Hot answers tagged

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'.


44

When flaps are retracted they do nothing, which is the whole point. The byproduct of lift is drag, a larger wing will create more lift, but more drag as well. More drag equals a slower cruising speed, or bigger engines to power past the drag along with higher fuel consumption. Flaps let airplanes cruise faster by getting out of the way.


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, unmated, ...


35

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 ...


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

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 ...


25

Your concerns about heavy flaps are well founded. The designers try to get away with as few high-lift devices as they can afford to. But not fewer! If you observe the trend over the years, flaps became more complex with every new airliner generation, starting from simple split flaps in the 1930s to triple-slotted flaps on the Boeing 747 in the late Sixties. ...


24

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

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 ...


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 ...


17

Indeed, when an aileron moves upward, it locally generates less lift and less drag. Assume we are talking about the aileron on the right wing. The reduced lift drops that wing, rolling the aircraft right. But the reduced drag accelerates that wing, yawing the aircraft left. On the left side the opposite happens, but it has the same effect. The aileron ...


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

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 \...


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 ...


13

Climb to cruise burns fuel. Adding additional drag burns fuel. Adding retractable mechanisms adds weight that burns fuel. More drag, even at higher cruise altitudes, requires larger engines for the same cruise speed. Larger engines burn more fuel (despite increases in modern engine fuel efficiency). Retracting high-lift, high drag devices reduces fuel burn ...


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

Blow-molded canopies (like the acrylic one on the P-51D) were relatively expensive in the late WWII and early post-War periods when the early-modern airliners were designed. Cost is an important factor in airliner design, and was even in 1950. Additional cost factors (contributing to the high cost of deeply or compound curved windshields) are that the ...


12

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 ...


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 ...


12

There are plenty of smaller power planes that achieve those numbers; motorgliders. And motorgliders with L/Ds in the high teens and low 20s are pretty efficient cruisers. So the real question is; why aren't all light aircraft made to be like motorgliders? Well, motorgliders have their disadvantages. The long wing span is a problem fitting in on the ...


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