Speed change with throttle change in a 152 Cessna.?
October 20, 2008
If your flying a 152 Cessna that is trimmed for level flight at cruise speed and you do nothing other than reduce power to idle, will your speed change and why.
Yes the airspeed does increase slightly. The question now is why?
Actually, your airspeed will increase.
Even if you take out the pitch oscillations with the yoke but don't touch the trim, the aircraft will settle down at a slightly higher airspeed than before.
If you don't believe me try it out for yourselves. You might be surprised!
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12 Responses to “Speed change with throttle change in a 152 Cessna.?”
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Two answers. Assuming level flight is maintained, your speed will decrease because you have removed some of the energy (thrust) in the equation.
If you don't touch anything else, the speed will remain the same, but the aircraft will descend, substituting potential (altitude) energy for kinetic (thrust).
To avio and the rest, I did not intend the airplane would stay precisely at the former airspeed, but the nose will drop and it will attempt to maintain somewhere near where it was trimmed. If you look back at some AOPA articles by Julie Boatman and Dan Gryder about pushing over when the engine fails, they are much more correct about the proper technique for engine failures than my answer.
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Obviously, when you reduce the power you will lose traction and your speed will decrease. That is because the upward force lifting the aircraft is a result of speed. Less power = less speed = less lift.
Even at full power, but with the brakes fully on, there is no speed and the aircraft does not fly, right?
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Private pilot
Warbird Pilot’s answer was textbook perfect.
An aircraft flys at the speed it is trimmed for, for a given power setting. Change the power setting and you either climb or descend, but the aircraft will seek its trimmed airspeed.
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Yes your speed will change and the aircraft will oscillate trying to find the speed for which it is trimmed. It’s called longitudinal phugoid oscillation. Sorry if I spelled that wrong but check it out in your text book.
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CFI
The simple answer is: no, your speed will not change.
Aircraft are generally thought to be trimmed for a certain speed. If you are trimmed in level flight and you reduce thrust, the aircraft will essentially maintain the same speed. It will do this by losing altitude and drifting down to the correct airspeed.
Of course there are some oscillations, as pointed out by one of the above answers, but all in all if you let it “settle in” it will stay at the same airspeed.
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Sorry–If you reduce to idle and do nothing else, the 152 will both slow down and lose altitude. The 152 is not longitudinally stable, and it will re-trim itself, but not enough to maintain airspeed. You will have to lower the nose to keep the airspeed up. So in a few seconds you will have a power-off stall if you just pull the power all the way off and drop your hands in your lap.
Somewhere along in there the 152 will enter a gentle descending spiral, and if you don’t wake up, you will do some agricultural work.
Get out there and fly some, guys!
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retired Boeing 747-400 Captain
Your speed won’t change. You will descend.
You trim the airplane for a certain speed, and the airplane will maintain that speed. If you increase power, the airplane will climb, decrease power and it will descend.
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Try it and see. Very simple.
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Reducing power reduces thrust, and thrust provides both the energy to counteract drag (maintaining forward speed) and the energy to sustain the aircraft in level flight. Therefore a reduction in power will produce a drop in airspeed and a descent. The aircraft will stabilize at a new altitude and airspeed when the forces acting upon it again reach equilibrium. However, with the engine idle, this equilibrium will not be attained before reaching the ground.
This remains true in some form for all aircraft. You need power to maintain level flight. If you reduce power, you must sacrifice some combination of altitude and airspeed. In theory, you could sacrifice only one of the two and maintain the other, but in practice, you lose a bit of both because of the interactions between different forces in the aircraft.
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Throttle controls altitude, pitch controls airspeed. Basic stick & rudder. If you bring back the power and do not adjust the trim for the elevator the plane will begin to descend.
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Actually, your airspeed will increase.
Even if you take out the pitch oscillations with the yoke but don't touch the trim, the aircraft will settle down at a slightly higher airspeed than before.
If you don't believe me try it out for yourselves. You might be surprised!
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For a 152, Cal's answer is correct.airspeed will (eventually)increase . I have demonstrated this to student pilots on more than one occasion.
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