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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