AOE 3104 Lecture 14:Takeoff Performance Continued –

FAR Part 25 and Examples

E. D Crede*

Aerospace & Ocean Engineering DepartmentVirginia 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.

VMCG V1 VR VLOF V2

Vstall

• Stall Speed: The minimum speed at which enough lift can be generated to maintain flight.

VMCG

• 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.

V1

• 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).

VR

• 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.)

VLOF

• 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 theground and continue the takeoff – not necessarily VR.

V2

• 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.

59’120’

Runway Centerline Markings

http://www.pilotfriend.com/training/flight_training/communication/rnwy_mark.htm

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• CLTO

• Density Altitude• Winds• 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 testdlevel: Takeoff distance, corrected for runway slopeµ: Runway slope from horizontal (+/- uphill/downhill)

Wind Correction

dcalm: Takeoff distance, corrected for windVW : Wind component along runway (+ for headwind)VTO : Takeoff groundspeed

Weight Correction

dWnom: Takeoff distance, corrected for weightWact : Actual takeoff weightWnom : 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).

The“Football”

Standardized Ground Roll Distance - dSL

Take

off A

irspe

ed -

V TO

NOMINAL CURVE

Example: T/O Results for T-38

References

• FAA:

http://ecfr.gpoaccess.gov/cgi/t/text/text-idx?&c=ecfr&tpl=/ecfrbrowse/Title14/14tab_02.tpl

• 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 all-volunteer crews, in the aftermath of the attack on Pearl Harbor.

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

• 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.

B-25B bombers on the deck of the CV-9, USS Hornet

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.

Jimmy Doolittle’s Raid

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

• http://www.specwarnet.net/miscinfo/doolittle.htm

• http://www.fighterpilotsusa.com/b25.html

• http://www.militaryfactory.com/aircraft/detail.asp?aircraft_id=81

• http://www.b25.net/pages/dooraid.html

• http://www.defenselink.mil/home/features/2006/DoolittleRaid_rev/Main.html

• http://www.youtube.com/watch?v=YWQ_fiaOToA

• http://www.chinfo.navy.mil/navpalib/ships/carriers/doolitl.html

What Next?

• Landing Distance

Week #5 Reading: • Anderson: Section 6.15

• Marchman: Chapter 7