# Actual non-axial airflow (sideslip) for a propeller driven aircraft

I am making a propeller model for a two-engine propeller driven aircraft (like a Dornier 228). I have expanded my model to include the non-axial airflow as well.  By non axial airflow, I mean that the air does not strikes the forward moving aircraft straight along the propeller axis $$v_x$$, but in some angle. Imagine the propeller rotating along the $$v_x$$ axis in the figure below:

I want to generate the data from the model for some meaningful range of the sideslip angles ($$\alpha ,\beta$$). I could obviously choose a range [$$-90^\circ 90^\circ$$]. But the simulation time increases exponentially with such a large number of datapoints.  Generally, I think, for a propeller driven aircraft, it should not be that high. I was thinking to take a range of [$$-15^\circ 15^\circ$$].  Can anyone who has a bit of experience in these kind of aircraft, please suggest a practical range of the sideslip angles that is generally encountered.

Thanks a lot in advance :)

• Are you simulating only the propeller, or also the engine and aircraft it will be mounted on? Commented Jul 3, 2023 at 20:01
• I am simulating only the propeller Commented Jul 5, 2023 at 6:31

If you consider the thrust-generating operation of the propeller, then yes, -15 to +15 degrees is sufficient as outside that range, the propeller blade will stall and be ineffective at generating thrust. For that reason, operation outside that region is not attempted practically. If you want to ignore reverse thrust i.e. focus on positive thrust only, then you can restrict yourself to only positive values. If however you want to include feathering operation of the propeller also, then you will have to take the high positive (+90) values into account.

• Are you sure that the blades will be stalled? I'm asking because in a helicopter, the main rotor as well as the tail rotor are almost 90° to the freestream airflow, but they are not stalled. So why would a propeller be stalled under similar conditions? Commented Jul 4, 2023 at 2:13
• @AdityaSharma Yes, Aditya. So there are a few angles to be taken care of here. Let q,d,o be a dextral orthogonal basis fixed to the acft with q pointing to starboard, d pointing forward and o 'upward'. In the case of the prop, the blades have a circumferential velocity in the q-o plane and the acft's velocity is along the d-axis (treating alpha as small). Consider a prop spinning CCW viewed from front and one blade instantaneously aligned along the o-axis. The blade velocity relative to acft is +q, so the blade's total velocity is 1st quadrant in q-d plane. Contd
– user69764
Commented Jul 4, 2023 at 2:51
• The angle of attack at the blade is the angle made by the chord line of the blade with this velocity vector. Since blade is airfoil, it generates lift if and only if this angle is small. Hence the blade chord must make a 'yaw' angle with the o-axis so that its chord is close to the total velocity mentioned above. To answer your helicopter questions. The main rotor is approximately in the q-d plane. Circumferential velocity has q and d components while acft velocity is d. Again, at any point on the rotor, the total velocity is a combination of q and d. Contd
– user69764
Commented Jul 4, 2023 at 2:59
• Now recall the formal definition of AoA. It is the angle from the chord-normal projection of the airfoil's velocity vector to the chord line. Consider a blade instantaneously along the positive d-axis, rotating CCW viewed from positive o. This is the configuration where there is greatest chance of confusion. Then, its total velocity has a -q and +d component. If the blade lies flat then span, chord and normal are +d, -q and +o respectively. AoA is angle from -q to -q which is zero. Give the blade a small 'bank' about d and you have a small angle of attack, not a stall.
– user69764
Commented Jul 4, 2023 at 3:05
• Thank you for such a detailed analysis, but I'm still not convinced that the propeller blades would be stalled at $α$/$β$ = 90°, while the helicopter rotor is not stalled under the same conditions. Take a look at this video of an RC stunt plane; the propeller appears to remain un-stalled at quite high "side slip" angles. Commented Jul 4, 2023 at 9:19