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How much side thrust and down thrust should be used?

Les Bond contacted me, asking about thrust lines in models. He wrote

Andrew, it is accepted as normal that an aeroplane with an i.c. (gas) engine has side thrust and down thrust build in as standard. But this does not always seem to be the case with aeroplanes with an electric motor. Some plans have side/down thrust built in but others do not. Indeed, some designs I have seen have what I would call an extreme amount of down thrust built in.

Can you advise me please, are there any rules that can be applied here to determine if side/down thrust should be used, and by how much. Also, what do you suggest should be done, if converting from an i.c. aeroplane to electric power? Would you retain the original thrust lines, or would you not? I’m hoping that you can shed some light on this matter.

Andrew's answer
Hello Les, and thanks for your interesting question. As a general policy, I would tend to stick with the original i.c. design's thrust lines. However, if you are installing significantly greater power than the designer intended, increasing the side and down thrust may be beneficial. In order to appreciate why the thrust line of aeroplanes is often slightly angled, it will be useful to discuss side and down thrust in a little detail. Let's start with down thrust:

Down thrust
Downthrust is applied to reduce the pitching up tendency that naturally occurs with an aircraft which has a thrustline below what can be called the 'centre of drag'. All airframe components produce drag, but the location of the wings is a particularly important variable in this matter, since wings are a significant source of drag. For example, the high wing of a Cessna 152 results in a relatively high 'centre of drag', which means that when power is applied the nose will tend to pitch up. So, a significant amount of down thrust may be added even though this is a low power design. Converesely, the low set wing of a typical WW2 fighter results in a lower 'centre of drag' line, so less down thrust may be required even though high power will typically be installed.

Many modellers see the pitching up characteristic under power as a 'fault' and seek to remove it with TX mixing and/or by adjusting thrust lines. However almost all of the types of full size aircraft I have flown, including training machines display this exact same characteristic so I don't necessarily see this as a fault in models. That said, it needs to be appreciated that while full size aircraft tend to be flown for long periods at a constant power setting, our models are often flown in a different way, so there's a good case for making their trim requirements less demanding.

Side thrust
Concerning side thrust, generally speaking the more power is installed in a model the more side thrust may be desirable in a given design. The reason for building in side thrust is to help counteract the asymmetric forces which result from using a propeller to power an aeroplane. One such force results from the fact on a conventionally powered model aeroplane (prop rotating anticlockwise when viewed from the front), the prop wash will hit the fin not straight on, but slightly from one side, which produces a yawing force.

This picture of a WW2 Corsair clearly shows that the prop imparts a swirl to the airflow, causing it to hit the fin at an angle. This produces a yawing force, typically causing the nose to want to swing to the left.

The yawing forces produced by a rotating prop can be very considerable. One illustration is that for many high powered WW2 fighters, where full power could not be applied during the take off roll until enough airspeed had been accumulated to allow the fin and rudder to achieve enough authority to counteract the yawing tendency (mostly, but not entirely cause by asymmetric propwash). I remember reading of one WW2 fighter where full power could not be used at all during the take off roll, otherwise the aircraft became directionally uncontrollable.

Some high powered full sized aircraft such as the WW2 Hurricane fighter used an offset fin and rudder to counter this yawing tendency, but this design solution applies a corrective measure regardless of the amount of power applied so in some senses it may not be such a good solution. However, given the high power of WW2 fighters and the relative inexperience of some of their pilots, a combination of side thrust and offset fin may well have been the best solution for the day.

The presence of a nosewheel will help to counter the yawing tendency, reducing the amount of corrective rudder required. However, taildraggers often need significant, carefully applied amounts of right rudder during take off, especially on very smooth surfaces such as concrete.

How much side and down thrust should be used?
Coming back to your question (at last!) we now have an appreciation of the main factors affecting the amounts of side and down thrust that may be appropriate in a given situation. A design with a good pedigree can be expected to accurately specify suitable values of down and side thrust. However, it’s worth being aware that published model aircraft designs do vary greatly in design quality, and sometimes the angles shown are not necessarily the optimum for that design.

Whether the model is powered by an electric motor or an i.c. engine will make no difference on its own to the model’s flight characteristics – the model has no awareness of how the prop is being made to turn. Modern electric power systems can be very powerful, sometimes even exceeding the power of a typical alternative i.c. engine. It may be worth allowing for the fact that the model’s thrust line may need to be adjusted after testing its flight performance, particularly if you wish to install a relatively high power system. Our models tend to be relatively more highly powered than full size aircraft, so if your model is a scale subject then it may not be enough to copy the full size example's thrust line.

Prop theory is incredibly complex, and for the purposes of this discussion I’ve confined my answer to a reasonably direct response to your question. However, bear in mind that there are several other prop issues which will affect the behaviour of models, such as torque reaction, gyroscopic precession and the P factor. These can be discussed on another occasion. In the meantime, I hope this discussion helps you out and answers your question.

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