Why does a plane climb slower than it descends, at the same trim configuration?

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

• angle of attack —> lift and drag coefficients
• airspeed and lift coefficient —> lift
• airspeed and drag coefficient —> drag
• vertical component of lift minus gravity —> vertical acceleration
• horizontal component of lift minus drag plus thrust —> acceleration in direction of flight

is still easy, but it gets really difficult if you include how the pitching moments then influence angle of attack

• thrust and thrust lever arm and center of gravity position —> pitching moment from power —> angle of attack
• airspeed, angle of attack and center of gravity position —> pitching moment from wing —> angle of attack
• elevator and trim position and airspeed and center of gravity position and angle of attack —> pitching moment from tailplane —> angle of attack

Wen you sketch this (and more) out, you get lots of funny loops, such as

• airspeed —> pitching moment from tailplane —> angle of attack —> airspeed…
  and that is BEFORE you take into account propwash…

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)

Trim for speed, throttle for pitch.
The other day I made a very long straight in final from 5500 ft in smooth air. Cruising at 100kt IAS on a fixed prop PA-28, I merely retarded the throttle from shy of 2400 to about 2000 rpm. IAS remained 100kts all the way down until it was time to take out flaps and get ready for landing. Not one more, not one less. I have not tried doing a 100kt cruise-climb to check the opposite.
Don’t mix transient effects ext or just shortly after the power change with the steady-state regime. From stable straight and level, a power change will change propwash and local aoa and can start a phugoid, which would look like a speed change, but it’s a temporary deviation.

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.