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Nose-down attitude during forward flight?


Luderbamsen
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During forward flight in a helicopter, does the fuselage pitch down?

 

I know that the pilot may point the nose downwards to increase acceleration, but what about cruising along at a normal pace? Is the nose generally pointed downwards even a little bit?

 

And if the helicopter is flown with the nose straight and level are there any general limitations on the top/cruising speed of a helicopter?

 

Are there any special circumstances, besides acceleration, (such as "flank speed" or others) where the nose might be pointing downwards during otherwise forward flight?

 

Obviously, I'm not a pilot myself, nor have I ever actually flown in a helicopter.

 

Thanks in advance

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It really depends on the helicopter and the speed at which you are flying. If you think about the rotor disc as a plane in which the thrust vector is perpendicular, then in a no wind hover, the rotor disc is parallel to the ground and the thrust vector is straight down. To accelerate in any direction, the disc is tilted in that direction. To go forward, the disc is tilted forward and the thrust is vectored down and aft. Tilting the disc forward creates more lift in the aft portion of the disc. This is translated through the rotor hub to the airframe, causing the tail to lift and the nose to pitch down. At a given amount of forward tilt on the rotor disc, the helicopter will accelerate forward until the drag becomes equal to the forward component of the rotor thrust. The aircraft will stabilize with a given amount of nose down attitude at a given airspeed. Most helicopters will employ some sort of horizontal stabilizer in the tail to produce a tail down force in forward flight so that nose down pitch is not excessive. The Blackhawk/Seahawk employs a large programmable stabilator to decrease the nose down pitch in high speed forward flight.

 

It isn't a bad idea to have some correlation between the nose attitude and the airspeed of the aircraft.

Edited by Rob Lyman
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Helos capable of some good speed (A109, S76 etc) have some mast tilt built in, so that when the fuselage is flat, the rotor system is tilted forward for forward thrust.

 

It makes the cabin more comfortable. The horizontal stabilisers in cheaper machines are fixed, and as price goes up, so does the complexity of the moving surfaces - though the 76 has a fixed surface.

 

In acceleration and deceleration, the cabin/fuselage attitude varies from -10 degrees to +20 degrees.

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Eric is right! Here are some pictures which might help to illustrate his point.

 

In cruise (about 144kts) I guess we have about -2 degrees pitch attitude. (So basically level.) During desceleration, I generally put about 6 degrees nose up, then go to about 11-12 degrees for the last part.

 

The Vertical Cat. A Heliport departure calls for -22 degress pitch down in 2 seconds! Normal runway departures...hmm about -10.

 

Just a note to remember, always approach the S76 from the sides...not the front. The disc can be as low as 6 feet on the ground!

 

Joker

 

 

288413.jpg

 

 

 

S76 on ground rotors stopped.

 

 

00352.jpg

pic1.jpg

S76 in hover!

 

01207.jpg

 

See the built-in mast tilt.

 

s76_helo1.jpg

356217.jpg

 

S76 in cruise.

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The Bell 407 has almost the same rotor system as the late model Kiowas. The difference? The Kiowa is designed for hovering and the mast is straight up and down. The 407 is designed to cruise at higher speeds so the mast has a 5 degree forward tilt to bring the nose up higher during cruise. Just like the other helos mentioned.

 

Jeff

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Thank you guys! This is turning into a treasure trove of info.

 

My reason for asking is a discussion I had with friends about how helicopter flight characteristics and controls are represented in computer games, notably the type of first-person-shooter games that aren't dedicated helicopter simulations, but do feature flyable helicopters.

 

Such games generally have a very liberal interpretation of how helicopters (and their pilots) go about their flying business, wich is probably a good thing, since most players of these games lack the joystick and other accessories necessary for a proper flight sim, and probably aren't all that interested in hyper-accurate flight simulation anyway.

 

But I digress. The discussion revolved around the more-or-less realistic flight characteristics in such games, some noting that many helicopters seemed to be flying around with a 45 degree nose-down attitude almost all the time.

 

The discussion was spirited (not the least because none of us knew very much about the subject) with some claiming that helicpters always fly around with the nose pointing at the ground, and others claiming that they always remain straight and level regardless of speed (other related subjects were discussed as well, but I'll leave them alone for the sake of clarity).

 

It would seem then that none of us were right (hardly surprising), but that the truth is somewhere in between. So once again thank you all for your contribution.

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The Bell 407 has almost the same rotor system as the late model Kiowas. The difference? The Kiowa is designed for hovering and the mast is straight up and down. The 407 is designed to cruise at higher speeds so the mast has a 5 degree forward tilt to bring the nose up higher during cruise. Just like the other helos mentioned.

 

Jeff

 

 

Actually the OH-58D Kiowa has forward tilt and also tilt to the left to compensate for translating tendency.

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Then just to throw a wobbly one at you, add a hook load!! Now you are going nose down, especially if you have something that you can get a good speed on with but still acts like a parachute. Try a fertiliser bucket with a 2 meter opening at the top. when its empty on the return trips you are just looking at the ground all the time.

 

Don't all helicopters have some forward rotor tilt?? i think even the old Bell Uh1s have a slight tilt, and they only truck along at 100k

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Helicopters are generally designed to have the fuselage as level as possible in cruise, because that's by far the most efficient attitude. If the fuselage is tilted forward, the result is more drag, thus more power required, and eventually a lower maximum cruise speed. From my experience, an aft CG, within limits of course, gives the fastest cruise. The tilt difference between the forward limit and the aft limit isn't very great, but even that results in perceptible speed differences.

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