# Tag Info

56

The idea that force can only be transferred/propagated at the speed of sound is a simplification. It's a very reasonable simplification, which works in a very wide array of cases, but it's a simplification. A fundamental concept in gas mechanics is the "mean free path length." This is how far a molecule can travel on average before it collides with ...

55

An 'aerodynamic force' (just one force...) appears when a body is immersed in a fluid stream. By convention, two components are chosen, one of them parallel to the stream direction, called 'drag', and the other one, perpendicular to that 'drag' is termed 'lift'.

41

Short Answer A procedure had been established to return from a Transoceanic Abort Landing (TAL) site to the Kennedy Space Center (KSC) using the Shuttle Carrier Aircraft (SCA). The maximum weight that could be ferried from Europe was approx. 87,000 kg, so so weight had to be removed from the orbiter prior to being mated. The SCA had a maximum range, ...

34

Because it's bloody difficult to make the curved shapes out of the materials used until recently, with the technology available at the time. The few aircraft that had it in the past generally were very expensive and labour intensive to build, compared to competitors, and therefore not economically successful. Now, with improved manufacturing techniques and ...

33

A propeller accelerates the air of density $\rho$ which is flowing through the propeller disc of diameter $d_P$. This can be idealized as a stream tube going through the propeller disc: The air speed ahead is $v_0 = v_{\infty}$ and the air speed aft of the propeller is $v_1 = v_0 + \Delta v$. The propeller effects a pressure change which sucks in the air ...

33

As you mentioned, drag is one of the reasons why retractable landing gears are used in the first place. But in order to use it, there are way more considerations than just drag. Scale: Size of the aircraft plays a big role here. Big aircraft have more room to keep the retracted landing gear. This is an issue as planes get smaller. Weight: Retractable ...

33

It is only the pressure wave that can propagate at the speed of sound. This means that a molecule of "air" that is ahead of a subsonic aircraft can get pushed out of the way without hitting that aircraft. It gets a push from another molecule, which is pushed by chain of molecules until it get to the one which is the one that hit the aircraft. The aircraft ...

28

Yes, it was possible and NASA had a plan. Enterprise was taken on a European tour in 1983, visiting the UK, France, Germany and Italy. To get there, it crossed the Atlantic on the back of the Shuttle Carrier Aircraft (the converted 747). According to Slate, the range of the SCA was about 1,000 nautical miles so it made refuelling stops at Goose Bay and ...

23

First, the Commando wasn't unique in having a 'stepless' cockpit design- the Boeing 307 Stratoliner, for example had them. Other military aircraft too had them, due to a few reasons like pressurization and the excellent visibility (He 111, Ju 388, B-29 Superfortress, the list goes on... etc) they offered. While it's true that having this design improves ...

20

With current technology the L/D might go up to 70 or 75, and going higher would require an almost impractically large wing span. Gliders need to fly in tight circles to use updrafts, and the larger the wingspan becomes, the bigger the speed difference between inner and outer wing will be. Also, landing such a wide wing without dropping a wingtip will be very ...

20

No, by definition it isn't. There is drag. Drag is caused by different physical phenomena. According to the cause, it is classified to: Induced drag Induced drag is side-effect of generating lift over finite wing span. Lift is an upward force that the air exerts on the wing. By principle of action and reaction (Newton's third law of motion), the wing ...

18

It is not "this causes that" - all is happening together. Let me explain: For me to understand aerodynamics, it helped to disregard all that talk of vortices and induction, but focus on the pressure field around a wing. When the theory of flight was developed, electricity was new and exciting, and it just happened that electric induction could be ...

18

Maximum dwell time or maximum endurance occurs when the power required is minimum. Hence, in this case, the maximum endurance speed is one where the power required is minimum, while in case of maximum range speed, the thrust required is minimum. For maximum endurance, we must minimize the fuel consumed per unit time i.e. the fuel flow. For maximum range, we ...

16

Strictly speaking, a 'pure' aerobatic aircraft doesn't need a retractable landing gear. For such an aircraft maneuverability is much more important than speed. In fact, the aircraft can't fly too fast or the spectators are going to miss the show. In case of air races, the weight penalty of having a retractable landing gear is huge- in addition to the ...

16

A 54 inch propeller would start to incur efficiency loss at approximately 4300 rpm. At 4300 rpm a 54 inch propeller would have a tip speed of 691 mph which, relative to the 767 mph speed of sound, is about the maximum which still allows propeller efficiency. You can calculate the variables using this tool: WarpDriveProps.com Propeller Tip Speed Calculator

14

Yes, there are. The Boeing B-17 FLying Fortress is one example of an aircraft designed with a retractable tailwheel. Source: USAAF via Wikimedia, Public Domain (USGOV-PD) Another example would be the Vought F4U Corsair: Source: Wikimedia, cc-by-sa-2.0 Doubtless there are also other examples of aircraft similarly equipped with retractable tailwheels.

13

Yes, it does increase drag. Anything that is not smooth, where the air flow will be disturbed, will generate drag. The 737 was originally designed only for short haul and space is at a premium. Adding doors and all the associated mechanisms adds complexity, weight, additional maintenance requirements and more possibilities for failure. Boeing have had ...

13

Introduction to Transonic Aerodynamics Roelof Vos, Saeed Farokhi (more) Written to teach students the nature of transonic flow and its mathematical foundation, this book offers a much-needed introduction to transonic aerodynamics. The authors present a quantitative and qualitative assessment of subsonic, supersonic and transonic flow around ...

13

You start from wrong assumptions, which explains your doubts. The line the induced drag is due to the tip vortices is as true as saying that wet streets cause rain. Also, the opinion that the tip vortices strength will be as same as the bound vortex is wrong. Unfortunately, many authors don't understand the topic themselves and copy what others have ...

13

How exactly does acceleration of air occur just by having a convex underside Just like the curved forward surface of a wing does it. Imagine for a moment that the air flows along a straight path: Now it would "see" that the surface below it curves away from its path of travel, and if that path would remain unchanged, a vacuum between the wing and the air ...

12

Concorde! Actually a "bumper". Obviously it was there to protect against over-rotation on takeoff, but the relevant item on the checklist was four greens, not three.

12

In the case of the Red Bull Air Race, the engine and propeller are specified by the race officials, and must remain unmodified, so the airframes are designed to get the most out of that combination. The (relatively) tight layout of the course benefits a highly aerobatic aircraft of the Extra 300 style. Fixed gear is also required. As a result of those ...

12

Your Taylor expansion only uses the first two terms, so it is only a crude approximation. But still, your observation is correct, however, whether it applies depends on the aerodynamic quality of the particular aircraft. Practical supersonic aircraft have been designed to minimize the Mach drag peak. The ways to do this should be familiar: Stretch the ...

11

Terminology first: Trim drag is the drag component added by adjusting the incidence of the horizontal tail rsp. the deflection angle of the elevator for trim. The increased parasitic drag of a larger tail surface itself is already part of the aircraft's zero-lift drag. Sizing criteria for tail surfaces The horizontal stabilizer surface must be dimensioned ...

11

Induced drag is caused by the backward inclination of the lift vector. Lift is defined as the aerodynamic force perpendicular to the flow direction, and since lift is created by deflecting this flow downwards, the resulting force is slightly inclined backwards. The formula for the deflection angle (called downwash angle) $\alpha_w$: $$\alpha_w = arctan \... 11 You're almost there. The difference lies in the kinetic energy required to move a certain mass of air, not in the air's viscosity. Copying freely from one of my earlier answers: The force to keep the object aloft is$$F=m_{object}g$$The force generated by downwards momentum transfer is$$F=\dot{m}v with $\dot{m}$ indicating mass flow (kilogram per ...

10

Polar curves for older airliners can be found in lecture notes or technical publications. The drag polars of the most recent ones, where no independent measurement is yet possible, are a closely guarded secret. If you see something, it will be restricted such that an important part of the puzzle is missing. For the polars to be meaningful, you need aircraft ...

10

In the flare you are already in ground effect. Maybe we should compare the final approach with cruise. The answer is simple: L/D is far worse in approach configuration. The wing buys the increased lifting capacity with a relatively higher drag increase. This is not all bad: If the L/D is too high, a precise landing becomes much harder. Also, some more drag ...

10

This is a very tricky topic. Basically, you need a clear definition of what is part of the engine-related drag and what is counted towards airframe drag. Splitting up drag between components will always produce an arbitrary result. An intake can generate thrust as well as drag. In case of the SR-71 at Mach 3, the intake contributed 54% of overall thrust, if ...

10

You must have read the wrong sources. Low pressure on the upper surface of the wing is really the major source of lift. The surrounding air sucks the wing up as much as it pushes it up from below. On a flat plate, the contribution of suction and pressure is about equal. On a wing with a thick airfoil, some additional suction is added on both sides due to ...

Only top voted, non community-wiki answers of a minimum length are eligible