Many customers prefer to imagine ideal squeaky weather in a squeaky blue sky without even a cloud in the sky. I have to say that as a helicopter pilot I find that rather boring. For me, clouds have a certain fascination due to the shapes and light interactions depending on the sunshine. They look even more interesting up close. Of course I mean only harmless cumulus clouds and less the sometimes dangerous thunderstorms that can cause gusts, heavy rain or even hail. A completely covered sky is not bad if the ceiling is high and there is good visibility underneath. Another advantage of clouds is the shadow they donate. The heat on the ground is not only caused by the hot air from North Africa or elsewhere. In our latitudes, the sun radiates more (and less in winter) perpendicular to the earth in summer and heats it. The surface can get so hot in midsummer that drivers can fry the famous fried egg on their bonnet. Even if the engine is off and the bonnet is not additionally heated by the engine that is boiling in a traffic jam.
Cumulus clouds - with the ability to grow into a tower
Since a lot of glass is usually built into helicopters to improve visibility, the passenger compartment heats up extremely quickly in strong sunlight. Before the helicopter actually takes off, I leave the doors open for as long as possible, otherwise the cell would become too uncomfortable for the guests. And I can do the typical checks shortly before departure with the door still open. During my apprenticeship in Florida it was even more of a standard to expand the doors on the student and also on the flight instructor side and to fly completely without doors. When I close the doors I often joke that the air in the interior now heats up a lot, but as soon as we get up to speed, the Mexican air conditioning starts - by that I mean the flaps on the nose of the helicopter that let in the airstream.
It is also becoming increasingly hot in Germany
The air is getting thin
But back to the road: If the sun now shines on the asphalt, it is heated depending on the existing initial and ambient temperature. The air above the asphalt heats up on the hot asphalt and becomes thinner as a result, because energy-charged air molecules move faster and thus keep more distance from one another. Thin air is therefore lighter and wants to go up. This is the principle of the hot air balloon - the air in a huge rubber bubble is heated by a gas burner until the bubble rises upwards with the basket dangling underneath. The important point about this story is that the air is getting thinner - so the density is decreasing. Warm air flowing upwards is what glider pilots and paragliders need and call thermals - the lift when I don't have a motor. Well - it's your own fault.
Thin air in and of itself is nothing particularly dangerous - we know that the air on a mountain peak is also thinner than on the valley floor and becomes thinner the further up we move away from the surface of the earth. If we keep going up, we will eventually be in space, where there is no longer any air. Commercial aircraft that fly at an altitude between 30.000 and 40.000 feet (about 10-12 kilometers) above sea level must therefore pressurize the cabin like a balloon so that the passengers do not pass out due to lack of oxygen.
Philosophy digression: selfishness
You all know the saying “In the event of a sudden drop in pressure, oxygen masks will fall from the ceiling in front of you…. First put one on yourself and then help any children traveling with you. " What? Should i help myself first? Doesn't it mean “women and children first” everywhere? Very honorable to think so, but: It may only take a few seconds for you to lose consciousness. If you use this time to keep yourself active, you can help others afterwards. For example the people who are sitting next to you and who did not manage to do the mask in time. I like to quote this saying in other life situations with “You can only share what you have”. The easiest way to understand this is with money. Anyone who has money can share it. The same also applies to less tangible things, such as energy, joie de vivre or time. You are of no use to anyone if you are burned out or sleepless. So first take care of yourself before you please everyone else and neglect yourself in the process. This is often formulated as egoism by those who demand, such as supervisors, spouses, children, friends, but is in truth pure self-care so that you remain capable of action and enjoyable for others.
Always this pressure….
So when we walk around on the surface of the earth, we always have a pillar of air above us that weighs on us and exerts a certain pressure on us: air pressure. At sea, the pillar is higher up to space and creates higher pressure. On the Zugspitze at 3.000 meters above sea level, the column is no longer quite as high up to space and generates less pressure. High pressure at sea level makes the air thicker and a little warmer.
Now the air pressure does not depend on the altitude alone. The weather that arises on earth, with all its winds, currents and temperature differences, creates phases or areas with higher air pressure and those with lower air pressure. Depending on where we are on the surface of the earth and what the weather is right now. Wind is essentially created by pressure equalization between high pressure areas and low pressure areas.
Another, albeit relatively minor, factor that affects the density of air is humidity. Compared to dry air, moist air is also lighter than dry air and also wants to go up. And what does all this have to do with flying? Well - when the air is thinner, more power has to be applied to aerodynamically take off. The power of the engines is limited. On the one hand, the engines generate less thrust when it is very hot, but on the other hand more power has to be applied so that the less numerous air molecules present in the hot, thin air generate the same lift. The aircraft must therefore be faster in the heat until it can take off and thus accelerate longer so that the speed required for lift is achieved and the lift force can let the aircraft rise into the air. If that sounds incomprehensible to you, I recommend our series "Why can a helicopter even fly?".
In hot weather, an airplane has to accelerate longer to take off
But at some point every runway is over. Honest. If the required speed is not reached by the end, the aircraft cannot take off. Fortunately, you can calculate something in advance and don't have to try it out. So you can calculate how much thrust the engine can bring to the air at a given temperature / density / humidity. Then it can be calculated how much time and therefore the runway all the existing engines need to bring the possibly fully loaded and fully fueled aircraft from the resulting weight XY kg to the speed necessary for takeoff. If the runway is not enough, there is still the possibility to reduce the weight of the aircraft. So let a few passengers get off or unload some cargo. Or not to fill up, just to take the fuel you need for the flight, plus a safety reserve, of course. This is already being done anyway. There are maximum temperatures and other environmental values for each aircraft type, up to which the manufacturer certifies the usual performance of the aircraft. Operation is prohibited if these values are not observed.
Since we don't need any runways or streets with helicopters (“Streets? Where we're going we don't need…. Streets!” - Doc Brown, “Back to the future”), the runway length factor simply disappears. The helicopter generates the lift solely through the speed of the rotor. In order to generate the necessary lift force, the rotor blades have to be pitched more steeply than otherwise in cooler conditions. That costs more power or performance, which we demand from an air-cooled motor, which from the outset also produces even less performance with thinner air. As a helipilot, we have to pay particular attention to the engine and how it is doing in such situations. When the engine is doing well, we're doing well too. Here, too, we calculate the feasibility in advance. We calculate the lift-off weight consisting of the unladen helicopter weight, refueling and passengers with luggage if necessary. It is therefore possible for a 20-minute sightseeing flight to take off with only 175 liters instead of 60 liters of full aviation fuel - that makes the helicopter a good 80kg lighter!
The boost pressure / manifold pressure
But how do we know how much power we are currently demanding from the engine? There is also a display on the instrument panel that is not that easy to understand at first - I'll try anyway: In one of the blog posts, we explained the 4-stroke gasoline engine in detail ("Today neither sightseeing flights ..."). In it we read that once the engine is running, it wants to suck in a combustible gasoline-air mixture by filling the cylinders. But we are mean now and we shut his throats so that he just survived and won't go out. Perhaps this is where the now very appropriate term “stall” comes from. So the engine sucks in like a vacuum cleaner and we hold its trunk almost completely closed. This creates a negative pressure in the vacuum cleaner tube…. sorry, in the suction pipe and this negative pressure is displayed on the lower row of the instrument on the far right with the label “Manifold Pressure”. In German, the corresponding term is “boost pressure” - this is the pressure with which the combustible gas mixture is loaded into the engine.
Part of the R44 instrument panel with the manifold pressure indicator in the lower right
In the picture above the engine is switched off - the pointer is on 30. Yes, ok. But what? 30 "inches of mercury" - or in German 30 inches of mercury. Since we in Europe work less with inches but more with centimeters and millimeters, that would roughly correspond to 30 millimeters of mercury (or mmHg for short - Hg is the symbol for mercury). In addition, in aviation we still use hectoPascals or hPa for short, which would then be around 762 hPa, just to have mentioned that before. We will come across this unit at least once again later. So the 1016 inches is simply the current ambient pressure. When the engine is not running, the probe in the intake pipe precisely measures the pressure that acts on it from outside through the various intake stages. As soon as we start the engine, it wants to suck in and a flap in the intake pipe will throttle the gas supply to the engine - hence the name throttle valve, which you may have heard of before. The engine draws in, does not get as much as it wants and a negative pressure is created - and this pressure is lower than the ambient pressure. Hence the colloquial “negative pressure”. The pointer moves to the left to values around 30 inches when the engine is idling. Incidentally, you also hold your car engine in the not very friendly-sounding stranglehold every day when you are at the traffic lights and the engine is idling. Unless you have a supposedly fuel-efficient thing that lets the engine die completely at the red light, only to laboriously revive it after a few moments at green. I love these puns.
Back to the helicopter. Then when we want to get to the nominal speed, we accelerate. We open the throttle a little by turning the throttle grip in a helicopter, similar to a motorcycle. When driving a car, ideally press the gas pedal with your right foot. If the helicopter pilot then turns on the blades during take-off to generate lift, more gas is automatically given to compensate for the air resistance and to maintain the speed. Meanwhile, the display in the manifold pressure display slowly moves up or to the right. As long as the pointer is in the green area - everything is in the green area. Yellow is seen as not uncritical and there must be a special reason for the red line. This is how easily colors mark a not so good condition.
The maximum continuous load
The engine is so constructed and designed that more power than the red line should never be demanded. If so, we may have to expect a slightly higher bill at the next engine inspection or, in the worst case, a drop in engine speed in flight because the engine can no longer deliver the required power. And when the speed drops, the lift force drops too and that means only one direction: downwards. So the motto is: avoid! Since we're on the subject of engine performance: The manufacturer Robinson Helicopters has for the R44 (see also "R44 helicopter in portrait“) Defines a diagram of the maximum power that can be demanded from the helicopter engine. A distinction is made between the maximum permissible continuous power in cruise flight (Maximum Continuous Power, MCP) and the take-off or landing process, which may require more power. The height above sea level and the temperature were incorporated as the two main factors of air density. The air humidity was omitted because of the minor influence. This diagram looks like this:
The pilot first determines the altitude at which he is. The Jesenwang airfield near Munich, for example, is 1861 feet above sea level, i.e. almost 2000 feet. The outside temperature is, for example, 18 degrees according to the display - so let's take the 20 degrees column. The table gives us a value of 22.3 inches that we are allowed to pull. To get started, it says under the table that we can add 5 inches for a maximum of 2.8 minutes. So we know that when the manifold pressure starts, it should not go above 25.1. During cruise, this value is then redetermined from the current flight altitude and outside temperature.