Ernie Twells' notebook



Ernie Twells' notebook


Ernie Twells' engineering notebook covering the theory of aircraft engines





One notebook of 70 pages of handwritten notes.


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Form 619
Notebook for use in Schools.
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[blank page]
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[page of calculations]
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[page of calculations and formulae]
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[formulae and a graph]
For constant boost and [one indecipherable word] RPM. IHP [one indecipherable word] to RPM for a constant boost RPM will determine weight of air consumed and I.H.P is ∝ to the weight of air consumed. The losses of power between IH.P and B.H.P will be
A/ Frictional losses witch [sic] are ∝ to RPM
B S/G witch [sic] are ∝ RPM at constant Boost
[example calculation]
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[page of calculations and graph]
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[underlined] SUPER CHARGING [/underlined]
Rate of air consumption by [underlined] weight [/underlined] governs the I.H.P
Supercharging is a means of obtaining higher power at S L or of maintaining cruising power to a higher altitude.
[formulae and calculations on super charging]
Power absorbed prop to square of RPM and weight of air consumed
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Performance of Supercharged Engines
1/ Normally asperated [sic] engine
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[page of formulae and calculations]
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[page of formulae]
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[calculations scribbled out]
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[blank page]
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For any given revs and boost there is a full throttle height - it is the height up to which the given boost can be maintained with the given revs
Full throttle height affected by
1/ Given boost higher the boost lower [inserted] FTH [/inserted]
2/ Given R.P.M higher RPM higher F.T.H
3/ Ram effect
4/ Intake efficiency
[underlined] Rated conditions [/underlined] are those that may be used for more than 5 mins and less than 1 hour (1/2) generally the max climbing conditions in rich mixture
Rated Power is the power developed with rated R.P.M and Boost at rated height
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[page of graphs]
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To find power required at any altitude multiply BHP by Altitude Factor for height wanted.
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[underlined] FUELS [/underlined]
1/ [underlined] DETONATION [/underlined]
[underlined] EFFECT O DETONATION [/underlined]
(a) Increase of heat losses to cylinder walls and pistons
(b) Maximum pressure exceeds normal max value.
Factors governing Detonation
1/ Nature of Fuel
2/ Compression Ratio
3/ Boost Pressure
4/ RPM
5/ Air intake temp
6/ Ignition advance
7/ Cylinder Cooling
Anti Knock Fuel the Property of the fuel ton resist detonation
[one indecipherable word] octane and [one indecipherable word] to get the octane No.
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[underlined] Volaldily [sic] [/underlined] Tendency of a fuel to Vapourise
Absent of vapour lock
[underlined] SPECIFIC FUEL CONSUMPTION [/underlined]
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[underlined MIX POWER [/underlined]
20:1 rough running combustion will not take place below this ratio
16:1 most ecconical [sic] used for cruising [one indecipherable word] [inserted] power [/inserted]
15.2:1 C . C
14:1 Weak mixture max power
12:1 Rated mixture streath [sic]
10:1 Take off
[underlined] REQUIRED [/underlined]
[one indecipherable word] rich
Cruising 16:1
High power 14:1
Take off 10:1
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[page of graphs]
[Graph showing S.F.C. against power at constant boost]
[Graph showing S.F.C. against power]
The S.F.C. Grid shows a variety of ways witch [sic] any given power output can be obtained. But there is only ONE seting [sic] for any given power witch [sic] gives envelope minimum S.F.C. this setting is that given on the S.F.C.
1. Up to full throttle height better S.F.C. same power can be obtained with less revs ie better mec effo [sic]
Above full throttle height S.F.C. increases as to maintain same power revs must be increased.
2. If operating at min R.P.M. to maintain constant power, throttle must be closed gradualy [sic] to full throttle heighjt and opened above full throttle height
[graph of P against A showing full throttle height]
Summary of Factors affecting S.F.C.
1. air fuel ratio (S.F.C. Loop)
2. Power developed.
3. R.P.M. (friction losses)
4. Butterfly opening (boost)
(power wasted in S/G)
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Venturi injection type Stromberg and Seco Venturi with float chamber. SU [two initials] non venturi injection no float chamber R.A.E. Hobson
S.E.C.O and Stromberg
Basic principles of operation
A-B chambers pressure [symbol] airflow
D-C chamber pressure [symbol] fuel flow
[sketch of carburettor]
Engine caracteristics [sic] of interest
1. Power – speed rate of climb
2. Fuel flow – endurance
3. Specific consumption – range
Factors affecting performance character
1. Boost
3. Barometric pressure altitude
4. Temperature
5. Mixture strength.
[table showing effect of changes of the above factors]
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1. Variation of power – const [sic] height temp mixture
a. increasing boost R.P.M const [sic] S.F.C. decreases
b. Increasing RPM boost const [sic] SFC increases
c. Increasing Power at full throttle SFC rough [sic] constant may decrease at low RPM definite increase at high RPM
2. Increase in height
a. constant boost and RPM S.F.C decreases up to full throttle height
b. Constant I.A.S increasing power
(i) at const [sic] RPM S.F.C. decreases
because increase boost
decrease in back pressure
decrease in temp
(ii) At constant boost S.F.C may decrease at low RPM otherwise increase.
(iii) full throttle – much the same as (ii) poss [sic] more favourable see (1c)
Increase in temp
(a) Const [sic] boost and RPM – S.F.C. increase more so in uncompensated carb
(b) Const [sic] IAS increase of power
(i) const [sic] boost S.F.C. increase more so in uncompensated carb
(ii) Const [sic] R.P.M. S.F.C. may decrease slightly with a comp [sic] carb, increase with uncomp [sic] carb
(iii) Full throttle. S.F.C. increase more so at high R.P.M. and even more so with uncomp [sic] carb (PR of supercharger [one word] for higher temp therefore not such a great increase of boost)
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General Range Flying Principles
Range is distance with a given amount of fuel
Specific Range dist [sic] with a unit amount of fuel
Specific Air Range SAR is A.M.P.G
[formulae for calculating SAR]
S.A.R is a measure of the overall eff [sic] of A/C since L = prop eff [sic] S gives measure of both E and airframe efficiency.
Speed and power can only affect SAR through there affect on E. S or D
Assumption of const [sic] S and E.
[graph of SAR against Speed]
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Since V1 and D remains constant for changes of height and temp best V1 for range will remain constant and also S.A.R
[graph of SAR against speed]
Best speed for range prop square root of weight
Range inversely prop to weight
Increase in parasite drag reduces the best speed for drag and reduces the range also the power required at the new speed is greater than the old speed.
For summary for const [sic]
Best V1 for range is the same V1nn independent of the ht [sic] and air temp and prop to the square root and weight
Percentage vari [sic] of best V1 =[formula]
Increase in parasite drag increases total drag decreases best V1 also increases power required
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SAR at the best V1 is also independent of height and temp and is inversely prop [sic] to the weight.
Although the assumption of cost [sic] E and S it is not generally true in practice it some times occours [sic] that the value of E over S is allmost [sic] [deleted] negible [/delete] neglidlle [sic] and the above conclusions hold, and in any case can be graphed on to the above results.
[formulae and graph]
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At V1MD drag is approx const a small decrease in speed giving but this small decrease in speed gives a decrease in S so D x S is decreased S.AR is increased. If speed is still reduced we shall reach a point where decrease in S and increase in D are balanced and we get best S further reduction in speed shows [deleted] decreasing [/deleted] D increasing much more rapidly than S is decreasing So D x S has a net increase S.A.R therefore fall off
Increase in speed above V1 MD shows both D and S increasing therefore D xS increases and therefore S.A.R decreases see graphs
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SFC decreases with increased power
An aircraft is said to be under powered for range flying when the power used at the best speed for range is greater than the power giving min [sic] S.F.C.
It can be seen that an aircraft can commence a sortie underpowered and finis [sic] overpowered.
Prop eff can generally be taken to be between 75 percent and 85 percent but in any particular case the variation in eff [sic] will be small and of the order of 2 to 4 percent. Above rated altitude however efdf drops off due to high angle of attack required to absorb the power. High angle may also give reduced eff when operating at high boost low RPM In a few isolated cases where prop eff does not remain aprox [sic] constant the variation in drag and S.F.C and the need for operating at certain conditions may override the low RPM high boost rule
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Fig A [graph]
The tables show that the rate of power around V1MD in [sic] generaly [sic] lower for lower weights in other words as weight is decreased the aircraft becomes less underpowered or overpowered.
We saw that for an underpowered a/c speed for range is less than V1MD in an overpowered aircraft speed for range is greater than V1MD
This difference depends on slope of S.F.C envelope. As the aircraft becomes less underpowered ie as power is reduced the slope of the envelope is reduced and speed for range becomes nearer V1MD As the aircraft becomes more overpowered ie power required still less the slope increases and speed for range becomes more and more above ViMD
Fig it shows that the variation of speed for range is less for ViMII and in practice we say the percentage of speed for range is
For a constant SFC and Prop eff. We saw that variation of height had no effect on range for as on best speed for range and so in practice variation in height will only effect range and speed for range where it effects S.F.C and prop eff
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Aircraft overpowered at Sea Level
Therefore operated at const R.P.M. increased power with height by increasing boost. S.F.C improves therefore therefore range improves therefore S.A.R improves up to the full throttle height At full throttle height aircraft correctly powered S.F.C minimum. If above full throttle height extra powered required obtained at higher RPM this gives greater S.F.C therefore smaller air range.
At sea level overpowered aircraft the best speed for range is higher than ViMD at full throttle height it is correctly powered speed for range ViMD
Above full throttle height high RPM required therefore reduce speed to avoid uneconomical engine settings
Aircraft underpowered at sea level
Operating at max boost increase power increase R.P.M S.F.C may improve at first giving slight increase of range with height, but when high R.P.M are required SFC increases more rapidly giving reduction in range. Speed for range aircraft underpowered Vi less than ViMD remains aprox [sic] constant up to full throttle height up to when heigh [sic] RPM make a reduction necessary to avoid unnecessary engine [word]?
Constant IAS RPM compensated carb SFC decreases slightly therefore range increase
Uncompensated carb SFC increases slightly therefore range decreases.
At constant boost and IAS full throttle range will allways [sic] decrease more so with uncompensated carb 2.5 percent.
With increase of temp aircraft become less overpowered or more underpowered (due to increase T.A.S) the effect in both casses [sic] on the best speed for range is the same and is a reduction
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Minimum speed for cruising is the speed at which a small change of speed the power required is sufficiently large to make the aircraft return rapidly to its mean speed. In rough air bigger changes in speed bigger margin required. Fly a little faster. Tendency to instability biger [sic] movements of control are required biger [sic] changes of speed fly a little fast.
Recommended speed is chosen suffice [sic] above the minimum speed for cont cruising to allow for all normal variations and C of G and stability and average rough air conditions
1. Speed – drag handling.
2. Weight – drag (C of G)
3. Engine settings – S.F.C. Prop efficiency
4. Height – S.F.C. (prop at height)
5. Air temp – S.F.C.
6. [word]? – average drag
7. Pilot efficiency – use of auto pilot
Range summary
1. Use max boost and low RPM to obtain optium [sic] IAS
2. Use M.S gear at an altitude such that the optium [sic] I.A.S. is obtained at full throttle but do not fly so high that RPM near the max must be used
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3 Roughly % change best speed =1/2 to 1/4% change in weight 1/4 (2/3) % change in range = 1 to 1/2 change in weight.
[underlined] EFFECT OF WIND [/underlined]
[calculation of T.M.P.G.]
[calculation of A.M.P.G]
[calculation of TMPG]
[calculations for the above]
Graph shows that best speed for range with a head wind is increased from 155 to 173 TAS aprox [sic] 12% Graph will lack would show a decrease in speed
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It will be seen that the head wind reduces T.M.P.G. by aprox [sic] 30%. Where as changing our speed for range we regain aprox [sic] 3% it similar effect and it is seen that the loss due to the wind is far in excess that than that gained by increase of speed. Thus it will be seen more probable to change height for more favourable winds were the gain will be a greater % than any obtained by changing speed or operating at full throttle.
[calculation for best INS for wind]
its empirical rule is usely [sic] found for each aircraft where by speed for range is changed 1 mile/hour for a given change in ground speed i.e after speed 1MPH for every 10MPH ground speed
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[underlined] CLIMB AND DESCENT
Climbing T.H.P. = T.M.P to overcome drag + THP for climb.
[underlined] Vertical Climb [/underlined] it dont [one deleted word] a where range is of no importance and heigh [sic] gained per gall is important factor.
Telling max climbing never and boost gall/hr are fixed so to climb max number of feet per gall we must climb as fast as poss.
[diagram of power for climbing]
Max climb obtained where max power is obtained for climb in threshold region but flight is not now uncomfortable since a good margin of power is available above that now required to maintain speed
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[diagram of climb/speed ratio]
RATE OF CLIMB “ say from 500FT/MIN to 1000FT/MIN.
[calculation for climb per gallon at lower power]
[calculation for climb per gallon at higher power]
And so increase of power in addition to giving greater rate of climb given more climb per gall and so is more eff in a vertical climb.
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[underlined] RANGE CLIMB [/underlined]
To climb to where the horizontal distant [sic] is important and there is ample time to available to get height.
The first aprox [sic] ratio would be to climb at best speed for range but a little extra power is required for the climb and so the aircraft is less overpowered or more underpowered than it would be when flying level this means a slight reduction in speed is required to obtain the best speed for range Therefore obtain rate of climb or 200/300 ft/m by a reduction of speed from 5/7MPH and an increase in power.
[underlined] Delayed Climb [/underlined]
Would be employed were [sic] is not [one deleted word] necessary.
[diagram for delayed climb]
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[underlined] Descent [/underlined]. Power to overcome drag. Power from engines + power from gravity.
[underlined] Vertical descent [/underlined] distant [sic] covered unimportant.
[underlined] Range descent [/underlined]
Will require [corrected word] most miles per galls. Less power required from engine therefore aircraft more overpowered or less underpowered there for best speed is slightly over best speed for range (in level flight) e.g. reduce power slightly and increase speed aprox [sic] 5 miles/hr to get rate of descent.
[underlined] ENDURANCE [/underlined] Max number of hours flying with min fuel.
Total fuel = G.P.H x No of hours
[diagram of fuel endurance]
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Thergical [sic] best speed would be the best speed for M.P. but this is not practical and so we fly as slowly as poss for comfort i.e the minimum speed for continuous cruising and as low as poss.
[Height for endurance formulae]
Since endurance flying is at low power 500ft should be taken as safety height.
Endurance decreases 1% per 1000ft of altitude.
[underlined] Speed [/underlined]
Speed is any speed in excess of speed for range flying
[formulae for speed / range / power]
Since V is large the important [indecipherable word] is a V3 and the term b/V has proportaly [sic] has much less affect on the power required. Thus changes in weight will have less effect on the power required to fly at speed than change
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(a) it standard simple method of calculating weight and c of G without use of appliances.
(b) Complete doc proof of safe loading.
(c) It standard prep of aircraft plan (i.e) list showing modifications to be carried of out by MVs and units combined with card index checking system.
[one indecipherable word] is absolute point laid down by the numb usely [sic] near the sect of the leading edge of the main plane but working with different aircraft.
[underlined] Weight [/underlined] Weight [one indecipherable word] to the nearest 16 Arm is the [one decipherable word] felt measured fore and aft parrlel [sic] to the dalium line if any item from the self line to the nearest 05IT.
[underlined] Moments [/underlined] is the weight of an [calculus].
Refference [sic] line. an [indecipherable word] line perpendicular to the aircraft for and aft dalian al [sic]some commence distance forward to the dalian point.
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[underlined] Tare weight [/underlined] The weight of the aircraft less all removeable [sic] items of equip (Col 9) but modified to a certain standard. Can be found in Vol 4 Sec 4.

1. Aircraft Tire condition and weight and moment only of any mods fitted but not included list of tare mods plus weight and moments of any command mods to give.
2. Weight modified tare condition. Ad [sic] weight and movements of all items of moveable equip. required when aircraft is operating to given role to give.
3. The gives a/c basic condition. The weight and movement of the crew and baggage oil and any other item of equip to give.
4. aircraft weight and cond [sic]less fuel and pay load ad [sic] all items of pay load this gives
5. Aircraft light condition.[Table to accompany above notes]
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The weight and movement of all fuel is add the given
6. Aircraft condition.
[underlined] Compartment loading [/underlined] procedure. Object to simplify loading of large number of small items of exp of known weight.
1. Aircraft is divided into 10 phicical [sic] compartment letted [sic] A to K no letter I.
2. Aprox [sic] mean arm in round feet is painted inside of each compartment this is known as the centract [sic]. When loading all items are grouped round the central in each compartment the arm of the centrical [sic]is then taken of [sic] all the items.
3. To find aircraft light conditions add weight and movement of all compartments to aircraft less fuel and payload. Weight and movements charts. To simplyfy[sic] calculations the chart consists of weight ploted [sic] against movements so that you can read the [indecipherable word] the C of G levels are also shown on the chart.
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in factors producing parasite drag. Fly at full throttle height for the power require because up to full throttle height power increases and S.F.C. decreases.
For a given required T.A.S. (to make good a required ground speed) the I.A.S will be less if we increase the altitude This means within increased altitude V1 will be nearer V1 MD That is we are approaching nearer to the most eff. Flying condition for A.M.P.G.
[underlined] ASSYMETRIC [sic] FLIGHT] Height will one or more [inserted] (engine failed [/inserted] if windmilling [sic] of feathered prop and the fact that the a/c is crabing [sic] (due to unsymetrical [sic] thrust) all lend to increase parasite drag therefore lest speed for range is reduced. In addition power required from the remaining engine(s) will be greater thus making the aircraft more underpowered or less [one deleted word] overpowered again reducing less speed for range (except in the care of the A/C very much overpowered) were range and speed for range will be increased). Generaly [sic] then in assymetric [sic]
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flight best speed for range is reduced and range is reduced.
[underlined] TAKE OFF [/underlined]
[Graph for Take Off – speed/time]
5% increase weight 71/2% increase in take of [sic] time
10% “ “ 15% “ “ “ “ “ “
Take of [sic] run is ∝ to W2.
i.e. 3% increase in weight 10% increase T.O. run
8% “ “ “ 16% “ “ “
[page break]
Compartment Loading Tables published with to give a quick guide of distribution of pay load between compartments when any given pay load is to be carried.
The section of the table are laid out in the same manner as the ap. section of the weight and balance clearance form to facilitate compiling.
Remember prove [sic] sal [sic] loading is the weight and ballance [sic] clearance form.
Weigh [sic]
This is intended to provide written prof [sic] of satif [sic] loading of [indecipherable word] a/c. With slide rules no such proff [sic] existed.
It will entail additional work for captain and load control officer but is felt justified with a view to safty [sic]. Loading data in course of prep. will greatly simplfy [sic] its completion. Pending issue of loading data the form [underlined] must be used [/underlined] in combination with ex data. Refference [sic] to data used will go under remarks collumn [sic] of weight and clearance.
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Compleation [sic] and distrabution [sic] W and B will be comp in Trup [sic], partly by captain and by load control officer. On compleation [sic] it will be certified by person 1/C loading party captain and load control officer.
[underlined] Distribution [underlined]
Original handed to load control officer next step [underlined] Duplicate [/underlined] retained by captain and attached to MC PAPER. Triplicate retained by local officer at place of departure.
1. Enter details of flight plan (part 1)
2. Crew baggage part 2 3 4 .
3. Enter details palload [sic] computation (T.MU. )
4. “ payload offered to trafic [sic] (lesser fig col U)
5. “ quantity weight and movement of fuel load TO Landing V to Z
6. C.G limits at bottom of part 2 and C.G limits prefered [sic] by pilot.
Action by load control officer.
1. He will decide load distribution from loading tables and will enter data on loading plan (fig 9) giving seperate [sic] details for pass mail freight
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2. He hands loading plan to I/C loading party to carry out.
3. On receipt of loading plan on comp of loading he checks the captains entrys [sic].
4. Enters details in sec A to K. on W.B from loading plan.
5. Enters details in col V V1 V2 Certification of W an [sic] B form by the loader that load has been dest as shown on the loading plan [underlined] 2 [/underlined] By load control officer that he has checked loading and is satisfied with the dist.

3. By the captain that O/C is safely loaded load is securely lashed and C of G is within limits.
Captain [underlined] is finely [sic] responsible for [/underlined] loading
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[underlined] AIRCRAFT PERFORMANCE TESTING [/underlined]
Much carefull [sic] prep on the ground by pilot and crew.
2. Necessary to prepare and complete a flight plan for each member of crew & aircraft is being tested for eco the pilot may require from 10 to 15 min to settle his speed when IDS has settled pilot should warn the crew that they may take reading he should note his height at com the run and endeavour to maintain that height during the run.
Equip required.
A. Standard equip
B. Flow meters
C. Air mileage unit for accurate of T.A.S.
D. Stop watches
E. [indecipherable word] sensitive altitude metres set to 10/3 MB
F. Instruments should be calibrated
G. Free air cannot be trusted see met. Take off weight must be assested [sic] as accurate as poss.
This may be done by carefull [sic] check of equip against loading chart And the fuel in the tanks should be checked as accurately as poss
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[Table showing ready to start run checks]
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[Table for readings and clock times]
Find out if the engine obeys H R L REVS Unless aircraft is to be tested it will be necessary to examing [sic] the power altitude [indecipherable word] at height is chosen al [sic] watch the throttle is likely to be open A series of reves [sic] and boost
[page break]
watch range of power and with what combination the best AMPG results are obtained.
In order to find out the mean weight for the best of series of combination are tested and then repeated in the reverse order giving an average result at a weight aprox [sic] for TO with half fuel consumed.
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[Page of calculations]
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Ernie Twells, “Ernie Twells' notebook,” IBCC Digital Archive, accessed July 20, 2024,

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