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Joey Course

Multi-Engine Aerodynamics

FAR Part 23 Certification of Normal Category Airplanes

FAR Part 23

From 23.2005

Certification levels

  • Level 1 - for airplanes with a maximum seating configuration of 0 to 1 passengers
  • Level 2 - for airplanes with a maximum seating configuration of 2 to 6 passengers
  • Level 3 - for airplanes with a maximum seating configuration of 7 to 9 passengers
  • Level 4 - for airplanes with a maximum seating configuration of 10 to 19 passengers

Performance Levels

  • Low Speed - For Airplanes with a Vno and Vmo ≤ 250 KCAS and a Mmo ≤ 0.6
  • High Speed - For Airplanes with a Vno and Vmo > 250 KCAS and Mmo > 0.6

Climb Performance for Two Engines Operating

§23.2120 Climb Requirements

a) with all engines operating and in the initial climb configuration

1) For levels 1 and 2 low-speed airplanes, a climb gradient of 8.3 percent for landplanes and 6.7 percent for seaplanes and amphibians and,

2) For levels 1 and 2 high speed airplanes, all level 3 airplanes, and level 4 single-engines a climb gradient after takeoff of 4 percent

Two Engines Operating

FAR 23.67 Feb 4, 1991- August 30, 2017

Old

(a) For normal, utility and acrobatic category reciprocating engine-powereed airplanes of 6,000 pounds or less maximum weight, the following apply:

(1) Except for those airplanes that meet the requirements prescribed in §23.562(d), each airplane with a Vso of more than 61 knots must be able to maintain a steady climb gradient of at least 1.5 percent at a pressure altitude of 5,000 feet with the -

(i) Critical engine inoperative and its propeller in the minimum drag position;

(ii) Remaining engine(s) at not more than maximum continuous power;

(iii) Landing gear retracted;

(iv) Wing flaps retracted; and

(v) Climb speed not less than 1.2 Vs1

(2) For each airplane that meets the requirements prescribed in §23.562(d), or that has a Vso of 61 knots or less, the steady gradient of climb or descent at a pressure altitude of 5,000 feet must be determined with the -

(i) Critical engine inoperative and its propeller in the minimum drag position;

(ii) Remaining engine(s) at not more than maximum continuous power;

(iii) Landing gear retracted;

(iv) Wing flaps retracted; and

(v) Climb speed not less than 1.2 Vs1

  • The steady gradient of climb at an altitude of 400 feet above the takeoff must be no lesss than 1 percent with the -
  • Critical engine inoperative and its propeller inthe minumum drag position;
  • Remaining engine(s) at takeoff power;
  • Landing gear retracted;
  • Wing flaps in the takeoff position(s); and
  • Climb speed equal to that achieved at 50 feet in the demonstration of §23.53.
  • The steady gradient of climb must not be less than 0.75 percent at an altitude of 1,500 feet above the takeoff surface, or landing surface, as appropriate, with the -
  • Critical engine inoperative and its propeller in the minimum drag position;
  • Remaining engine(s) at not more than maximum continuous power;
  • Landing gear retracted;
  • Wing flaps retracted; and
  • Climb speed not less than Vs1

6000 lbs or less

Table

  • Steady gradient of climb or descent at a pressure altitude of 5,000 ft or less with the -
  • Critical engine inoperative and its propeller in the minimum drag position;
  • Remaining engine(s) at not more than maximum continuous power;
  • Landing gear retracted;
  • Wing flaps retracted; and
  • Climb speed not less than 1.2 Vs1
  • Must be able to maintain a steady climb gradient of at least 1.5% at a pressure altitude of 5,000 feet with the -
  • Critical engine inoperative and its propeller in the minimum drag position;
  • Remaining engine(s) at not more than maximum continuous power;
  • Landing gear retracted;
  • Wing flaps retracted; and
  • Climb speed not less than 1.2 Vs1
  • **For aircraft type certificated prior to Feb 4, 1991, the performance requirement is based on .027 Vso squared versus 1.5 percent

61 knots or less

Part 23 Performance Requirements - 8/30/17 and beyond

After a critical loss of thrust on multiengine airplanes-

  • For levels 1 and 2 low-speed airplanesthat do not meet single-engine crashworthiness requirements, a climb gradient of 1.5 percent at a pressure altitude of 5,000 feet in the cruise configuration(s);
  • For levels 1 and 2 high-speed airplanes, and level 3 low-speed airplanes, a 1 percent climb gradient at 400 feet above the takeoff surface with the landing gear retracted and flaps in the takeoff configuration(s); and
  • For level 3 high-speed airplanes and all level 4 airplanes, a 2 perecent climb gradient at 400 feet above the takeoff surface with the landing gear retracted and flaps in the approach configuration(s)

New

Calculating Single Engine Performance

Calculating our performance loss on a single engine is vitally important to multi-engine operating. There are several different ways to do so.

Performance

Calculating Performance Loss Rule of Thumb

  • 360HP available
  • Assume it requires 40% of the available HP to maintian S&L at Vyse
  • 360 * .4 = 144. Thus, 144 HP required to maintian S&L
  • 360 HP - 144HP = 216HP available for "performance"
  • With an engine out, we lose 180 HP. Therefore, 216HP - 180HP = 36 HP left for "performance" on one engine
  • 36HP/216HP = .16
  • 1 - .16 = .84 or 84% performance loss

Rule of Thumb

ROC Calculation

ROC

ROC x Weight = EHP

33,000

1650 fpm 2 engines 185 fpm 1 engine TO wt. 3730

1650 fpm x 3730 / 3000 = 186.5 185 fpm x 3730 / 33,000 = 20.91

20.91 / 186.5 = 11.2% remaining - 100% - 11.2% = 88.8% performance loss

Via a Drag Diagram

Drag Diagram

Using Performance Charts

A more accurate and calculatable way to do single engine performance is using your performance charts for comparing climb rates. For example,

2 Engine ROC = 1650 fpm 1 Engine ROC = 185 fpm

185/1650 = 11.2% remaining or 88.8% loss

Performance Charts

VMC

Vmc is the calibrated airspeed at which, when the engine is suddenly made inoperative, it is possible to maintain control of the airplane with that engine still inoperative, and thereafter maintain straight flight at the same speed with an angle of bank not more than 5 degrees. The method used to simulate critical engine failure must represent the most critical mode of power plant failure expected in service with respect to controllability.

VMC

Factors Used for Determing VMC

Critical Engine Inoperative

Operating Engine at Max Takeoff Power

Most unfavorable weight and CG

Most critical mode of powerplant failure represented

Maintain control (no dangerous attitudes, no more than 20 degrees heading change

Inoperative Engine propeller in Takeoff Position

NOT more than 5 degrees bank

in ground effect

more than 150 lb rudder input at max power

Flaps in Takeoff Position

Landing gear retracted

Trimmed for Takeoff

Vmc not more than 1.2 Vs1

Factors affecting VMC

There are several different factors that will either raise or lower VMC, making it important for you to understand how each will affect VMC while you're flying with one engine INOP

Factors Affecting VMC

Power & Density Altitude

  • Greater power = Greater Asymmetry
  • Lower temp/altitude (lower density altitude = more power or greater asymmetry
  • Higher temp/altitude (high density altitude) = less power or less asymmetry
  • Less power or higher density altitude = better controllability, but less performance

Power & Density Altitude

Windmilling Prop

  • Windmilling prop = huge amounts of drag
  • πr² can be used to figure out the surface area of the windmilling prop.
  • 74'' diameter prop = 6.16' in diameter.
  • 3.08' radius
  • 3.14*(3.08)² = 29.78 square feet of area.
  • Essentially a wall of drag being caused by a windmilling propeller

Windmilling Prop

Impact of Gear and Flaps

CG Position

Rudder effectiveness increases with a foreward CG vs an Aft CG due to the longer arm

Impact of CG Position

Impact of Bank Angle

Bank angle is introduced to eliminate the sideslipping condition peresent when only rudder is used to maintain a constant heading

Impact of Bank Angle

Loaded weight

Loaded weight has no bearing on the directional controllability of the aircraft until a bank angle is introduced.

Impact of Loaded Weight

Table of Factors

Factors Table

Critical Engine Factors

  • P - P-Factor (yaw)
  • A - Accelerated Slipstream (roll)
  • S - Spiraling Slipstream (yaw)
  • T - Torque Effect (roll)

Critical Engine Factors

P-Factor

P

Accelerated Slipstream

A

Spiraling Slipstream

Spiraling slipstream is dependant the direction of rotation of the propellers. The effect of an inoperative engine is different based on whether or not we a conventional twin design or counter-rotating propellers.

S

Right Engine Inop

Left Engine Inop

Counter-Rotating

Torque Effect

T

VMC vs. Stall Speed

VMC vs Stall

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