Notes on Airframes

MNealeETH1395951-150731-042.pdf

Title

Notes on Airframes

Description

Ted Neale's notes on aircraft.

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41 handwritten sheets

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IBCC Digital Archive

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This content is available under a CC BY-NC 4.0 International license (Creative Commons Attribution-NonCommercial 4.0). It has been published ‘as is’ and may contain inaccuracies or culturally inappropriate references that do not necessarily reflect the official policy or position of the University of Lincoln or the International Bomber Command Centre. For more information, visit https://creativecommons.org/licenses/by-nc/4.0/ and https://ibccdigitalarchive.lincoln.ac.uk/omeka/legal.

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MNealeETH1395951-150731-042

Transcription

[underlined] Wings.
Function of wings [/underlined]
In order that an aircraft may fly it is necessary that it be supported, the forward movement of the wings through the air produces an upward force called the lift, counteracting the weight of the aircraft and thus supporting it in the air.
[underlined] Angle of Attack [/underlined]
in flight the wing is slightly inclined to the direction of motion and hence to the airflow, this small angle (usually 2 to 4 degrees in straight and level flight) is called the angle of Attack.
[underlined] Airflow over the Wing. [/underlined]
Experiment shows that the airflow is smooth over both top and bottom

[page break]

surfaces and that the streamlines are closer together over the top surface than over the bottom. The airflow is faster over the [und] top surface [/underlined] than over the bottom, the upwash in front of the wing and the downwash behind it should be noted.
[underlined] Pressure Distribution [/underlined]
The result of this characteristic airflow is to produce (a) a slight increase of pressure on the lower surface, (b) a slight decrease of pressure on the upper surface, the actual differences from atmospheric pressure are very small. Typical values are 1/200 of atmospheric pressure for (a) and 1/100 for (b). note that approximately 2/3 of the total lift is provided by the upper surface.

[page break]

[underlined] Sketches [/underlined] 1/ (a)(b)(c)

All the forces acting on the wing, both above and below may be combined into one “total reaction”, which acts through [underlined] the centre of Pressure [/underlined], it is inclined backwards.
[underlined] Lift and Drag. [/underlined]
Sketch (2).

[page break]

The total reaction (R) is split into two components, one parallel to the airflow, the other perpendicular, the first force (that parallel to the airflow) resists motion to the air and is called [underlined] DRAG. [/underlined] The second (perpendicular to the airflow) supports the aircraft and is called [und] LIFT. [/underlined]
The designers object is to obtain the greatest possible lift with the least possible drag; the pilot on his part must keep the wings at the angle of attack which gives the best results.
[underlined] Effect of Increasing Angle of Attack. [/underlined]
(Speed Remaining Constant)
[circled 1] [underlined] Drag [/underlined] as angle of attack increases from about 0 (Drag Minimum) Drag increases gradually at first and more rapidly later.

[page break]

[circled 2] [underlined] Lift [/underlined] as angle of attack increases from about –2 (Lift 0) Lift increases, [underlined] but does not continue to increase indefinitely. [/underlined] At a certain angle (roundabout 15) Lift reaches a maximum and begins to decrease with further angle of attack. The angle of attack which gives maximum Lift is know [sic] as the [underlined] stalling [/underlined] angle.
Sketch (3)
[diagram]

[underlined] Airflow at the Stalling Angle. [/underlined]
Experiments show that the decrease in Lift at the stalling angle is caused by the airflow leaving the top surface of the wing and forming eddies or

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turbulence above & behind the wing and spoiling the downwash, thus it is the loss of effectiveness of the [underlined] top [/underlined] of the wing which causes stall
Sketch (4)

[underlined] What Lift and Drag depend upon. [/underlined]
[circled 1]. Shape of the wing (a) cross section (b) plan
[circled 2]. Angle of attack
[circled 3]. Density of the Air
[circled 4]. Wing Area (Projected)
[circled 5]. Air Speed.

[page break]

Theory in Practise
[circled 1] Biplane versus Monoplane
Low speeds demand large wing area in order to obtain sufficient lift (hence biplanes & triplanes in the last war). The biplane however has the following disadvantages
(1) Interference between the two wings this results in loss of lift
[diagram]
Interference may be partially overcome by (a) Increasing [underlined] gap [/underlined] (b) [underlined] Stagger [/underlined] (although this is largely employed to increase the pilots field of vision
(2) [underlined] Increased Drag due to struts & wires [/underlined]

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The monoplane has developed mainly because of (a) higher speeds and consequent smaller wing area (b) improvement in materials and methods of construction (eg) cantilever)

Wing Loading = TOTAL WEIGHT / PROJECTED AREA OF WINGS usually measured in lbs per sq ft.
Low wing loading – slow, light biplanes eg Tiger Moth.
High wing loadings – fast monoplanes e.g. Spitfire
Typical figures High Stirling 49 lbs sq ft Dornier 217E 64 lbs per sq ft
Low Tiger Moth. 7.6 lbs per sq ft

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[diagram]

[underlined] Wing Drag. [/underlined]
[circled 1] Profile Drag (Form Drag)
Dependant on the shape of the cross section of the wing, also on the angle of attack.
[circled 2] Skin Friction
Can be reduced by making the surface smooth. It becomes relatively more important as form drag is reduced and speed becomes greater.
[circled 3] Wing Tip Vortices (Induced Drag)

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[circled 3] Is really part of the lift, it can never be entirely eliminated, but the loss of lift can be partially restored and the induced drag, at the same time decreased by using,
[circled 1] High Aspect Ratio
[circled 2] Tapered wings.
[circled 3] Rounded or Raked Tips.
[underlined] Parasite Drag. [/underlined]
(Drag of rest of Plane)
[circled 1]. Profile Drag
Dependant on streamlining, note: although a retractable undercarriage much reduces drag when up, it is usually worse than the non retractable type when down, this does not help take off.
[circled 2] Skin Friction.
The smaller the area, the smaller

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is skin friction, hence the cramped quarters in an aeroplane.
[circled 2] Cooling Drag
Greatly reduced of late, may even be negative.
[underlined] The Four Forces [/underlined]
Considering now the whole aeroplane in straight & level flight, all the forces acting on it can be summed up in four
1/. [underlined] Lift. [/underlined] acting vertically upwards from the [underlined] Centre of pressure [/underlined] of the plane as a whole
2/. [underlined] Weight [/underlined] acting vertically downwards from the [underlined] Centre of Gravity [/underlined] of the Plane.
3/. [underlined] Thrust [/underlined] acting forwards along the propellor shaft, usually horizontally
4/. [underlined] Drag. [/underlined] of the whole plane acting

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horizontally backwards along a line that we can call a [underlined] line of drag. [/underlined]

The plane is not altering its height or speed and is therefore in mechanical equilibrium. The condition for equilibrium are.
Lift must exactly equal weight (L = W) to keep it from sinking or rising
2/. Thrust must exactly equal drag (T = D) to keep it from speeding up or slowing down
Note L & W are much bigger than T or D eg L =W = 10,000 lbs T =D = 1,000 lbs
3/. The four forces must act in the right places so that the plane may be neither nose heavy nor tail heavy.

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These conditions might be fulfilled thus but it would be impossible to keep them like this because
[diagram]
(A) C.G moves with alteration in bomb & petrol load and the position of the crew
(B) C of P moves with alterations in angle of attack:
(C) T & D alter their lines of action with various angle of attack.
A more usual arrangement for normal conditions is
(1) To have the lift slightly behind the weight line giving a diving twist
(2) To counteract this by having the drag line above the thrust line giving a stalling twist.
[diagram]

[page break]

This has the advantage that if the engine fails and thrust ceases the L. W twist puts the nose of the plane down in the correct attitude for the glide.
[underlined] Trimming Devices [/underlined]
As already stated the designer may arrange to have the plane in perfect equilibrium at one speed & angle of attack, but as soon as a new angle of attack is needed for a new speed the disposition of the forces is upset. trimming devices enable the pilot to restore equilibrium & eliminate the alternative of using the elevator with a constant & tiring pull on the stick

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[underlined] Stalling
Stalling Angle. [/underlined]
Is the angle of attack of a wing (at any particular speed) which gives maximum lift.
[underlined] Stalling Speed [/underlined]
Is the least speed that can be maintained in straight and level flight.

A pilot wishing to decrease speed must still maintain lift = weight. The drop in lift due to decrease of speed is therefore compensated for by a greater angle of attack. This may continue until a further increase in angle of attack gives no increase in lift – Stalling Angle & Stalling Speed have been reached. Controls are now sloppy & ineffective & any further attempt to decrease speed will result in loss of control, the dropping of the nose or of one wing. Stalling Speed in Straight and Level Flight may not always be the same for the same aircraft. [underlined] An increase in total weight [/underlined]

[page break]

will raise the stalling speed, while an increase in [underlined] wing area [/underlined] (see later “Flaps”) will lower the stalling speed, thus the [underlined] wing loading [/underlined] of an aircraft will indicate its probable landing speed – No wing loading, land slowly; High wing loading high landing speed.
[underlined] Decrease of Air Density [/underlined] will increase the stalling speed, but the [underlined] indicated [/underlined] airspeed on the ASI for stall is the same for all densities & all height
[underlined] Stalling in turns & manouvers [sic] [/underlined]
[diagram]

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[diagram]
If the aircraft moves in a curved path the ‘effective weight’ becomes greater and more lift is necessary, in other words stalling speed rises. In turning the increase in stalling speed is not great, at angles of bank up to 45, but with steep turns it rises considerably, similarly, when pulling out of a dive the stalling speed may be very high if the stick is pulled back strongly.

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[underlined] Example. [/underlined]
Aircraft with level stalling speed 60mph
Angle of bank 60 stalling speed 85mph
Angle of bank 70 stalling speed 105mph
Angle of bank 75 stalling speed 120mph
Same aircraft pulling out of 300mph dive
Loss of height 1500 ft, stalling speed 105mph
Loss of height 700 ft stalling speed 125mph
Loss of height 400 ft stalling speed 170mph
(7G Blackout)
Loss of height 100 ft “ “ 200mph
(10G)
[underlined] Slots & Slats. [/underlined]
Decreasing [deleted] in [/deleted] stalling speed & delaying stall.
If a small auxiliary wing (slat) is placed in front of the main wing, with a suitable gap (slot) between the two –
1/. The smooth airflow over the wing is

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maintained beyond the normal stalling angle by a further 10 or more.
2/. The maximum lift at this postponed stalling angle is 50% to 100% higher than without the slot
3/. The higher the maximum lift, lowers the stalling speed.

The slot usually is arranged to open automatically just before the main wing stalls, the disadvantage of slats lies in the [underlined] large angle of attack [/underlined] necessary for landing, producing
a/. poor visibility
b/. High undercart
Hence slats by themselves are rarely used now except when 1/. linked flaps
2/. Fitted only at wing tips to improve control (see later0.

[page break]

[diagrams]

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[diagram]

When an aircraft glides with the engine stopped the forces acting are 1/. Lift at right angles to the glide path. 2/. Drag, backwards along the glide path. 3/. Weight, acting vertically downwards. At a steady gliding speed L & D balance W.

It will be seen that the angle of glide [symbol] is the same as the angle between the lift & the vertical. This angle varies according to the ratio of lift to drag, ie with small D & large L [symbol] is small therefore for a very small glide it is necessary to have L/D as large as possible. Now L/D for a wing may be about 20 to 1

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but for the whole aircraft may be only 10 to 1. It is a measure of the [underlined] efficiency [/underlined] of the aircraft, aerodynamically & from a measure of L/D the flattest glide angle may be calculated, conversely improvement in streamlining may easily be tested by finding what improvement has resulted in gliding angle.
[underlined] Best Gliding Angle [/underlined]
It is possible for an aircraft to glide in different attitudes. It does [underlined] not [/underlined] necessarily glide along its longitudinal axis. In order to have the flattest possible glide, (ie to glide as far as possible) it must be controlled so that the ratio L/D is a maximum – ie so that the angle of attack

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is about +4 degrees (depending on the type of aircraft). If the angle is [underlined] greater [/underlined] or [underlined] less [/underlined] than this, the [underlined] glide path will be steeper. [/underlined] Only one Air Speed will correspond to this particular angle of attack, so that for optimum glide the pilot must watch his A.S.I. The instinctive tendency when wanting to glide as far as possible, is to put the nose up to far, this lowers the air speed, but [underlined] steepens [/underlined] the glide path.
A rough guide for possible distance is 1 mile for every 1,000 ft of height, this may be greatly exceeded with an efficient aircraft, efficiently operation, e.g. [symbol] = 3.6 degrees, which is equal to 3 miles per 1,000 ft.

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[diagram]

1/. [underlined] Problems of Landing
Flaps. [/underlined]
Modern high speed A/C of high wing loading necessarily have a high stalling speed and therefore a high landing speed. Increases in speed & wt carrying tend to increase landing speed.
[underlined] Flaps [/underlined] make it possible to reduce landing speed & length of run with –

[page break]

out sacrificing performance. In their simplest form the rear portion of the wing near the trailing edge is hinged so as to be movable downwards.

[diagram]

[underlined] Effect on Lift. [/underlined]
Flap angle 0 to 45 lift increases steadily, flap angle 45 to 60 lift increase more slowly. Flap angle 60 and onwards, practically no increase.
[underlined] Effect on Drag [/underlined]
0 - 45 small increase in drag
45 - 90 large “ “ “
[underlined] Effects of Flaps. [/underlined]
a/. [underlined] Reduction of stalling speed [/underlined] due to increased lift. This gives 1/. Slower Glide

[page break]

2/. Slower landing speed without increasing angle of attack (compare slats)
B/. [underlined] Steepens gliding angle. [/underlined] when landing
C/.
[underlined] Shortens the hold off period. [/underlined] (increased drag)
D/.
[underlined] Shortened landing run. [/underlined] by increasing drag (large flap angle)
E/.
[underlined] Shortened take-off run. [/underlined] by increasing lift (small flap angle)

2/. [underlined] Wheel Brakes [/underlined]
These are used to shorten landing run
[underlined] Disadvantages [/underlined]
a/ Tend to cause nose over.
b/. Tend to cause kite to swing. (CG behind main wheels) ie wheel brakes tend to make A/C [underlined] unstable [/underlined]

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on landing run.
Both a & b may be eliminated by
3/ [underlined] Tricycle Undercart. [/underlined]
In this case the C.G. is in front of the main wheels (thus preventing swing) & the nose wheel prevents nosing down, therefore the A/C can [underlined] land fast with tail up. [/underlined] There is also less tendency to bounce.
[underlined] Stability [/underlined]
An aircraft is said to be stable if, when disturbed from straight and level flight it returns to normal without any action on the part of the Pilot.

[underlined] Longitudinal Stability about the lateral axis or stability in pitch. [/underlined]
[diagram]

[page break]

[diagram]

If an aircraft flying level is suddenly disturbed so that the nose is raised, the following events occur
1/. Inertia will cause the A/C to persist in its previous path of flight. Thus producing
2/. An increase in the angle of attack
a/. The wings
b/. The tailplane
b/. produces
3/. An upload on the tail which gives for a nose down to its previous position or attitude.
[underlined] Note [/underlined] The increase in lift on the

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tail
is small compared with that on the wings but its effectiveness is due to greater leverage
[underlined] For good stability in pitch the C of G must be well forward.

Lateral Stability about the longitudinal axis, or stability in roll. [/underlined]
Methods of securing this are
1/. Dihedral on the wings
a/. a/c rolls therefore :-
B/ Lift is no longer vertical, this produces
C/. Sideslip toward the lower wing. This alters the direction of the relative wind, the angle of attack on the lower wing being greater than that on the lower wing, this gives:-
D/. More lift to the lower wing :-
e/. Roll back to horizontal.

[page break]

2/. [underlined] Sweepback. [/underlined] (Tiger Moth).

High Wing & Low CG (Pendulum) e.g. Lysander.

[underlined] Directional Stability. [/underlined] about the normal axis, or stability in yaw. (Weathercock Stability) When an q/c yaws inertia tends to keep it moving on its original flight path ie for a short distance it travels crabwise in the air, this means that the relative wind now strikes the side of the fuselage, the [underlined] tail fin [/underlined] & [underlined] the rudder [/underlined], the effect of this wind on the fin & the rudder & on that part of the fuselage behind the C of G is to return the a/c to its original attitude. The thrust on the front

[page break]

part of the fuselage tends to increase the yaw, but the large & efficient area of the fin & rudder, plus the greater leverage, easily overcome this adverse force.
[underlined] Connection between rolling & yawing. [/underlined]
When an q/c rolls it sideslips, hence dihedral restores it to an even keel, [underlined] but [/underlined] The sideslip will cause the relative wind to strike the fin & hence turn the a/c in the direction of the roll, thus it is normally necessary for this to be corrected by the pilot by a kick on the rudder.

[page break]

[underlined] Control. [/underlined]
Control surfaces
1/. Rudder.
2/. Elevators
3/. Ailerons
4/. Flaps
Effectiveness of a control surface depends upon a/. area
b/. Distance from turning axis (leverage)
c/. Speed of Airflow
[underlined] Aileron Drag [/underlined]
In order to bank the aileron one wing is lowered (thus increasing lift & the wing rises) & on the other, is raised (decrease in lift & wing falls)
But drag is also affected & the wing, with the aileron [underlined] down [/underlined] (outside wing) has its drag increased more than the other, this tends to turn the A/C

[page break]

in the wrong direction.
[underlined] remedies: [/underlined]
1/. Differential ailerons
[diagram]
[underlined] Frise. [/underlined]
[diagram]

[underlined] Balanced Controls. [/underlined]
The amount of physical strength necessary to operate the control column

[page break]

may often be very great, in order to make it smaller control surfaces are balanced by making part of the surface project in front of the hinge line & thus help in moving the controls.
[diagram]

[underlined] Mass Balance [/underlined]
Control surfaces which are slightly flexible, plus slackness or play in cables or control rods may produce flutter, which may become violent enough to break the controls, this is prevented by the addition

[page break]

of mass balance weights as shown below.
[diagram]
[underlined] Trimming Tabs & Balance Tabs. [/underlined]
[diagram]
Tab goes down, control surfaces goes up. Tab goes up, control surface goes down.
[underlined] Uses. [/underlined] a/. as a [underlined] balance. [/underlined] in the same way as horn balance or inset hinge.

[page break]

b/. as a bias or correction to [underlined] trim [/underlined] the A/C e.g. on elevators when C.G. alters, this results in releasing strain on control column & trim tabs have superseded the adjustable tail plane for this purpose. Trim tabs may be
1/. Adjustable from pilots seat (bombers)
2/. “ only on ground (most fighters)
[table]

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[diagram]

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[diagram]

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[table]

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[diagram]

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[diagram]

Collection

Citation

Ted Neale, “Notes on Airframes,” IBCC Digital Archive, accessed December 3, 2021, https://ibccdigitalarchive.lincoln.ac.uk/omeka/collections/document/16381.

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