Transcript Introducing a New Product

POWER & DRAG FOR THE REST OF US
A WORKING MODEL
www.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!
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 test data – accuracy

Bust some myths along the way

Q&A
The Universal Drag Curves
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
Go see John
McGinnis's
presentation
Wedn. 11:30
Finding Your Key Point
The Key Point is defined by:
speed x and minimum drag y

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
New stuff!

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 power
Demonstrated equal to Zero Thrust


Closed throttle or engine off NO GOOD
Use corrected
IAS for CAS
and GPS for
vertical
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.
Finding Drag
Mix 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
Use GPS not VSI
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.

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 Drag
Summary

Mix Power & Gravity
• Constant 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

Prop(ulsive) Efficiency
see 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”

PE
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

SFC
BHP
Working from THP:
• Example: 0.5 * (135/0.85) /6 = 9.56 gph

SFC
THP
PE
What is My BHP?
Multiply factor by fuel flow (example: 15 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!
Using the SpreadSheet
Entering 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 SpreadSheet
Validating 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 pounds
Glide 9.37:1
645 ft/min, 32.6 THP,
Drag = 177.2 pounds
C-152 POH & calculated data
Propulsive Efficiency
THP computed from my Power&DragV4.c152.xls
Which uses CAFE drag curve(s)
Data from Cessna POH and as calculated
Effective
RPM
TAS mph KTAS hp% BHP GPH SFC
THP
Prop Eff%
Pitch Inches
2000
2400 116.251
101
75 82.5
6.1 0.444
53.5
64.85%
51.2
2000
2300 110.496
96
66 72.6
5.4 0.446
47.3
65.19%
50.7
2000
2200 104.741
91
59 64.9
4.8 0.444
42.6
65.59%
50.3
2000
2100
98.986
86
53 58.3
4.3 0.443
38.2
65.44%
49.8
2000
2000
92.08
80
47 51.7
3.9 0.453
33.6
65.07%
48.6
4000
2450 118.553
103
75 82.5
6.1 0.444
53.8
65.20%
51.1
4000
2400 116.251
101
71 78.1
5.7 0.438
51.5
65.98%
51.2
4000
2300 109.345
95
63 69.3
5.1 0.442
45.1
65.02%
50.2
4000
2200
103.59
90
56 61.6
4.6 0.448
40.6
65.83%
49.7
4000
2100
97.835
85
51 56.1
4.2 0.449
36.5
65.04%
49.2
4000
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%
51.0
6000
2400
115.1
100
67 73.7
5.4 0.440
48.6
65.89%
50.6
6000
2300 109.345
95
60 66.0
4.9 0.445
43.7
66.27%
50.2
6000
2200 102.439
89
55 60.5
4.4 0.436
38.6
63.83%
49.2
6000
2100
96.684
84
49 53.9
4.0 0.445
35.1
65.10%
48.6
6000
2000
90.929
79
45 49.5
3.7 0.448
32.1
64.85%
48.0
8000
2550 123.157
107
75 82.5
6.1 0.444
54.3
65.84%
51.0
8000
2500 119.704
104
71 78.1
5.8 0.446
51.0
65.30%
50.6
8000
2400 113.949
99
64 70.4
5.2 0.443
46.0
65.37%
50.1
8000
2300 108.194
94
58 63.8
4.7 0.442
41.6
65.24%
49.7
8000
2200 102.439
89
52 57.2
4.3 0.451
37.8
66.08%
49.2
8000
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%
50.1
10000
2400 112.798
98
61 67.1
5.0 0.447
43.9
65.39%
49.6
10000
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%
48.6
10000
2100
94.382
82
46 50.6
3.9 0.462
33.1
65.36%
47.5
12000
2450
115.1
100
62 68.2
5.0 0.440
45.5
66.76%
49.6
12000
2400 111.647
97
59 64.9
4.8 0.444
43.0
66.29%
49.1
12000
2300 105.892
92
54 59.4
4.4 0.444
39.1
65.89%
48.6
12000
2200 100.137
87
49 53.9
4.1 0.456
35.9
66.55%
48.1
12000
2100
93.231
81
45 49.5
3.8 0.461
32.6
65.86%
46.9
0.4458 average
0.6543 average
0.0059 std dev
0.0062748752 std dev
0.4364 min
63.83% min
0.4625 max
66.76% max
0.0261 spread
2.92% spread
5.85% spread%
4.47% spread%
2.93% Plus-Minus
2.24% Plus-Minus
Altitude
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

Propeller Efficiency
Pure Prop Efficiency (PPE) is thrust / power.
 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 q
 Props have extreme q 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
