In a descent, various power settings produce almost exactly the same speed. Well, once you have stabilised the phugoid oscillation…
But if you apply more power in level cruise, you don’t get the same speed. You get a bit less.
Peter wrote:
But if you apply more power in level cruise, you don’t get the same speed. You get a bit less.
I don’t understand. You’re saying that if you increase power in the TB20, the aircraft decelerates?!
ortac wrote:
I don’t understand. You’re saying that if you increase power in the TB20, the aircraft decelerates?!
Normally one would expect a conventional airplane to maintain its trimmed speed regardless of the powersetting. This works reasonably well for any airplane I have flown, but I have never bothered to check if climbs and descents yield the exact same results. If they don’t, as Peter says, I would have no explanation for it.
That is generally dependent on the aerofoil used and the effect of speed and alpha on the pitch moment.
The NACA 23000 series seem to be pretty stable in that respect and is widely used on many factory and amateur built designs with maximum camber at about 15% chord.
It would be quite a challenge to get an aerofoil fully speed stable at all power settings and not really necessary.
http://m-selig.ae.illinois.edu/ads/aircraft.html shows how many types have chosen that airfoil for various reasons
Presumably there is less prop wash in a descent (lower power settings)?
According to section 1.3.2 of this site most aircraft will slow down when power is added (and of course climb) and speed up when power is reduced (and of course descend). I would attribute that to changed propwash over the stabiliser. The stabiliser always produces a downward force. If you increase (decrease) propwash, the downward force will increase (decrease) and the aircraft will pitch up (down) which will decrease (increase) the trimmed speed.
Has gravity suddenly lost importance?
Gravity is fairly constant….
There are several factors that influence each other, and if you draw them out on a piece of paper it is quite complex, even at constant air density
The basics
is still easy, but it gets really difficult if you include how the pitching moments then influence angle of attack
Wen you sketch this (and more) out, you get lots of funny loops, such as
and by the way – in climb, the angle of attack for the same airspeed is LOWER than in level flight, same in the descent. and despite what articles say – the same trim position does NOT give a constant angle of attack, see above. There are many more factors.
The point I am trying to make is: the reason that most well behaved aircraft are reasonably speed stable when you change power is not some law of physics, it is carefully designed into the aircraft.
And even the same aircraft will behave differently depending on CoG position; for example a tail-heavy aircraft will pitch up and down a LOT more with power changes, and hence be less speed stable – the forces required to affect pitch are lower, but the engine power and lever arms remain the same.
Airborne_Again wrote:
According to section 1.3.2 of this site most aircraft will slow down when power is added (and of course climb) and speed up when power is reduced (and of course descend)
Arne wrote:
Trim for speed, throttle for pitch.
Shrieeek! Not this discussion again… (the internet and the flying magazines must contain one month’s full time reading on this) – and btw. I was taught it the other way round, always flew the other way round and teach it the other way round.
The reason why I use the trim is to get rid of residual control forces. As simple as that.
And regarding the original question: My simple explanation is the the angles are different (between trust line, AoA, gravity) as is the AoA. Therefore the amount of of drag created for the same lift changes. Different drag, different speed.