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High-altitude flying - check my reasoning


Fishbreath
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I'm setting up a mission in which I fly as AFAC for my A-10 buddy. We'll be up in the mountains northeast of Tskhinvali in the wintertime, which puts me at about 2200 meters with deicing required, which is conditions where the engine governors are getting a serious workout.

 

Here's the scenario: I'm in a hover with a heavy load (six Vikhrs and a launcher's worth of smoke rockets, plus two fuel tanks for added loiter time over the target area). The overhead panel warning lights say the governors are limiting engine speed, and I'm sinking at maybe one meter per three seconds or so. Blade angle is about 13 or 14 degrees, and rotor RPM is sitting at around 82%-83%. Is it better to reduce collective? That would mean the rotors are turning faster and the engines aren't working as hard, but is it generally the case that the increase in rotor RPM would be sufficient to counteract the lower blade angle?

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That is quite the question. I do not have an answer, but the fact that question could even be asked in a flight simulator, makes me want to go fly the shark right now. I freaking love this sim.

 

Sorry for the OT.

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In some cases, but generally not. Just like with aircraft wings the helicopter rotor blades have some certain angle of attack that generates lift most efficiently. This angle is mostly relevant in hover as other things become more relevant in forward flight. You would need a fuel flow or torque meter to see how much power you are using and then slowly changing the rotor RPM and collective in unison to keep the altitude you would seek the minimum in required power. When hovering with light load you might find that lower RPM actually gives you more lift with less power but in general the difference wouldn't be great and you would be sacrificing flight safety in the process. In single engine flight it's very easy to choke the engine with too much collective and drop out of the sky as a result. The default rotor RPM is a result of optimizing multiple variables to get a happy compromise and generally not adhering to it isn't going to do you any good but usually just the opposite.

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In some cases, but generally not. Just like with aircraft wings the helicopter rotor blades have some certain angle of attack that generates lift most efficiently. This angle is mostly relevant in hover as other things become more relevant in forward flight. You would need a fuel flow or torque meter to see how much power you are using and then slowly changing the rotor RPM and collective in unison to keep the altitude you would seek the minimum in required power. When hovering with light load you might find that lower RPM actually gives you more lift with less power but in general the difference wouldn't be great and you would be sacrificing flight safety in the process. In single engine flight it's very easy to choke the engine with too much collective and drop out of the sky as a result. The default rotor RPM is a result of optimizing multiple variables to get a happy compromise and generally not adhering to it isn't going to do you any good but usually just the opposite.

 

Right, but at high altitudes the calculus changes a little: with the engine governors limiting engine RPM, the rotor RPM would (I would think) be below where the flight control system wants it to be for the given collective setting.

 

Some of it might be the wind in this mission, too—it's pretty intense at 2000 meters, and the shape of the valley might be causing updrafts and downdrafts. At one point I couldn't hover at full collective, and at another I was hovering with less collective than I normally need down near sea level.

Black Shark, Harrier, and Hornet pilot

Many Words - Serial Fiction | Ka-50 Employment Guide | Ka-50 Avionics Cheat Sheet | Multiplayer Shooting Range Mission

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I'm speaking my mind here but, hovering counter-acts your loitering time and adding more weight with extra fuel and fueltanks counter-acts your hovering performance.

 

Apart from the general engine performance questions, I'm assuming the bottom line is that you want to stay at that altitude for some time. If that's the case I think you'll be better of flying a figure 8 pattern with the fuel tanks.

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Keep 130kph IAS. No hovering. Do multiple passes if you must, but don't hover where hovering is difficult. In fact, don't hover ever when you can avoid it, just do multiple passes and keep your distance.

 

At altitude, you can't hover anyway as you've discovered. 130kph is where this heli uses the least power to fly, and that's where you want to be at when you're flying in difficult conditions.

 

Right, but at high altitudes the calculus changes a little: with the engine governors limiting engine RPM, the rotor RPM would (I would think) be below where the flight control system wants it to be for the given collective setting.

 

Some of it might be the wind in this mission, too—it's pretty intense at 2000 meters, and the shape of the valley might be causing updrafts and downdrafts. At one point I couldn't hover at full collective, and at another I was hovering with less collective than I normally need down near sea level.

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One note, you won't be able to use Vikhrs and smoke rockets unless you have some strange loadout of a fuel tank on 1 and 3 and smoke rockets/ vikhrs on 2 and 4. There are only switches for inboard and outboard. I don't even know if that would work.

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Right, but at high altitudes the calculus changes a little: with the engine governors limiting engine RPM, the rotor RPM would (I would think) be below where the flight control system wants it to be for the given collective setting.

 

Some of it might be the wind in this mission, too—it's pretty intense at 2000 meters, and the shape of the valley might be causing updrafts and downdrafts. At one point I couldn't hover at full collective, and at another I was hovering with less collective than I normally need down near sea level.

 

Engine and rotor aren't connected mechanically but only aerodynamically. Rotors are connected to the gearbox which is connected to two turbines behind each engine. The engine exhaust makes these turbines to spin and provide torque to the rotor. So in effect the RPM limit only limits the power of the engine ie. torque to the rotor but the rotor RPM depends also on the drag of the blades. You can have the engines limited by RPM but still have nominal rotor RPM if you your collective setting doesn't provide too much drag to the rotors.

 

DCS doesn't model any kind of updrafts or downdrafts currently besides some turbulence.

 

I'm sorry I got somehow confused what you were asking originally and gave you the wrong answer in previous post. The proper answer is: "In general the nominal rotor RPM is the most efficient, so in general you should have less sink rate with less collective." Everything after the first sentence should be correct. With light load having lower than nominal rotor RPM could be more efficient but there's no proper control in Ka-50 for that except the low RPM switch that might or might not put the RPM to proper range. To really understand what's going on with the rotor and the engines you will need to read some aerodynamics like this:

 

www.cnatra.navy.mil/pubs/folder5/th57/p-401.pdf

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[quote=Fishbreath;1797455

 

Here's the scenario: I'm in a hover with a heavy load (six Vikhrs and a launcher's worth of smoke rockets, plus two fuel tanks for added loiter time over the target area).

 

Also the decision to carry smokes when playing overwatch is questionable because smoke-markers can be set only within a range of ca. max 6 km from your position.

Give him instead lat/long coordinates of enemy positions.

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