http://www.eaa.org/experimenter/articles/2009-08_drag.aspOr go to www.N17HH.net

Power & Drag for the Common ManSome old things and new ways of seeing them

All drag curves are the same shape

A “model” can be deduced

New methods of getting correct data (vs CAFE)

A new spreadsheet integrates it all

Using them to get & validate test data

Q & A

The Universal Drag Curve

The key

point

Drag Curve Key Facts

At Vld, parasite and induced drag are equal

Therefore if you know total you know each

Each changes (up or down) as speed squared

From key point all points are known

Vms x 1.31607 = Vld

THP is drag times speed (more later)

Finding Vld – “Method X”Your best L/D speed: x axis

CAS: find with GPS & density altitude

Propless glide

Use Norris's Zero Thrust Device (CAFE)

GPS glide ratio, partial power

Fly at Vms, level flight (min. pwr.)Demonstrated equal to Zero Thrust

Find the Sink-Break point (Norris's R/mile)?

Closed throttle or engine off NO GOOD

Use corrected IAS for CAS

3 Methods: Find Drag by Sink

Direct

Propless glide

Zero Thrust Device

New Sink-Break-point method ?

Place Curve on Y axis

Thrust and Gravity

Gravity works like thrust.At any given speed drag is same.

Thrust plus ? Equals drag

3 Methods: Find Drag by SinkMix power & gravity: 1 example

3 Methods: Find Drag by Sink

Cheat: use fuel flow at known TAS

Top cruise, => 8,000' DA best

Assume / Estimate SFC (lowest or best power)

Assume / Estimate PE

Change sink to match actual fuel flow

Use for reality checks Use to find where best PE (series)

Place Curve on Y axis

SFC Estimating

Specific Fuel Consumption Pounds per HP per hour 6 pounds per gallon – close enough

Best Power: 0.50, usually, .48 for mine No EGT? Lean cruise: about 0.45 LOP: as low as .38 but .40 works best Peak – what manufacturer says? .43 for me

Prop(ulsive) Efficiencysee www.PropellersExplained.com PE is not constant

PE is lower than pure prop efficiency

PE is (thus) lower for Luscombe than RV

Luscombe PE: 63~75%, C-150: 66~80%

RV PE often =>85%

We cannot measure pure prop efficiency No wind tunnels available anymore Airplane/propwash factor always there We CAN approximate PE with this “model”

Power, Drag, THP vs BHP

Thrust Horsepower is drag x speed Thrust Horsepower is sink x weight Gravity & thrust work equally!

THP = weight x sink ft per min / 33,000 THP = TAS ft per min x drag / 33,000

BHP is THP / PE (always < 1.0) In other words, BHP is larger number

Fuel Flow

Working from BHP: Fuel flow = SFC * BHP / 6 Example: 0.5 * 100 / 6 = 8.33 gph Example: 0.45 * 75 / 6 = 5.625 gph

Working from THP: Example: 0.5 * (135/0.85) /6 = 9.56 gph

PETHP

Using the SpreadSheetEntering Data

The spreadsheet implements the “model” Any change also changes something else Data for “Start Here” sheet

Vld in CAS mph Weight ( for Vld and for given test) SFC estimate Propulsive Efficiency estimate Pressure altitude & OAT Data for “my plane” sheet: sink at Vld

Using the SpreadSheetValidating Data

The spreadsheet implements the “model” Any change also changes something else

Built-in calc's for weight, altitude

Accuracy is critical! - use GPS, etc.

Check Vld by using >1 method

Use multiple SFC's, PE's, speeds, alt's for level flight data

Results for drag & PE must be reasonable If known, compare to similar planes

Summary

Drag/power curves are universal

Results should conform to that model

New methods placing yours: X & Y

CAFE for the rest of us – reasonable

Spreadsheet tool integrates it all

Open invitation

Q&A

Back Up Slides

Finding Sink Rate:Sink-Break (untested)

All glides to be at Vld, Use GPS for VSI

With throttle closed, note RPM, sink At low power, note RPM, sink Reduce power by 10 rpm, note, repeat.. Find the point at which the prop goes from

thrust to drag and note sink rate.

3 Methods: Find Drag by Sink

Place Curve on Y axis Mix Power & Gravity

Fuel flow proportional change Constant Power, vary airspeed Computed power change (with MP only)

Finding Sink Rate:Power Proportion & Gravity

All glides to be at Vld, Use GPS for VSI

Stabilize in level flight, note power data Reduce power about 1/5 to 1/7 & note Stay level & note change in airspeed &/OR (better) hold IAS, note sink rate Compute! Refer to curve

Finding Sink Rate:Constant Power & Gravity

Fly at Carson's (Then faster..) For each speed, note power data Change pitch while holding power constant Note change in airspeed and sink/climb Refer to curve and compute

Airspeed Key Facts

CAS is what your ASI should read, doesn't TAS increases with altitude for same CAS CAS easy to find with GPS & density altitude Drag curve is normally plotted in CAS terms IAS corrected is CAS; fix once, use many Vld higher when dens. Alt. higher Vld higher when weight higher, equal angle

Accuracy -the data must be accurate

TAS – use 2, 3 or 4 leg GPS runs IAS/CAS – make a correction card / graph OAT – check with verified thermometer OAT – placement matters, check speed Δ Altitude – IFR certified best M.P. - at least check with engine off Sink – use GPS, not baro VSI Fuel Flow – calibrate RPM – fluorescent light, digital, etc.

1977 C-152 P.O.H.69 mph (CAS)Approx 1650 poundsGlide 9.37:1645 ft/min, 32.6 THP,Drag = 177.2 pounds

C-152 POH & calculated dataPropulsive Efficiency

We Need a New Term

Traditional: thrust vs drag No power means no thrust In power-off glide, what opposes drag? New term: push? pull? anti-drag? Critical thought: [push + thrust] = drag Why: in partial power glide THP = [P+T] x V