Author Topic: Helicopter flight  (Read 1970 times)

0 Members and 1 Guest are viewing this topic.

Offline Adrian Chitan

  • Pilots
  • Full Member
  • **
  • Posts: 153
  • Popularity: +17/-1
    • View Profile
Helicopter flight
« on: November 24, 2014, 04:27:30 PM »
 

Hi guys,

me again :))). I know it's not a big thing in our VATSIM environment but it is loads of fun and very awarding once you do it right. While a helicopter generates lift in the same way as an airplane (theoretical point of view), controlling a helicopter is totally different because of the engineering setup.

I will however start with the bad news (at least for us, the simmers). There are very few good helicopter simulations out there. Just to set the record straight:
FSX/Prepar3D - none
X-Plane - Bell's, Eurocopter's and Robinson's
Yes, FSX/P3D have no real helicopter sim (they say so, not me). As with any study sim, FSX/P3D considers performance tables when modeling the response of an aircraft. While for airplanes this can cope quite well with the real feel of a flying machine, it does not for a helicopter. Helicopters are about acute corrections of the lift plane, while on airplanes this remains static for entire flights. When you also put the weather in the equation, you realize how complex flying even the simplest helicopter (i. e. a Robinson) is over flying a simple plane. And to finish up the FSX/P3D part, I have two ATPL-H friends and both of them said that FSX/P3D is good for the first year training when they all fly planes at the academy. Just adding torque deviation doesn't make a helicopter.

Looking at X-Plane, it is much better at simulating helicopters (actually, any object in a airflow stream - that's what it was made for). But even so, helicopters take a little bit more twiking and real pilot testing before they are released as "realistic flight model". But being able to recompute the forces on an object at least 20 times every second in a given airflow does help a lot. And the weather in X-Plane is perfectly simulated as you know (the atmosphere is divided into little cubes of air, and each and every one's effect on the overall physics is computed). That is why even the default helicopters on X-Plane are "acceptable".

Just to complete the helicopter sim picture, I have to add that the best helicopter simulators today are published by Digital Combat Simulator: Ka-50 (superb coaxial-rotor Russian military helicopter - airborne "heavy artillery" class), UH-1H Huey (the US work horse during the Vietnam War and onwards), Mi8-MTV2 (large Russian transport helicopter). As an advice (only if you have these), from experience I would recommend the Huey for your helicopter flying lessons (clean and fast to respond). Having all these three beauts, I would not recommend the Ka-50 because it is not a classical helicopter but a coaxial rotor one and it is controlled differently than a normal heli (to fly it in a coherently manner you are helped by some A/P channels); nor I would recommend the Mi-8 because it is a bit large for the initial feel of a helicopter (slow response and quite a lot of A/P channels to make it stable). Unfortunately, DCS has nothing to do with VATSIM, they just fly in their little war infested region around the Crimean peninsula. Nevertheless, they are the best in helis.

Lift generation

As opposed to airplanes, a helicopter has three forces acting on it at any given time: lift (/thrust), drag and weight. In a normal configuration, a helicopter has a main rotor and a smaller tail (/anti-torque) rotor. The main rotor generates the lift and it controls the motion of the helicopter, while the tail rotor opposes the opposing torque effect of the main rotor so keeps the helicopter from spinning (this effect can be felt at prop-airplanes too, but to a smaller scale because of the smaller propellers and it is easily counteracted with the application of opposite rudder input).

The blades of the main rotor generate all the necessary lift in something called rotating airfoil concept. In a normal airplane, the airflow washing the wing generates lift in a combination between Newton's third law of motion and Bernoulli's venturi principle. In a helicopter the airflow is generated by spinning the "wing". Because the lift generated by a wing is directly proportional with the squared speed and along the length of the rotor blade (wing) we have different speeds (the tip is moving the fastest while the segment near the hub is the slowest), the lift distribution generated by a helicopter (no transverse motion and no weather) looks like the figure below


Because the main rotor has to generate lift for both keeping the machine airborne and moving it through the air, the rotational speeds are quite large for the dimensions of the rotor. For example, a Huey helicopter has a two blade main rotor with huge steel blades (it can easily cut through small tree branches). Its rotor spins at 324 rpm. Knowing that the main rotor has a diameter of 14.63 meters, we get a tip speed of 893.07 kmph (482.22 kts). And here lies the first problem of a helicopter. There is no wing in the world that can generate efficient lift in supersonic airflow, but a helicopter's blade tip can easily get to sub/super-sonic speeds (add to the above values the transverse motion of the chopper) and loose lift. That is why helicopters move slower than planes. The fastest helicopter in the World is the Westland Lynx recorded at 400.87 kmph (but it uses angled blade tips to counteract the supersonic flow just as an airplane uses wing sweeping to counteract the same problem).

To accomplish this, the blade has four angular degrees of freedom: lag angle, bend angle, collective angle and cyclic angle. Lag angle comes in to help with the supersonic airflow by adding a sweep angle to the entire blade based on the drag the blade experiences at any given moment. It is not a controllable angle by the pilot but intrinsically built in by the engineers. The bend angle is another built in angle and it refers to the up down motion of the blade based on the lift in generates at any given moment. The collective and the cyclic are the angles a pilot uses to fly a helicopter.

The collective

The collective is best described if you associate it with the power setting on an airplane. While there is a power lever on a helicopter, after starting the engine it is set to the governed/max setting and remains there for the entire flight. A helicopter's rotors ALWAYS spin at the same rpm throughout the flight. Depending on the collective angle, an electronic governor automatically sets the power output of the engine to maintain the same rpm (higher collective angle, higher power output - in laminar flow, not stalled blades).

The collective angle is the angle at which each and every blade attacks the air (AOA of the blades). It is called the collective because it refers to all blades at once. Because the lift generated by a wing is directly proportional to the AOA, the higher the collective, the larger the lift generated. Of course, as any other wing, a blade can stall. Actually, along the blade's length there will always be a certain portion of it in a stall condition. Depending on the model, the maximum achievable collective angle is theoretically calculated so that the blade's separation angle will never be reached, and a maximum of 14 degrees is usually found.

The collective is usually controlled by an up-down lever (called collective) on the left of the pilot. It has a blocking mechanism so that it is not moved accidentally and as I said, it controls the total lift generated by the rotor.

The cyclic

The cyclic can be associated with the ailerons and elevator of an airplane. The cyclic moves tilts the lift plane around the rotation hub by modifying the blade's AOA through the rotational circle. So, while spinning, the same blade can have a certain AOA (let's say unmodified AOA given by the collective setting) while passing above the nose of the helicopter and a slightly modified AOA 90 degrees later on the circle (while passing above the right door) and then back to the unmodified AOA while passing above the tail. So this is a cyclic change of the AOA and that's why it is called the cyclic.

By this slight change in AOA at certain positions around the rotation circle, an asymmetry in the lift generation is induced which will tilt the lift plane and consequently the lift vector's direction. And if the lift vector is not pointing straight up then, if we decompose it, we will get a new transverse force beside the weight opposing one which will make the whole chopper move in that direction. As a secondary effect, the body of the helicopter will pitch and roll because of the this asymmetry so that's why it can be associated with the ailerons and the elevators on an airplane.

The cyclic is controlled with the stick between the pilot's legs (called cyclic) and controls the cyclic change of lift around the lift plane.

The tail rotor

The tail rotor is there to counteract the induced effect of opposing torque of the main rotor. If a propeller spins (and creates thrust!!!) in one direction, then there is a spinning force in the opposite direction transmitted to the platform. In the case of the helicopter, given the size and thrust generated by the rotor, this opposing spinning force is huge. So whole new propeller is needed to counteract this effect. Don't be fooled by the smaller size of the propeller. To counteract the opposing torque of the rotor a similar rotor would be necessary spinning in the opposite direction and creating the same amount of thrust, that's how massive the torque induced to the body of the helicopter is. But because it is about torque, engineers can use a smaller force (propeller) but with a longer application arm (that is why it is mounted in the tip of the tail and that is why, actually, a helicopter needs a tail) to achieve the same torque and counteract it.

The tail rotor's AOA (and thus the force generated) is controlled by the pedals and it acts in the yaw axis of the helicopter. That is why the tail rotor can be associated with the rudder of an airplane.

Helicopter flight

Given these control inputs, the pilot learns to fly a helicopter by conditions and transitions and practices them until they become reflexes because they are not always intuitive (as most things are in airplane flight - a helicopter can climb and descend in the same nose up attitude and turn in 0-bank angles or even opposite bank angles). The basic conditions of flight for a helicopter are: hover, rolling takeoff, climb, descent, cruise, rolling landing, landing, autorotation. The transitions are all possible permutations between these basic conditions and there are checklists to follow for each and every one of them (each of them involving more artistry than actually following up a set of steps - try getting from cruise to hover :))

Hover

This is one of the hardest condition a helicopter can achieve. It is the difference between an airplane and a helicopter. It's when the helicopter floats above a fixed point at a constant altitude (actually, hovers are more about height than altitudes). The pilot, using the cyclic, the collective and the pedals, has to keep the machine in a constant spot in the sky. There aren't many maneuvers that can achieve a more dramatic effect than this (maybe a plane landing with just one wing - possible, seen it on youtube). You have to actually achieve and keep a perfect balance between the lift generated by the rotor, considering wind and load configuration. There is no trim for hover and hands off the stick, because the lift can get asymmetric even with a small local change in air temperature.

The hover consumes the most fuel per hour in the same load configuration because it needs more power than any other condition. Furthermore, because the engine needs rushing air to cool down, the maneuver heats up the engine the most. In reality, there is no helicopter in the world that can stay in the hover for prolonged amounts of time. The exception is the Ka-50 which is the only one that can cool down very well during a hover so it can stay in that condition until its fuel runs out (but the Ka-50 is one of the best helicopters in the world and can do lots of things that other choppers can't).

This maneuver is most efficiently (least amount of power used) done in ground effect and the "hover check" is the usually mandatory for any helicopter before actually taking off. The hover check shows if the helicopter can cope with the load and also shows the pilot how the helicopter will feel once airborne. This check is done a few feet above the runway/helipad.

Rolling take-off

A helicopter can take off by doing a climbing hover from a helipad. But when its load exceeds a certain amount, to save fuel the pilot will do a rolling take-off from a runway, just as a plane would. The forward motion of the helicopter will add speed to the lift plane so this in turn will add lift, thus making the heavy take-off possible.

Climb

A helicopter will usually climb in motion and not in a hover climb. Just as in the case of the rolling take-off, the rotor will generate more lift when aided by the oncoming air.

Cruise

Cruising a helicopter is an equilibrium between the upward lift and forward lift (if you decompose the lift vector). There has to be enough upward lift to keep the helicopter airborne and enough forward lift to pull the helicopter to a normal cruise speed. If the speed is to low, than the range will be decreased and the same happens if the speed is to high (the drag increases with the square of the speed). Because the forward motion will add additional lift, these parameters have to be managed by the pilot.

Descent

The descent with a helicopter is tricky. It can descent with or without forward motion but there is a condition in a rotating airfoil where during the descent, because the air is coming from below, can stall the entire lift plane. It's not a normal stall given by a high AOA, but it is induced by a negative AOA. When this is reached, the helicopter drops and the descent can become unstable. Recovery involves increasing the negative AOA (to get to positive) on the blades and increasing the forward motion to further increase the AOA by pushing the cyclic forward. The problem is that if there is not enough altitude, there is nothing you can do.

Rolling landing/landing

Besides managing the descent, the pilot also has to masterfully put the helicopter on the ground, be it a rolling landing or a hover touch-down.

Autorotation

The autorotation is the combination of all the skills a helicopter can collect. Despite popular believe, a helicopter that looses engine power will not fall out of the sky. The is no air machine in the world that does that because it would not get certified ;). When a helicopter looses engine power, it will use incoming air to create lift, thus making the rotor a salvage parachute. The physics behind this condition is quite complex (as is every other condition in helicopter flight) but the main idea is that every helicopter is built with a carefully calculated autorotation angle. If the pilot can set this angle on the collective and make the necessary adjustments to achieve the perfect free airflow, the rotor will be rotated by airflow going through the middle segment of the rotor, while the outer part of it will generate the lift necessary to descend in control and make that perfect flare before touchdown. Of course there are some technical procedures to do before going into autorotation (i.e. disconnecting the rotor from the reducing gearbox and engine because the rotor has to be free of any additional friction) but it saves your skin. The only requirement for autorotation is time/altitude. You need to be able to reach the autorotation condition before slamming into the ground. Anyway, it's an extremely rewarding maneuver (I've never done a successful autogiration in DCS, while in XP I have no payware helicopter - it has to say that the model is autorotation capable otherwise the model was not developed to autorotate, all payware helis are).

While in autorotation, there is no adverse yaw effect (opposing torque) because the rotor doesn't disturb the air around it. This means that autorotation is useable even during loss of tail.

As a final word on autorotation, there are helicopters that autorotate better than others. It depends on the rotor's inertial momentum and the designed blade load.

Engines

A helicopter can use both cylinder and gas turbine engines. Nowadays, only very small and old choppers use cylinder engines. The majority use turboprop engines because of their high power to weight ratio. Even the 1952 Huey is turboprop powered, while many helicopters have two turboprops.

Biggest

The biggest helicopter in the world (and most powerful) is the Russian Mi-26. It can take off at 56 tons (22 of which are payload tons) and it is powered by two Lotarev turboprops which can output a total power of 22'798 shp (shaft horsepower - the power that actually reaches the rotor shaft). The diameter of the main rotor reaches a staggering 32 meters (a Huey has 14).

Maximum ceiling of the Mi-26 is 15'000 ft which is quite high for a helicopter because helicopters are highly dependent on the density of the air - so are planes, but they can play in a high range of speeds, while a helicopter can only do so much before reaching supersonic speeds at the tip of the rotor combined with less lift on the blades than on the huge wings on a plane.

To top this off, the wing area of an Airbus A380 is 845 squared meters while the disc area of the Mi-26 is 804 squared meters. The word is: HUGE.


Conclusion

If you want a taste of helicopter flying you can read the DCS UH-1H Huey flight manual (take it from https://www.dropbox.com/sh/r0gvzec1rj8qyry/AAAH2WGe53sGIMib8xT3TWT2a?dl=0). Of course, helicopter flight is immensely complex and there are 800-page books covering only the main rotor conditions. I will try to get into this (on VATSIM, because I have a lot of training on DCS but none in XP) shortly to widen my virtual horizon :D and that's why I have started this post here. Hope you like it!
« Last Edit: November 25, 2014, 09:21:52 AM by Adrian Chitan »

Offline Bogdan Ghincea

  • Captain
  • Administrator
  • Sr. Member
  • *****
  • Posts: 273
  • Popularity: +5/-1
  • Flying is a way of life
    • View Profile
    • Air Danubius - virtual airline company
Re: Helicopter flight
« Reply #1 on: November 25, 2014, 07:44:00 PM »
Nice! Keep us posted!

Offline Adrian Chitan

  • Pilots
  • Full Member
  • **
  • Posts: 153
  • Popularity: +17/-1
    • View Profile
Re: Helicopter flight
« Reply #2 on: December 09, 2014, 07:57:32 PM »
Hi again,
just did my first helicopter flight in X-Plane on VATSIM around Vaclav Havel airport (LKPR). I did not file any plan because I just wanted to take off, go north of the airport and do some turns, altitude controls and certain special heli conditions. It was just a 10 minute flight for it was pitch dark because of those fairly low stratus clouds out there.

The test was done using the default Bell 206 helicopter in X-Plane. Even so, the model is quite complex but it didn't give me the feel of flying a helicopter as DCS does. While X-Plane is capable of simulating vortex ring conditions (the helicopter descends so rapidly that the updraft interferes with the flow through the rotor plane and the machine losses all it's lift suddenly), I found it very hard to enter VR which should be no problem at all (low forward speed - less than 30 kts - and high descent velocity). I could easily get out of high negative VVs just by adding collective which is the worst thing you can do while in VR. In this respect the helicopter acted like it had a vertical turbo fan.

Another problem were the high G turns. A helicopter is not capable of loading up to 9 G (rotor strike of the fuselage, structural damage, low blade load compared to a 9G fighter capable of huge wing loads, etc.). But in X-Plane's Bell 206, no problem :))). I took turns just like in a fighter aircraft. One last point I should add is that in a helicopter, turns are negotiated differently than in an airplane. Pedals are used actively in the turn as opposed to airplanes where the pedals coordinate the flight path of the plane. This didn't happen in my test.

The hover was un-achievable, at low forward speeds the helicopter is very jerky in controls (I didn't set up a good trim method for helicopters). An advice I saw about piloting helicopters in X-Plane on the internet was to put up the flight model information and try to keep everything in the green (don't drive the model into fierce computation). But again, this was a default helicopter, my first flight with it and I saw people on the net piloting it quite nicely. Furthermore, there are paid add-ons out there that address the exact issues I mentioned, so there is definitely lots of room for improvement in flight modelling.

On the good side, the model was quite challenging at takeoff, with massive correction in the yaw axis because of main rotor adverse yaw. This felt like a helicopter.

The METAR for LKPR was LKPR 091800Z 21005KT 160V250 0550 R24/0700N R30/0650N FZFG VV001 M03/M03 Q1029 NOSIG.

Offline Bogdan Ghincea

  • Captain
  • Administrator
  • Sr. Member
  • *****
  • Posts: 273
  • Popularity: +5/-1
  • Flying is a way of life
    • View Profile
    • Air Danubius - virtual airline company
Re: Helicopter flight
« Reply #3 on: December 13, 2014, 11:27:06 AM »
SO if you want real virtual heli experience DCS is the way then.

Offline Adrian Chitan

  • Pilots
  • Full Member
  • **
  • Posts: 153
  • Popularity: +17/-1
    • View Profile
Re: Helicopter flight
« Reply #4 on: December 18, 2014, 11:56:21 AM »
No doubt about that!

If you read one of their manuals (Ka-50, Mi-8 or UH-1H) you'll see how extraordinary the physics is implemented. They present helicopter physics with "everything here is implemented". And when you test it, it works. Just to set the record strait, DCS (actually not just DCS/Eagle Dynamics but also the companies who started bringing these superb modules to DCS World - Leatherneck, Belsimtek, etc.) implement real torsion and swash plates to helicopters, so even the control of the blades is done realistically. Add to add, they have realistic material constraints (i. e. the blades on a Hwey were built from heavy steel to be able to cut small branches out of the way when flying through jungles). Too bad it's only in DCS' world :) and too bad it's only military (though you can take an Mi-8 on a police surveillance mission around a city - but then again you can put some rockets on the pylons and scare even the biggest and meanest tank with it :)) ).

Anyways, I like DCS. <<'O lot!>>

Cheers.
« Last Edit: December 18, 2014, 11:58:06 AM by Adrian Chitan »

Offline Adrian Chitan

  • Pilots
  • Full Member
  • **
  • Posts: 153
  • Popularity: +17/-1
    • View Profile
Re: Helicopter flight
« Reply #5 on: January 27, 2015, 07:52:46 AM »
Hi again, Bogdan

I have just found out that X-Plane now has the best helicopter simulation in the desktop sim market with its Dreamfoil Bell 407 at 34.95 USD. So, as you can see, the battle is between XP and DCS (both use Blade Element Theory to simulate flight). FSX/Prepar3D don't even enter the competition.

Cheers.

Offline Bogdan Ghincea

  • Captain
  • Administrator
  • Sr. Member
  • *****
  • Posts: 273
  • Popularity: +5/-1
  • Flying is a way of life
    • View Profile
    • Air Danubius - virtual airline company
Re: Helicopter flight
« Reply #6 on: January 27, 2015, 01:52:35 PM »
Unfortunetly you're right when it comes about Heli. I sow a movie when GTA V and Battlefield were entering heli competition also. Strange aah?

Offline Adrian Chitan

  • Pilots
  • Full Member
  • **
  • Posts: 153
  • Popularity: +17/-1
    • View Profile
Re: Helicopter flight
« Reply #7 on: January 29, 2015, 07:20:07 AM »
Hi, Bogdan

I was kind of intrigued by your GTA 5 and Battlefield...choices so I looked them up (flight model, flying lessons, how it handles in youtube clips, etc.). Conclusion: point and shoot arcade games (emphasis on "games" and not simulators).

GTA5: the last GTA I played was GTA4. Having said that, it is definitely a fun game but even the physics behind a car's behaviour are NOT well modelled there so what do we expect from helicopters flying near skyscrapers where updrafts should play a major role (not that calm weather flight is well modelled but just to give you an idea of the challenges). Also, if you watch the videos on youtube, you will see lots of things that are missing like stationary lift plane, no vortex ring effect on harsh descents, no inertial momenta in any axes, etc.

Battlefield: I kind of studied this title because at some point I wanted to buy it (every 5 years or so I like to get up to date with tactical shooters - unfortunately Battlefield 3 was nothing more than a long war movie for which you had to press the mouse button a few times...anyways). The flight model is non existent because it is a linear action game. So even if it mimics take-offs and turns, it's not because it's a simulator but because it doesn't know how to do anything else. Think of it like this: they filmed an Apache taking off, shooting something on the training grounds and coming back; you take this movie and generate an animation of it for which instead of training targets you draw insurrectionists with bazookas hidden behind some clay wall or some other bullshit. Of course some things have to be extrapolated to let you shoot from any angle to give you the impression that you have control, but there's nothing simulated in that.

Now, do you remember Tom Clancy's H.A.W.K.S. (1 or 2 - they were actually made by Ubisoft Romania from where Silent Hunter 3 and 5 also came)? Some people were actually putting it in the simulator category even though it was nothing more than what Novalogic was doing like 15 years ago but with improved graphics and gameplay. Like having the Tom Clancy "stamp" on the title meant they were highly detailed technical simulators. Tom Clancy was a novelist and that what HAWKS was, a linear novel for which you have to press a button once in a while to get to the next chapter.

And as long as we're talking about helicopter simulation (or any simulator for that matter), we cannot discard the impact of well coded systems. Even if the flight model is great, we still like to see highly accurate avionics and other systems. None of the above mentioned titles accurately simulate any systems or even try to do that. Their target market are users that press the "let's get into the action" 5 seconds after they have installed the game.

Cheers.
« Last Edit: January 29, 2015, 07:23:58 AM by Adrian Chitan »

Offline Bogdan Ghincea

  • Captain
  • Administrator
  • Sr. Member
  • *****
  • Posts: 273
  • Popularity: +5/-1
  • Flying is a way of life
    • View Profile
    • Air Danubius - virtual airline company
Re: Helicopter flight
« Reply #8 on: January 31, 2015, 10:58:33 AM »
You are totally right.

Offline Adrian Chitan

  • Pilots
  • Full Member
  • **
  • Posts: 153
  • Popularity: +17/-1
    • View Profile
Re: Helicopter flight
« Reply #9 on: February 17, 2015, 02:35:01 PM »
Hi again,

I found out a little more about FSX and helicopter flight. As I said, helicopter flight is not modeled in FSX at all. But there are good news. Aerosoft created something called Huey X. This tries to model the Huey helicopter in FSX. Because FSX knows squat about helicopter flight a special new library had to be created for the Huey. This library adds torque induced yaw (now you can see the extent of how poorly FSX is when simulating helicopters, it doesn't even have the torque induced yaw effect), loss of tail effectiveness (when you make the tail blades stall) and retreating blade stall (this may be one of the most encountered phenomenon in helicopter flight). What is NOT yet modeled is the vortex ring state which is quite important for helicopters (I think it has to do with the fact that FSX doesn't care about up-drafts and down-drafts on wings, being a pseudo-parallel flow simulator) - so you can descent the Huey like a jet fighter with no repercussions. In any case, the vortex ring effect can be simulated by the pilot if he limits his descent rates based on forward speed (the faster you go the faster you can descent without being caught up in your own up-draft).

The problem is that the library is only made for the Huey and it is not a general FSX library for other developers to use.

Cheers.

Offline Bogdan Ghincea

  • Captain
  • Administrator
  • Sr. Member
  • *****
  • Posts: 273
  • Popularity: +5/-1
  • Flying is a way of life
    • View Profile
    • Air Danubius - virtual airline company
Re: Helicopter flight
« Reply #10 on: February 17, 2015, 06:07:48 PM »
Let's hope maybe someone will implement a general module trough fsuipc so will work on all FSX helicopters. Will see.

 

Sitemap 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27