takeoff testing of aircraft does not occur in an
idealized world.
• Standardization refers measured takeoff distance
to a standard altitude and weight, with zero wind
on a level runway. (Empirically derived.)
• Four steps, applied in order:
Runway slope correction
Wind correction
Weight correction
Altitude correction
Slope Correction
dslope: Takeoff distance found from aircraft test
dlevel: Takeoff distance, corrected for runway slope
µ: Runway slope from horizontal (+/- uphill/downhill)
Wind Correction
dcalm: Takeoff distance, corrected for wind
VW : Wind component along runway (+ for headwind)
VTO : Takeoff groundspeed
Weight Correction
dWnom : Takeoff distance, corrected for weight
Wact : Actual takeoff weight
Wnom : Nominal takeoff weight
Density Correction
½SL : Sea-level, standard day air density.
½TO : Air density during takeoff.
dSL: Takeoff distance, referred to SL ISA conditions.
Standardized T/O Distance
Putting it all together…
• Given a measured takeoff distance dmeas, compute
the standardized takeoff distance dSL.
• This is the takeoff distance in nominal conditions
(level runway, no wind, standard weight, and
standard, sea-level density).
Takeoff Airspeed - VTO
T/O Results for T-38
Standardized Ground Roll Distance - dSL
• FAA:
• Shevell, Richard S., Fundamentals of Flight,
Second Edition, Prentice Hall, 1989
Jimmy Doolittle’s Raid
• Captain Frank Lowe (USN) predicted that Army twin-engine bombers
could be launched from a carrier, under the right conditions.
• Lieutenant Colonel Jimmy Doolittle (USA) planned and executed the raid
using 16 modified B-25B's of the 34th BS, 17th BG flying from the deck of
the USS Hornet (CV 8) - 650 nm from Tokyo
• The raid took place after two months of planning and training with 16 allvolunteer crews, in the aftermath of the attack on Pearl Harbor.
Jimmy Doolittle’s Raid
The two key impacts of the raid, according to J. Doolittle, were:
– “[To] give the folks at home the first good news that we'd had in
World War II [and]
– From a tactical point of view, it caused the retention of aircraft in
Japan for the defense of the home islands when we had no
intention of hitting them again, seriously in the near future. Those
airplanes would have been much more effective in the South
Pacific where the war was going on.”
Jimmy Doolittle’s Raid
B-25B bombers on the deck
of the CV-9, USS Hornet
• 2 US Aircraft carriers were sent on the mission.
• To protect the carriers, and save weight on fuel, the bombers did not
plan to return to the carriers (and give their position away): Crews
were to ditch the aircraft in China and Russia, saving fuel weight.
• A Japanese boat discovered the fleet on the way to the takeoff point.
The planes had to launch 400 miles farther away than planned.
Jimmy Doolittle’s Raid
• The premature mission launched in 30 ft seas.
• 15 of the 16 bombers were able to attack their target;
none were shot down.
• Of the 80 aircrew on the mission, 64 survived and were
able to fight again in WW II.
The Technical Challenge
• How can a B-25 Mitchell take off from an aircraft carrier?
• Early carrier -- no steam catapults.
• Nominal takeoff run for a B-25 : 1,400 ft
• Maximum takeoff run on the carrier: 467 ft
Normal B-25 Weights
• Nominal takeoff weight: 32,000 lbs
• Crew: 6 men (165 lbs per man) : 1000 lbs
• Payload (bombs): 3200 lbs
• Fuel (1000 gallons): ~6750 lbs
(note these numbers are rounded)
Raiders Weights
• Actual takeoff weight: 31,000 lbs
Crew: 5 men (165 lbs per man) : 825 lbs
Payload (bombs): 2000 lbs
Fuel (1241 gallons): ~8375 lbs
Except for one gunner’s station, all guns were replaced
with painted broom handles.
• The heavy Norden bombsight was removed and replaced
with metal crosshairs.
Trimming Takeoff Distance
• Only 1000 lbs of T/O weight removed.
• Where did the T/O distance reduction come from?
– 30 mph headwind
– Full flaps on T/O (increasing CLmax from 1.92 to 2.92)
Standard B-25 Takeoff
Using Anderson’s “constant force” approximation for
T/O distance (in US units):
Lower Flaps to Full B-25
Lowering flaps increases CLMax from 1.92 to 2.92.
Headwind Takeoff
VW = 30 ft/sec (20 MPH)
Mission safety requirement: VW > 20 MPH.
Jimmy Doolittle’s Raid
B-25 References
What Next?
• Landing Distance
Week #5 Reading:
• Anderson: Section 6.15
• Marchman: Chapter 7

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