Can an aircraft climb with the thrust line below the line of flight?

Question

"Downthrust" is a common engine mount feature of propeller driven aircraft, models and full scale alike.

From model building experience and flight, downthrust, angling the (tractor) engine mount down a few degrees seems to relax static stability a bit when full power is applied by counter-acting tail down force and enabling flight at various speeds with less trimming.

The effect in a full power climb would be a higher trim speed than a power off glide at the same trim.

Yet many people on this site strongly feel that lift must be less than weight in a climb.

I think it can be (rockets do this well), but using the wing to climb seems far more efficient, as witnessed by superior climb and distance performance at Vcc (cruise climb) and Vy compared with Vx as seen in this reference

The larger slower wing produces much more force than the smaller, faster propeller. Additionally, at Vx, increased angle of attack, with thrust used to counter additional drag may be a better strategy than reducing wing lift and trying to replace it with a vertical thrust component. Most GA planes will lose airspeed at this point. At cruise climb speeds, the thrust vector may well be slightly below the line of flight, yet less fuel is consumed per distance than even Vy!

any other climb path is just another point in the ratio of lift + thrust + surplus thrust for climbing

Let's apply our "surplus thrust" and see what happens:

1. Increasing thrust increases airspeed
2. Increasing airspeed causes aircraft to pitch up
3. Airspeed stabilizes (to trim airspeed) at higher pitch (relative to the horizon).
4. $cosine$ of lift vector (vertical lift) decreases

In order to maintain vertical lift we:

a) speed up b) increase angle of attack c) pitch up even more

Here lies the crux of the issue. Most GA aircraft have a thrust to weight ratio of 1:4. Attempting to replace a 300 lb weightlifter with a 75 lb one will have predictable results.

"Lift is less than weight in a climb" is based on the assumption that lift is perpendicular to the thrust line...and the (upward pointed) thrust vector is carrying part of the weight

That's not the best way to do it, and is known as "helicoptering". the wings job is to produce enough $vertical$ $lift$ to make the aircraft "weightless", so the engine thrust only opposes drag. It is extremely wasteful to use thrust to "lift weight", unless the aircraft is pitched at so high an angle as to make the wings drag greater than its vertical lft contribution.

a) and b) seem to be the correct answers, as seen in the "real world" as Vx, Vy, and Vcc.

Is there a way to reconcile this academic quandary?

Answer 1

Yes, it can. As you say, that lowered thrust line adds to the lift requirement, but not dramatically so.

I guess the technical term you mean is called downthrust. Together with sidethrust, this offset alignment of the propeller axis helps to reduce retrimming when thrust is changed.

Downthrust has two effects:

1. It increases the angle of attack on the tail slightly when thrust is increased, thus trimming the airplane for a higher speed. The angle of attack on the tail is the sum of free flow, downwash from the wing and propeller slipstream. More thrust increases the contribution of the propeller, and if that slipstream is coming at a positive angle, the resulting angle of attack on the tail is also increased, adding some lift and lowering the nose for faster flight.
2. It helps to shift the propeller slipstream up so the vertical tail "sees" a more symmetric slipstream. Without downthrust, the tail would fly in the upper half of the slipstream and swirl would lead to a sideslip angle there. By lifting the slipstream up, the lower vertical tail now flies in the lower part of the propeller slipstream and experiences an offsetting sideslip angle to the one hitting the upper part of the vertical.