After mathematically ensuring the performance of the jet the next step should be to control the stall speed in order to avoid surprises at the take off and landing phase.

A usual criterion for estimating smooth and trouble-free handling of the model on normal airfields is the well-established method of measuring the wing loading. This is applicable when two similar models are compared and should not be applied with a strongly different airfoil, a very different aspect ratio or a divergent proportion of wing and fuselage.

Besides, the airfoil depth has an impact on stall speed. In order to consider all of these parameters at least to some extent, we provide you with the following formula.

Horizontal velocity of the jet:

\({v=\sqrt{2 \cdot m_{ges} \cdot g \over \rho \cdot c_a \cdot A}}\)

In here \(g\) equals the gravitational acceleration of \(9,81 m/s^2\) and rho equals the air density of \(1,2 kg/m^3\). \(m_{ges}\) means the overall weight of our model in kg, \(A\) is the wing surface in \(m^2\) and \(c_a\) is the maximum lift coefficient, depending on the Reynolds number and the used profile, which is still flyable in the landing phase. The following table illustrates parameters for the maximum lift coefficient \(c_a\). (\(c_a\) should be derived from the L/D polar curve of the used profile and the corresponding Re number if available)

- model type, airfoil: lift coefficient \(c_a\)
- Airliner, thick, high cambered airfoil: 1,2 – 1,3
- moderate speed jet, airfoil e.g. HQ 2.0/10: 1,1 – 1,2
- sporty jet, airfoil e.g. RG-15: 0,95 – 1,1
- speed jet, airfoil e.g. RG-14, HQ1.0/8: 0,8 – 0,95

These values, of course, also depend on the Re number and the airfoil depth. If the model has a small wing depth the smaller \(c_a\) value should be used.

e.g. Model Pampa, \(m_{ges}=2,4 kg\), \(c_a=0,95\), \(A=21,5 dm^2=0,215 m^2\)

\({v=\sqrt{2 \cdot 2,4kg \cdot 9,81m/s^2 \over 1,2kg/m^3 \cdot 0,95 \cdot 0,251m^2}}\)

\(v\)=12,75m/s

The stall speed resulting from these calculations is already quite realistic but still slightly too high as the lift effect of the fuselage has not been considered yet. At extremes (e.g. with the Starfighter, SU-27…) that approach would not work as the fuselage generates about 40% of the overall lift.

In that case it is necessary but also more time consuming to calculate the lift of the fuselage or to make use of empirical values. Typical minimum velocities for Airliners are at about 9 – 11m/s. Moderate speed models are a bit heavier and have a value of about 12 – 13m/s and small speed jets can operate in an area of 14 – 16m/s.

These estimated values are considered for informational purposes only and enable the estimation of a maximum weight for our corresponding jet. However, in some cases the value for the wing loading is going to be extraordinarily high while the model still flies smoothly.