View Full Version : Out of trim in a Robbo!!
helimutt
29th Jan 2001, 14:08
As an instructor once said to me, when flying out of balance, the airspeed decreases which is why we have to stay in trim at all times. mmm.
The pitot tube is not facing directly into the air flow!! There is more surface area so not as streamlined hence more wind resistance!!
Now I hear that the Robbo shouldn't be flown out of trim or it will fall apart!!
Nice work if you can get it I guess.
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Two factors going on there -
1. As you rightly say, drag increases as the Heli. is not so 'slippery' when flying sideways so true airspeed will decrease.
2. Less of the air molecules that form the incoming airflow (resulting from directional flight) actually go down the pitot tube to exert dynamic pressure where it counts, inside the ASI. The true airspeed could be 60 Kt but the ASI reading could be 30Kt, (for instance). The amount of underread will be directly proportional to the amount the fuselage is out of balance. This is positioning error.
Helimutt, I doubt the robo will fall apart in a hurry if its out of balance for a bit, but NO helicopter should be flown out of balance - what would be the point? (Unless you're filming or something)
The main dis-incentive to having your ball out of the window is that the other pilot/ instructor will want to hit you because its uncomfortable!
My CFI used to ask me early on why in light rain I would slip to his side.
"Well I'm not going to get wet ... silly!"
Was the answer hehehe ...
Part of the real point of flying anything, one certainly wouldn't do it intentionally.
Another set of pics a friend took after I had had my PPL for a year, my current CFI loved them, meaning he must have taught me well, not for the view, but in every shot, banked or level, he pointed out that the yaw strings were "Proudly" straight up next to the canopy rail ....
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Marc
rotorque
30th Jan 2001, 06:14
Helimutt,
Probably not as relevant to helicopters, but in some fixed wing aircraft when you fly out of balance you may encounter fuel starvation as the fuel sloshes to one side away from the inlet. It's rare but it has caused a few tense moments.
Cheers.
offshoreigor
30th Jan 2001, 14:38
Rotorque:
Oh Contraire, the S76A C30 Allison, when flown at high speed with more than a half a ball deflection has been known to make all the oil mysteriously hide in the back of the accessory gear box never to be heard from again followed by a very silent and siezed C30!
Cheers, OffshoreIgor http://www.pprune.org/ubb/NonCGI/eek.gif
212man
30th Jan 2001, 20:15
Don't forget that with the measured static pressure will increase leading to a false (reduced) airspeed as well.
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Another day in paradise
Inherent Sideslip, which is the result of tail rotor (anti-torque)thrust in forward flight means that most balance balls fitted to helicopters are not indicating streamlined flight and hence minimum drag. If you have a piece of string or wool as a balance indicator it is more accurate except in a climb where the swirl of the downwash tends to deflect it slightly.
If you are using a balance ball and fly wings perfectly level with the ball smack in the middle your heading will gradually change as you are in fact slipping sideways through the air very slightly (to the right in the case of an anti-clockwise rotor).
To maintain an accurate heading you must fly slightly left wing low with the ball slightly out to the right.
The Gazelle has both balance indicators and they frequently disagree!
Skycop
30th Jan 2001, 23:30
Yeh but the Gazelle instrument panel isn't level when the aircraft is so, due to the soggy "Barrymounts" and the weight of the right hand panel extension. Many a Gazelle has flown an IF trip with an Enroute Supplement shoved under the right hand end of the panel! Only then does it fly straight and level.
Off at a tangent a little but following the
slip indicator's need to be set to show level when truly 'wings level'(not in the hover) it is also interesting to note that an AH cannot be taken from F/W (although it looks identical) and fitted to a Heli. It has to be adjusted (or built) to show 'wings level'although one skid is low due to TR drift/roll.
So true, on a heli AH you find not only a adjustment to place the indicators on "level" but one to adjust roll as well.
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Marc