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Hot answers tagged

49

Interesting question. Purely empirically, it is the lift-to-drag ratio you are looking for. If you take this value as given for any particular aircraft, you have a direct answer for how much more effective wings are. It is the ratio of the lift to the total drag. The engine only needs to overcome the drag. With L/D equal to unity you would need the same ...


31

It's a Russian YAK-141 This was the VTOL prototype meant to compete with the Harrier and perform carrier-based work (note: edited for clarity) Designers from the Yakovlev bureau found out, that the double engine scheme of the Yak-38 and Harrier was not suitable for the new plane. Instead they created a layout with a single engine, that could turn 95° ...


29

With regard to energy expenditure and power, for a given amount of force that is to be produced by accelerating an air mass, more power is required when you accelerate a small air mass in each period of time than when you accelerate a large air mass. This is because the force is proportional to the change in momentum of the air mass, whereas the power is ...


26

In a traditional aircraft the majority of the power from the engine is used to keep the aircraft moving forward at a certain speed. Very little of that power is actually needed to create lift. Consider a simple paper airplane. It flies for a long time with no engine at all, until the drag on it causes it to slow down and if loses lift and descends to the ...


24

They can climb vertically, but this works best if they are several tons below their maximum take-off mass. Fighter jet engines need a lot of fuel, and at the beginning of the flight the aircraft will be too heavy for vertical climb. Also, the landing gear would need to be rearranged if the plane is to take off from any airport. Even a thrust/weight ratio ...


23

It isn't done because a moving control surfaces is easier to design and build than an engine mount that rotates. Plus the associated structure needed to accommodate the thrust, p-factor, and gyroscopic loads is going to be a lot heavier than ailerons, spoilers, moving tail plane, etc. Another issue is if one loses one or both engines, your roll and pitch ...


19

VTOL aircraft can have more than one engine. It is just that the design process of the Harrier and F-35 led to a situation where only one engine was required. The main challenge in any VTOL aircraft is to have the engine produce thrust in two different phases of flight: during cruise and during takeoff/landing. This basically requires the powerplant on the ...


16

To try and change lift by changing fan blade speed would be too slow to modify the lift for "tiny" and "delicate" changes in how the plane hovers. Like a helicopter, the F-35 is able to maneuver in very small movements and change very fast (in fact likely better than a helicopter). So the pilot can "nod" to you, and move around the front nose with tiny, ...


15

Your assumptions (constant thrust for both lift fan and exhaust) are wrong. From wikipedia: "For pitch control, the areas of exhaust nozzle and LiftFan inlet are varied conversely to change the balance between them while maintaining their sum, and with constant turbine speed" So yes, the thrust is modulated to accomplish this.


12

Under the perfect wind conditions, could a normal airplane ever land (safely) with a vertical or nearly vertical trajectory? Yes, watch this video: Alaska Super Cub, 10 foot landing, 10 foot take off. Had the wind been a bit stronger the plane would have landed at a null horizontal speed.


12

Yes. I assume that you do not have a good understanding of aerodynamics. I apologise if that is not the case. The way to think about these sort of questions is to ask "does the aircraft know that it's moving in relation to the Earth". The answer to this is "no". The only thing that matters, and the only thing that the aircraft cares about is airflow (...


12

The closest to what you ask was maybe the Do-31 experimental VTOL transport. Dornier Do-31 in hover. Picture Source It was developed to fly supplies to VTOL fighter bases. Makes no sense to have a VTOL fighter fleet if you cannot supply them, right? That was what the Do-31 was for, and the technically successful program folded when NATO abandoned the VTOL ...


12

From what is available on the internet, the F35B's lift fan is more than just a simple shrouded fan. There is at least some variable guide vanes functioning as nozzle. So to answer your question, no, the lift from front and back seem to be completely (lift fan by variable guide vanes, engine nozzle by, well, a variable nozzle) adjustable.


10

A big part of making an aircraft that can take off and land vertically is the thrust to weight ratio has to be greater than 1 so that it can work against the force of gravity. The Harrier Jump Jet can only take off vertically when it isn't loaded to full capacity (in which case STOL is used). For a commercial airliner to perform VTOL, taking the A320 as an ...


10

Yes, they can accelerate straight up (even at max weight in some cases), but to accelerate straight up from 0 airspeed requires some kind of control to keep the aircraft stable. All of the aircraft's normal control surfaces only work with air flowing across them, so if you stood it up and pushed the throttles forward it would simply tip itself over. This ...


10

This seems to be a hot topic - in principle, the V-22 should be able to fly and land in autorotation, but tests so far did not demonstrate this. The manufacturer's position is that autorotation was never part of the specification. What it comes down to is the inertia of the rotating parts relative to the aircraft's mass. The requirement to fold the rotors ...


9

The F-35 has what is called the Rolls-Royce LiftSystem. This Contains one lift fan in the middle of the aircraft, and a rotating jet exhaust to provide vertical thrust, and two rotating nozzles to provide stability. The pilot pushes a button, and doors on the aircraft open for the lift fan, while the exhaust nozzle rotates downward. The pilot controls the ...


9

If you think about it, your question applies in exactly the same way to a conventional helicopter. You might expect that it would pitch nose-down if the weight is forward, and nose-up if the weight is aft. In fact, it's even worse for a conventional helicopter, because with only one rotor it would roll (tilt left-right) as well as pitch. The answer is the ...


9

Yes, Eric "Winkle" Brown took delivery of a Sikorsky R-4 and flew it after reading the flight manual and getting a briefing from a USAF Sergeant. He had no experience or training in helicopters. He wrote about it in "Wings of the Weird and Wonderful".


9

Because when selecting aircraft the requirements were for close air-support and to be able to operate from amphibious assault ships like the USS Tarawa and the USS Nassua. These ships have very small decks that aren't geared towards launching larger aircraft like the F-18.


8

What you describe is called a tiltwing. It has wing-mounted engines which swivel with the wing so their thrust points up for the VTOL mode. Building a tiltwing airliner will face two challenges: Normally, the static thrust force is only one third of the weight force, so the tiltwing airliner needs to add three times as many engines - at least. Better make ...


7

Russian YAK-141 aka YAK-41 VTOL (vertical takeoff and landing). https://en.wikipedia.org/wiki/Yakovlev_Yak-141


6

There is nothing that prevents tilt rotors from performing an autorotational landing, in theory. AugustaWestland AW 609 has already demonstrated this. The Bell XV-3 also did this. However, V22 has not demonstrated autorotation in any practical sense. The descent rate is too high for safe landing. The failure of V 22 to autorotate is due to the high wing ...


6

The V-22 was designed to be operated by the US Marine Corps on-board US aircraft carriers. Any airplane that operates off of a carrier has to be relatively compact in order to be able to fit onto the carrier's elevators and short enough to fit into the hangar deck. A twin tail spreads the elevator authority over 2 smaller control surfaces instead of 1 big ...


6

The Osprey's engines drive the rotors at 412 RPM in heli mode or 333 RPM in forward-flight mode, according to this article. With a rotor radius of 5.8m, that puts the tip velocity at 250m/s, or about 75% the speed of sound, at the higher RPM. So they aren't near supersonic yet because the blades are short enough. But there is a possibility of reducing blade ...


6

The Harrier's NATOPS flight manual (A1-AV8BB-NFM-000) is available online. Both the text and the diagram in Part III Normal Procedures confirm the opposite. Thrust is idled when "positively down". And not before. Which agrees with the dozens of soft landing videos on YouTube (base and shipboard alike). And as with shipborne helicopters, in rough seas the ...


5

The closest the US ever got to it is the ZEL program for the F-100 Super Sabre. Basically, put the F-100 on a cruise missile launch rail, strap one hell of a big rocket booster to its arse, and run for cover. Used to have a 90 minute VHS tape about the project, they got so far as to design underground launch ramps with nuclear blast proof doors to launch the ...


5

Well, engines and rotors obey the same laws regarding induced drag (induced power in this case) as wings: larger fan/propeller/rotor is more efficient, but limited to lower speeds. And from that, the advantages should be obvious: Tiltrotors (V-22, AW609) are only a little less efficient in hover than normal helicopters, but they can only go around 300 knots ...


5

The main problem with VRT is the airflow through the rotor which is disturbed because it is ingesting its own downwash. The large area of air accelerated downwards keeps being sucked back in to the rotor because the helicopter sinks back into it. In this state the rotor provides very little thrust - it acts as a sort of divide-by-zero state, as the ...


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