Warning:- Brain activity required to read the following,otherwise you will face a severe eye sight fade and a slight headache!
Small paragraph about VTOL aircrafts:-
VTOL transitioning aircraft combine a helicopter’s ability to take-off and land almost anywhere, with the speed, range, endurance and load carrying capability of a fixed wing aircraft. Some people say that they're far superiour than a CTOL(conventional take-off and landing) BUt VTOL aircrafts also have a lot of disadvantages:-
- They are mechanically complex,and maintaining them in service usually costs a lot.
- They're difficult to control in hover and transition.
- Combining VTOL with conventional forward flight is weight critical and raises costs significantly, that's why a lot of bright brains made a lot of progress in lightweight composite construction, load and stress optimization methods,
computerized flight controls and high performance propulsion systems, have introduced new VTOL concepts, particularly for unmanned systems.
- Very sensitive to any changes to Thrust-to-weight ratio ( For a lot of reasons the most common T/W ratio is between 1.2 and 1.5 -multicopters can exceed 1.6 and sometimes it goes up to 2.0 as a fail safe option in " quad- or octocopter ")-some mad people have a jet assisted multicopter that can score a T/W around 6.0 ..
- Engine failures....The aircraft must not only be capable of sustaining sufficient vertical thrust, but also balance this thrust about the center of gravity. To achieve redundancy, a massive surplus of power and/or a
complex driveshaft and gearbox system must be used,leading to unreliability.
Transitioning VTOL Configurations :-
I'll discuss the most relevant of them:-
This is the simplest method to enable VTOL capability for any aircraft is to add lift engines to the airframe,this takes up internal volume, requires some effort to avoid excess drag in cruise, and causes a considerable weight increase. However, the big advantage lies in the possibility to size the main propulsion system for efficient cruise,thus reducing the fuel fraction for that part of the flight. Consequently, also the lift engines can be designed for a single high power operating point.
We will discuss the configuration that falls under the premise that exclusive propulsion systems are installed for hover and flight.
This Picture is a simple way to distinguish a L+C configuration.
1.1 Multicopter + Conventional Airframe:-
Fusing a multicopter (quad/hexa/octo-copter) with a conventionally laid out aircraft is the easiest way to give a fixed-wing aircraft VTOL capabilities. Each system is decoupled and used in its most effective state. Using the multicopter system the aircraft ascends to obstacle height and then uses the regular propulsion system to accelerate and sustain wing-borne flight. Of course, the landing procedure is reversed.
sometimes a hybrid propulsion options (meaning electric lift motors and combustion engines for endurance flight) are used frequently and minimize the weight impact, But in reality the weight reduction is useless because of the necessity of using heavy batteries, So why do some engineers prefer it?,well Another benefit of using electric motors for short durations , is their ability to operate in overload conditions for a short time.
While this heats up the motors significantly, this can be of benefit in case of an engine failure. This is not possible for a traditional combustion engine.
this type of configuration can be and "Add-on" so you can make the aircraft a VTOl and a CTOL whenever you want!
Examples of a Multicopter + Conventional Airframe
2.Arcturus Jump 20
1.2 Jets + Conventional Airframe :-
For a small UAV this can be an "Add-on" like the previous one ,but since it's a small UAV it'll need a perfect design to stop hot gases from going into the intel again going through the compressor, resulting in a very big power reduction, But for For manned aircraft, the Lift + Cruise approach was used successfully for the supersonic French Mirage III V. However, the eight lift engines imposed a severe range and payload penalty on the aircraft and it was never able to take off vertically and successfully attain supersonic flight during the same flight ( But the X-35 did it)
2.Lift = Cruise :-
Another method to enable VTOL capability for aircraft is to use the same propulsion system for both cruise and hover. This eliminates the need for an additional propulsion system, which would only represent dead weight during the cruise or loiter part of the flight. Unfortunately, the powerful engines cause low efficiency and a very high fuel consumption during forward flight. Additionally, I can't find a UAV of this kind as they are rarely used on missions requiring very high speeds or very high climb rates, so the excess power cannot be coined into an advantage at all - I'm actually sad about this-
I will discuss the configuration that falls under the premise that the propulsion system is sized by hover requirements and also used for cruise flight.
This picture is a simple way to distinguish a L=C configuration
2.1 Tiltrotor (tilt-nacelle and tilt-duct are arguebly the same) :-
A tiltrotor use the same motors for both vertical and horizontal flight. It can rotate its propellers, rotors, ducts or nacelles in such a way, that the thrust vector can support the aircraft’s weight in vertical flight, and provides forward thrust during level flight. In some cases, the entire nacelle with the engine is tilted or using swiveling rotors, and that poses a great danger of sudden loss of control which is countered by a well mechanism that synchronizes the movement, Because The wings and fuselage stays level during the transition maneuver. This allows any sensitive payloads to be carrier like humans, sensors....etc, The wings can be small and sized to cruise requirements, because low speed flight is achieved in the hovering configuration -Great example: the V-22 - , tiltrotors are usually equipped with full span flaps, which will deflect to a 90° down position during hover,Conventional take-off is usually impossible for propeller driven aircraft, because the Diameter of the rotors prevents a forward tilt on the ground. This can be avoided using a jet propulsion system (Tiltjets),but this is at the expense of reduced hover efficiency,Tiltrotors usually use wing-mounted propellers. When a tandem-wing or lifting-canard layout is used(don't worry I'll give you some examples), it allows multicopter-like control in hover and a very desired High T/W.
1. Tilt Rotor (Bell-Boeing V22 Osprey)
2. Tilt Prop - a Tandem-wing tiltprop -(Curtiss Wright X-19)
3. Tilt Duct/Nacelle ( Bell X-22)
2.2 TiltWing :-
A tiltwing aircraft is quite similar to the tiltrotor, as it is able to rotate the thrust vector of its main engines. However, not only the nacelle and rotor are swiveled, but the entire aircraft’s wing, with the engines fixed relative to the chord.I hear you saying "WHY?" well, that have some advantages like:
-A single rotation mechanism is simpler and it will make tracking error and repairs more easier and it saves some weight too, but here comes the disadvantages which is:- during transition from hover to forward flight, the wing reaches very high angles of attack and will stall. This can be controlled by submerging the entire wing in propwash( Propwash:-The disturbed mass of air pushed aft (or fore when in reverse) by the propeller of an aircraft ) and running the engine at a high power setting,this is "OK" for take off but when you need to slow down for a landing oh boy oh boy,and don't forget that the whole wing is a offering a very big surface for the air Minimizing hover control,For propellers without swahplate ( the thing that makes helicopter blades swivel) actuation an additional method to control the pitching motion during hover is required-otherwise, good luck trying to do any STOL take off or landing -. This can be either a reaction control system (very complicated to be done with props) or an additional rotor, typically mounted in the tail
Look at rotor of the LTV XC-142
and as I mentioned you need to cover the whole wing in propwash, so you will use smaller propellers instead of few large rotors as for tiltrotors,and this allows the Tiltwing to function as a CTOL because of the reduction is propeller's diameter!
Some morons like NASA created a concept of distributed propulsion (GL-10) and some crazy guys thought it would be funny to make a distributed electric fan system that consists of 24 separate motors(Aurora XV-24), so the Germans thought it was funny to steal their work and they did steal it (startup Lilium)
there is some "hyprid between the tiltrotors and tiltwings but I won't discuss them because they're the same but the quarter of the wing will move instead of only the propeller or the whole wing a but if you want to read about them search for .. Boxwings..
2.3 Thrust Vectoring and Flow Deflection:-
Thrust Vectoring and Flow Deflection are very related to each other,the difference is thrust vectoring happens directly at the engines exhaust nozzle, while a flow diversion involves a movable device that redirects the flow through ducting to a different outlet. It is therefore found in aircraft propelled by ducted fans or jets,take the Hawker Harrier’s Pegasus turbofan engine as an example it allows the vectoring of fan-airflow and core-airflow through swiveling nozzles, but this results in ejecting very hot air along the aircraft fuselage, not to mention that the engine must be positioned really far forward, creating an unpleasant trans-sonic performance.
You can still find Propeller planes which used deflected slipstream,but they weren't that successful and The US army knew the hard way that it was best suited for STOL not VTOL operations.
1.Vectored thrust (Hawker Harrier)
2. Flow Deflection prop (Ryan VZ-3)
2.4 Tail-Sitter :-
It uses the same system for take off and landing, but it differs a lot from other L=C configurations,because The whole aircraft is tilted, not only a part of it,it sits on its empennage during take-off and landing, and transitions by pitching the entire aircraft, If propeller propulsion is used,either large flaps or a coaxial rotor system must be used to control propeller torque, using a propeller comes with a great great disadvantage of not being able to optimize the aircraft for either vertical or horizontal flight, making it very hard to land an control in hover.
Jet propulsion can also be used for tail-sitters. Afterburners are not used at take-off and landing for such a concept since the re-heated exhaust gases will melt any landing pad, so a high or ultra high bypass turbofans are more preferred, Unfortunately, this will restrict such aircraft to subsonic speeds and shows how difficult it is to combine supersonic flight and VTOL requirements, which was the goal for the US navey, many of the concepts for a tail sitter was either a delta or a flying wing.
1.Convair XFY-1 pogo
3. Lift + Lift/Cruise :-
The best overall system performance is obtainable by combining the power of the cruise propulsor and a dedicated separate system for hover. However, if the systems are not properly laid-out, the system’s performance can easily be degraded to the point where it performs worse than a well thought-out Lift + Cruise configurations. But please don't get confused between some L=C and L+L/C aircrafts
This picture is a simple way to distinguish a L+L/C configuration!
3.1 Jet L+L/C :-
These are jets which is usually use a vectoring nozzle on the main engine in the back, to deflect its thrust vector vertically during VTOL operation. This thrust is then supplemented and balanced by an additional lift engine in the forward part of the aircraft. This configuration is similar in operation to jet L+C but with reduction in weight and Volume making them more versatile then a L+C aircraft,That's it.
3.2 Augmented Jet powerplants:-
If a jet powerplant isn't producing a powerful enough thrust,You can give it an... augment!, just like video games! anyway:- For level flight we know a really powerful augment which is Afterburners ,but for hover, this is an impractical solution, because of the high exhaust temperature,SO what about extracting shaft power from the main engine and use this energy to drive a horizontally oriented fan system buried in the aircraft’s fuselage(I'm talking about Lockheed XV-4),this method is very very attractive,due to the reduction in exhaust gas temperature of the primary jet, since the work for the fan is extracted from the engine’s hot section,so no energy is wasted(not really)
1. This disgusting piece of SH*t
2,NOTE:-This plane (XFV-12) Doesn't use the method mentioned above, But I'll discuss it in the near future.
AND THE LAST ADVANTAGE:- With VTOL you can Teabagg Your kills.