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BoredCop t1_ix8gobv wrote

You don't really slow the rotor, changing its speed would likely bring other problems as well. You change the pitch of the blades instead, for the same effect but in a more controllable fashion with quicker response than changing RPM.

As for doing that to compensate for weight reduction while firing, I suspect it is irrelevant because they're actively steering the aircraft all the time anyway and the weight difference isn't that great. Plus it would usually be bad tactics to shoot while absolutely motionless, you'd be an easy target if hovering.

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rankorcankor t1_ix8mejz wrote

Pretty much the above. Helicopter controls have a lever called the collective (as in collective pitch), usually with a throttle built into the lever to allow the pilot to compensate for changes to the rotor RPM caused by increasing/decreasing drag from altering the pitch. But most modern civil helicopters have various systems that would also handle keeping the RPM static without pilot intervention. I imagine military helicopters would always have those systems or even more advanced ones.

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DLBaker t1_ixbgbs1 wrote

Weapons and ammunition is heavy and most helicopters have a stores jettison avaialble if they're low on power. The Apache helicopter as an example has two Turbines to carry everything. If you're down to one engine and need to enter a hover for any reason you may need to drop stores. If you're on the ground with one engine and a rolling liftoff is not available you won't get off the ground unless you jettison some or all your ammo. Missiles, rockets, etc.

The short answer is yes, as you fire, you will reduce your collective as needed. Ammo is heavy.

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eye_spi t1_ixbh9w1 wrote

>If you're on the ground with one engine and a rolling liftoff is not available you won't get off the ground unless you jettison some or all your ammo.

How does a rolling lift off help a helicopter?

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TinKicker t1_ixcdm9q wrote

It’s called “translational lift”.

By moving forward, clean (undisturbed) air begins flowing over the rotor blades, greatly increasing their efficiency.

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eye_spi t1_ixci2nw wrote

Thanks, that's a very clear explanation.

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GarbageTheClown t1_ixbozcq wrote

Rough guess is that in a hover you are fighting your own prop wash / ground effect, which would reduce lift.

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tim36272 t1_ixc5rzp wrote

The rotor acts as a wing in forward flight, providing some of the lift. That was actually wrong: it’s because the vertical speed of the “fresh” air you’re flying into during forward flight is 0 (in the absence of external forces) whereas in hover the air above and below the rotor is already moving down, thus it takes more energy to accelerate that air further.

Same reason it takes less power to fly forward than to hover.

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CisseV t1_ix8mayl wrote

Helicopters generally operate with a fixed range of rotor RPM, lift generated is changed through pitch (collective) of the rotor blades. Weight does impact performance a lot, especially at hover. at higher weights and altitudes, hover might actually not be sustainable.

taking the Apache as a model, wikipedia gives us a weight of

Empty weight: 11,387 lb (5,165 kg)

Gross weight: 17,650 lb (8,006 kg)

Max takeoff weight: 23,000 lb (10,433 kg)

for gun ammo i've taken M789/M788 which ways around 340 grams per cartridge. x1200, which is maximum ammo capacity gives us 408 kilograms of weight in gun ammunition. which is a not insignificant percentage of the weight.

whilst in a hover, weight loss is generally slow because of fire rate (unless you jettison other external stores or fuel tanks), so collective change will be very slow, but will be needed if you dump all your ammo in one go from the same position in a hover.

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Wonderful_One5316 t1_ixa1p7e wrote

Does ammo play a part in your flight path and loadout?

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CisseV t1_ixa3cj1 wrote

More ammo means more weight, thus less range. For the Apache specifically, the internal ammo bay can be fitted with an auxiliary fuel tank. In order to extend range or loiter time.

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tim36272 t1_ixc5xc6 wrote

In addition to what the other poster said: it also impacts performance planning. The flight computer takes weight into account and will determine things like whether or not you can climb over that mountain and how fast you can climb.

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Droidatopia t1_ix9xeku wrote

There is a military helicopter weapon launch scenario that does involve significant enough loss of weight to affect control positions:

Hover launch of a torpedo. Torpedoes weigh ~750 pounds. When that much weight comes off the aircraft, controls will need to be adjusted.

That being said, if a military helo is launching a torpedo in a hover over water, most likely it is a coupled hover, i.e., the autopilot is keeping the helo in a steady hover.

Even more interesting, is that the only time a helo is dropping a torpedo in a hover is if it is using a dipping sonar for targeting, meaning the pilot has to be careful not to drop the torpedo on top of the dipping sonar cable! The pilot will usually give the helo a small lateral drift towards the side of the aircraft that the torpedo is launched from to ensure the sonar cable isn't in the way. It has happened a few times where the pilot has mistaken which direction to drift and instead actually increased the odds of the torpedo striking the cable. At least one dipping sonar that I can remember was lost at sea due to an incident like that.

Immediately after launch, the autopilot will sense the upward motion and slightly lower the collective to compensate, causing the RPM to stay the same, but the engine output to decrease by a few percent.

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nickeypants t1_ix9on7n wrote

No, the blades rotational speed always remains constant.

Helicopters have to balance aircraft altitude with the main rotor's collective pitch (controlled by the collective handle), the aircraft pitch and roll with the main rotor's cyclic pitch (controlled with the cyclic joystick), and yaw with with tail rotor's pitch (controlled by the foot pedals). Each part of this balancing act effects the other, and all must remain in balance to hover. The rotational speed of both the main and tail rotors remains constant, it is the pitch of the blades that changes rapidly to produce the amount of lift required for the balancing act. if you mounted a camera on the main rotor that spun with the blade, you would see the blade's pitch or angle of attack change rapidly during each rotation.

Spending ammunition does have the effect of reducing aircraft weight and producing recoil force. Both of these forces would have to be countered by the three controls mentioned above, all of which affect the blade's pitch, not the blade's rotational speed.

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tim36272 t1_ixc63s2 wrote

It's worth pointing out that in order to maintain the same RPM while blade pitch (and thus load) changes: the engine throttle is adjusted. This is (almost?) always automatic.

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iamajellydonught t1_ixacdjl wrote

As others have said, yes there is compensation but it's done with blade pitch rather than speed.

Planes do this too when they drop a payload, especially wing mounted loads. The pilot will select what to fire with consideration to weight and balance. Some fancy planes like the B1 will actually adjust the CG ahead of a scheduled drop. The A-10 will decelerate slightly when firing the GAU because there's so much mass being shot out at such a high rate.

Physics always apply, the pilot (or plane) must oblige.

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Puppy-Zwolle t1_ixblg6l wrote

Not really. It's all done by computers. This is a weapons platform and the execution of a shot is why it's there.

Remember Luke Skywalker disabling his targeting computer before destroying the Death Star? Yeah, not gonna happen on a military vehicle. Any influence on the vehicle is compensated by computer to get the optimal result.

Besides, slowing down an engine is the worst way to compensate. Angle of the blades is used to get the optimal lift. And it sure needs compensating. Not only the weight, but also the violent way way the mass leaves the vehicle. You could not compensate for that vibration by hand.

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