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HeliTorque Forum Index » Ground School

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TRT Question
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paddywak
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PostPosted: Sun Dec 18, 2011 2:27 pm    Post subject: TRT Question Reply with quote

OK guys and girls , another question for you:

When a helicopter is in an ideal free air hover the TRT is acting vertically and is equal and opposite to:

A.) weight and rotor drag
B.) weight and parasite drag
C.) engine torque and rotor drag
D.) parasite drag and rotor drag

So ruling out C and D because TRT is equal to the weight of the helicopter in the hover, leaving A and B

Rotor drag is overcome by torque
Parasite drag is zero with no forward airspeed

Any guesses?

The answer they want is B. Just not sure why Rolling Eyes
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LoachBoy
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PostPosted: Sun Dec 18, 2011 10:08 pm    Post subject: Reply with quote

Parasite drag results from airspeed, which in this case is zero, so the horizontal component of TRT equals (and is opposite to, in theory) zero.

Weight = mass x g, and in this case the machine is neither climbing nor descending.

So, vertical component of TRT = 'weight', but in the opposite direction.

Hence B. I think.

If in doubt, sketch it out!
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Ascend_Charlie
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PostPosted: Mon Dec 19, 2011 3:09 am    Post subject: Reply with quote

The parasite drag comes from the airflow blowing downwards over the fuselage. It adds to the apparent weight, so you need a little extra thrust to overcome it.

Looking at forward flight, you have four main vectors-
weight acts vertically down
"lift" acts vertically up to counteract weight (yeah, yeah, calm down, we all know it is perpendicular to the RAF, blah blah, just let me finish)
Drag acts backwards
Thrust acts forwards and is balanced with drag when at a constant speed.

As you slow down, and the downwash slowly changes from being rear-and-down to just being down, the vectors move around a little - lift and weight stay the same, drag moves to being a bit down, and thrust has to move a bit up.
In the hover, weight and drag coincide, and lift and thrust coincide, so in the hover you get
lift+thrust=weight +drag.
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paddywak
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PostPosted: Mon Dec 19, 2011 7:13 am    Post subject: Reply with quote

So the parasite drag curve should not indicate zero with no forward airspeed because the induced flow is creating some parasite drag on the fuselage albeit in a downwards fashion?

I went for the Rotor drag as although opposed by torque, assumed no parasite drag in the hover.
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LoachBoy
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PostPosted: Mon Dec 19, 2011 8:11 pm    Post subject: Reply with quote

Ahaaaaa....... AC makes a good point..... I forgot about parasite drag from the downward airflow...... D'OH!

Sorry chap. But the principle stands! Wink
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paddywak
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PostPosted: Tue Dec 20, 2011 11:52 am    Post subject: Reply with quote

Thanks for that chaps, on a different tack, has anyone got any good ways of memorising DME separation minima?
Killer Headache

Maybe it's time for a Cheers! or 2

Wish I had a photographic memory Laughing
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