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Power & Drag for the Common Man Some 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 = Vld THP is drag times speed (more later)

Finding V ld – “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 Sink Mix 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) Efficiency see 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 = gph Working from THP: Example: 0.5 * (135/0.85) /6 = 9.56 gph PE THP

Using the SpreadSheet Entering 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 SpreadSheet Validating 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 pounds Glide 9.37:1 645 ft/min, 32.6 THP, Drag = pounds

C-152 POH & calculated data Propulsive 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