POWER & DRAG FOR THE REST OF USA WORKING MODEL

www.n17hh.netwww.n17hh.net

"We can lift ourselves out of ignorance, we can find ourselves as creatures of excellence and intelligence and skill. We can be free! We can learn to fly!"

-J.L. Seagull

Reasons For This Project

Most modern engines cannot use the Norris-Bauer ZT device.

I'm compulsively curious.

Experimental aviation is supposed to be about education.

You can't design the perfect prop without a good drag curve.

You must isolate power from drag for airframe development.

Make near-CAFE level of testing available to nearly all of us.

Some Benefits of This Project

Understand Best L/D (Vld) and Minimum Sink (Vms) speeds

Understand and predict flight profiles easily.

Potentially improve propeller design & matching to airframe.

Improve Phase One results and safety while testing.

Understand how various measurements must fit together.

A solid basis for validating test data – helps find the “bad” data.

Know what your horsepower really is.

The spreadsheet eliminates all the tedious work!The spreadsheet eliminates all the tedious work!

THANK YOU!

I could not have done this without the help I got from Kevin Horton and Jack Norris.

No endorsement is implied. The errors are all mine, but much of the credit is theirs.

Agenda:Power & Drag for The Rest of Us

Some new ways of seeing them

All airplanes' drag curves are the same math shape(s) A working “model” can be built New methods of getting correct data (vs CAFE) A new spreadsheet integrates it all – drag, power, fuel, prop Using them to get & validate& validate test data – accuracy Bust some myths along the way Q & A

The Universal Drag Curves

The key point

This is a

ll of it

!

This is a

ll of it

!

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

Go see John McGinnis's

presentation Wedn. 11:30

Finding Your Key Point

The Key Point is defined by:speed xx and minimum drag yy

First, find the Vld - CAS for best L/D

Then, find the drag for that speed.

The model provides methods for both.

Finding Vld: the X Axis

Propless glide - impractical

Use Norris's Zero Thrust Device (CAFE) – not newer engines

Closed throttle or engine off NO GOOD

Finding Vld: the X Axis

GPS best glide ratio, low power, gets Vld

Fly at Minimum Sink (Vms), low power (x 1.32 = Vld)

Fly at Vms, level flight, minimum powerDemonstrated equal to Zero Thrust

Closed throttle or engine off NO GOOD

Use corrected IAS for CAS and GPS for

vertical

New stuff!New stuff!

Finding Drag Directly

Propless glide - impractical

Zero Thrust Device – not newer engines

We need new methods.

Finding Your Drag: the Y Axis

You must find your minimum drag – which is at your Vld, NOT at your Vms.

Several new methods for using sink or climb and/or power.– Using more methods gives better answers.

Remember that drag x speed = power; power/speed = drag

Thrust and Gravity

Usually seen as level flight.We need to understand sinking flight.

Gravity works like thrust.Thrust plus ? equals drag.

New way

to see it

Finding DragMix power & gravity: 1 example

Finding Drag: Climb/Cruise Differential

Principle: Once you know Vld you know the THP%Δ between Vld and any other V (without knowing drag)

Application: For the same power, 2 different speeds, the THP difference is climb rate x weight. THP / V = drag.

The rest is basic algebra.

Finding Drag:Constant Power & Gravity

Fly level at a medium cruise speed. Note weight and air data. Note power data. Change pitch while holding power constant (tricky!) Note change in airspeed and sink/climb. Refer to curve and compute THP , then drag.

Use GPS not VSI

Finding Drag:Power Proportion & Gravity

Use GPS for VSI

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

Methods: Find DragSummary

Mix Power & GravityMix Power & Gravity

• Constant Power, vary airspeedConstant Power, vary airspeed

• Fuel flow proportional change

• Computed power change (with MP only)

• Other similar methods are possible

Finding Drag: S.W.A.G.

S.W.A.G.: use fuel flow at known TAS

• Assume / Estimate SFC (lowest or best power)**

• Assume / Estimate PE**

• Change drag on spreadsheet to match actual fuel flow

• Use for reality checks• Use to find where best PE

How it all Fits

All TAS's are CAS corrected for density altitude Drag is equal at equal CAS's Drag changes when CAS changes Vld increases with Density Altitude ( at same CAS)

At any CAS/TAS, the ratio of THP to THP for Vld is known Therefore the drag Δ is known, too

Power, Drag, THP vs BHP

Thrust Horsepower is drag x speed Thrust Horsepower is sink or climb x weight Gravity & thrust work equally! Mix.

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

BHP = THP / PE (which is always < 1.0) In other words, BHP is larger than THP

thp

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~.42 works best Peak – what manufacturer says? .43 for me

PE

Prop(ulsive) Efficiencysee www.PropellersExplained.com

PE is not constant, but can be close to it

PE is lower than pure prop efficiency PPE

PE max is not at highest effective pitch

PE is lower for Luscombe than RV or ”fast glass”

Luscombe PE: 63~75%, C-152: 63.83~66.76%

We cannot measure pure PPE in flight Airplane / propwash / geometry factors always there We CAN measure PE with this “model”

Fuel Flow & SFC

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

PETHPSFC

SFC BHP

What is My BHP?

Multiply factorfactor by fuel flow (example: 1515 x 9.0 = 135)(determine your factor(s) from SFC)(ignores altitude corrections)

Manufacturer's charts (usually at best power mixture)

Complex formulas – Atkinson, Lipps, etc.

You need EGT & MP for most of these but “by ear” can work for best power and can be close for “lean mixture” (not=LOP)

Accuracy - The Data Must be Accurate

TAS – use 2, 3 or 4 leg GPS runs, Use kilometers to nearly double resolution

IAS/CAS – make a correction card / graph

OAT – check with verified thermometer; placement - check speed Δ

Altitude – IFR certified best

M.P. - at least check with engine off

Sink – use GPS or altimeter + stopwatch, not baro VSI

Fuel Flow – calibrate

RPM – fluorescent light, digital, etc.

Accuracy – The Envelope

Must use good test conditions

Not over 200 Kts.

Not over 10,000'

Not for special cases or helicopters

As good as CAFE? Yes and no

The model is largely self-checking!The model is largely self-checking!

Using the SpreadSheetEntering Data(demo here)

The spreadsheet implements the “model” Any change also changes something else Each page easy to use, leads to next :

– IAS-CAS, locating x, finding y, results

• Vld in CAS mph (auto calc's TAS)

• Weights ( for Vld and for given test)

• SFC estimate

• Propulsive Efficiency estimate or deduction

• Pressure altitude & OAT auto calculates DenAlt

Using the SpreadSheetValidating Data

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

Built-in calc's for weight(s), altitude, drag

Accuracy is critical! - use GPS, etc.

Check Vld and drag by using >1 method for each

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 – Chapters, Individuals

Q&A

Back Up Slides

BUSTING MYTHS Vx and Vy – not where you thought

Glide in wind – too big a thumb

Best L/D – not best power-off glide

RPM Cubed rule – very, very approximate

RPM & MP – there is more to power

Universal Drag Curve – not 100.00% true

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

The right # = 1.13975

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

THP computed from my Power&DragV4.c152.xlsWhich uses CAFE drag curve(s)Data from Cessna POH and as calculatedAltitude RPM TAS mph KTAS hp% BHP GPH SFC THP Prop Eff%

2000 2400 116.251 101 75 82.5 6.1 0.444 53.5 64.85% 51.22000 2300 110.496 96 66 72.6 5.4 0.446 47.3 65.19% 50.72000 2200 104.741 91 59 64.9 4.8 0.444 42.6 65.59% 50.32000 2100 98.986 86 53 58.3 4.3 0.443 38.2 65.44% 49.82000 2000 92.08 80 47 51.7 3.9 0.453 33.6 65.07% 48.64000 2450 118.553 103 75 82.5 6.1 0.444 53.8 65.20% 51.14000 2400 116.251 101 71 78.1 5.7 0.438 51.5 65.98% 51.24000 2300 109.345 95 63 69.3 5.1 0.442 45.1 65.02% 50.24000 2200 103.59 90 56 61.6 4.6 0.448 40.6 65.83% 49.74000 2100 97.835 85 51 56.1 4.2 0.449 36.5 65.04% 49.24000 2000 92.08 80 46 50.6 3.8 0.451 33.1 65.42% 48.6 6000 2500 120.855 105 75 82.5 6.1 0.444 54.2 65.75% 51.06000 2400 115.1 100 67 73.7 5.4 0.440 48.6 65.89% 50.66000 2300 109.345 95 60 66.0 4.9 0.445 43.7 66.27% 50.26000 2200 102.439 89 55 60.5 4.4 0.436 38.6 63.83% 49.26000 2100 96.684 84 49 53.9 4.0 0.445 35.1 65.10% 48.66000 2000 90.929 79 45 49.5 3.7 0.448 32.1 64.85% 48.08000 2550 123.157 107 75 82.5 6.1 0.444 54.3 65.84% 51.08000 2500 119.704 104 71 78.1 5.8 0.446 51.0 65.30% 50.68000 2400 113.949 99 64 70.4 5.2 0.443 46.0 65.37% 50.18000 2300 108.194 94 58 63.8 4.7 0.442 41.6 65.24% 49.78000 2200 102.439 89 52 57.2 4.3 0.451 37.8 66.08% 49.28000 2100 95.533 83 48 52.8 3.9 0.443 34.0 64.30% 48.0

10000 2500 118.553 103 68 74.8 5.5 0.441 48.2 64.49% 50.110000 2400 112.798 98 61 67.1 5.0 0.447 43.9 65.39% 49.610000 2300 107.043 93 56 61.6 4.5 0.438 40.1 65.11% 49.1 10000 2200 101.288 88 51 56.1 4.2 0.449 36.5 65.06% 48.610000 2100 94.382 82 46 50.6 3.9 0.462 33.1 65.36% 47.512000 2450 115.1 100 62 68.2 5.0 0.440 45.5 66.76% 49.612000 2400 111.647 97 59 64.9 4.8 0.444 43.0 66.29% 49.112000 2300 105.892 92 54 59.4 4.4 0.444 39.1 65.89% 48.612000 2200 100.137 87 49 53.9 4.1 0.456 35.9 66.55% 48.112000 2100 93.231 81 45 49.5 3.8 0.461 32.6 65.86% 46.9

0.4458 average 0.6543 average0.0059 std dev 0.0062748752 std dev0.4364 min 63.83%min0.4625 max 66.76%max0.0261 spread 2.92%spread5.85% spread% 4.47%spread%2.93% Plus-Minus 2.24%Plus-Minus

Effective Pitch Inches

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

Pure Prop Efficiency (PPE) is thrust / power. How efficient can a prop be?How efficient can a prop be?

A prop is a rotating wing. The best wings (sailplanes) have L/D around 60:1 That's a thrust of 60 for a power of 1 = 98.3% The Virgin Atlantic Global Flyer's L/D is 37 (97.3%) Those wings have extreme aspect ratios and uniform qq Props have extreme qq at the tip and low aspect ratios (~13) – very high induced drag My guess is that 90 or 91% is the limit for PPE Propulsive efficiency is lower than that by necessity

Propeller Efficiency