Articles of Interest
How important is Thrust?
as pertaining to powered paragliding
written by Keith Pickersgill
Thrust is probably the single most important factor to consider on a PPG motor, alongside comfort, strength, safety and price.
(A) The amount of STATIC thrust a motor develops, will affect how easily you can takeoff in low winds. If the motor is under-powered for a particular pilot at a particular location, that pilot will have to run VERY hard, and probably very far, before getting airborne. Also, there will be no room for error - if he gets airborne too early, he will gradually sink back into the ground BEFORE gaining sufficient airspeed to start climbing.
With a bit of extra thrust, the pilot will get airborne after a shorter run, and may have more room for error or poor technique. i.e. if he lifts his legs too early, the extra thrust will get the pilot and wing up to climbing speed quicker, hence less descent while accelerating, less risk of sinking into the ground at full power.
It stands to reason, that the takeoff field length becomes more critical with less thrust. A field that is more than long enough for most, may be too short for an under-powered motor.
(B) Furthermore, more thrust will deliver a higher climb rate.
This will be appreciated under various cicumstances:
- A higher climb-rate will allow more safety clearance over trees, fences or other obstacles surrounding the takeoff field.
- Higher climb-rate is great for getting through horizontal layers of turbulence quicker. In fact, an under-powered unit may not be able to climb at all in some types of turbulence.
- A higher climb-rate allows for more safety during low flying, in the event of unexpected obsctacles.
(C) More thrust is required to maintain level flight (or climb) when flying a wing in accelerated mode, either with trim-tabs dropped, or with speedbar applied. An under-powered unit may not be able to maintain level flight at all when the wing is in accelerated mode.
Think about this when you consider that most PPG flying is done with relatively small wings, often way over the placarded (unpowered) weight range.
Also, it is becoming trendy to do long flights, covering vast distances, at higher speeds. What happens when you are faced with a headwind?
(D) A motor with more thrust will be able to cruise (maintain level flight) at a relatively lower power setting, extending the motor's life, plus making less noise, less vibration, and less stress on the motor, wing and pilot.
(E) It is great to always have some surplus power in reserve for those unexpected situations which tend to occur surprisingly frequently.
e.g. sudden headwind develops on your way to the landing field, or you need to climb over a hill, ridge or mountain (or just some trees or powerlines), or you find yourself in a shear layer with unwanted turbulence and decide to climb up over the layer, or you suddenly realise you need much more height to maintain a safety glide to a suitable landing area in the event of an engine failure.
There are so many times when extra thrust is most welcome.
Too often, pilots chose a less powerful model due to lower price, only to regret this later when they realise the benefit of additional thrust.
Fortunately, some manufacturers are compensating for this, by making some models "upgradeable". This has the benefit of spreading the cost, although in the long run, it usually proves more expensive to buy small and then upgrade gradually.
The amount of thrust required, depends on a number of factors, chief among them being:
- The pilot's body weight
- The wing's size, lift generated, and performance under power.
- The altitude of the takeoff field ( the higher, the more power required)
- The air density, which depends on air-pressure, temperature, and humidity levels.
- The type of takeoffs done. Zero-wind, Zero-slope, are the most difficult, where every bit of extra thrust helps.
- The type of flying preferred by the pilot. Level cruising, with some soaring legs, requires less thrust than many climbs and /or aerobatics, or sports-flying.
The amount of thrust developed by a motor, depends on a number of factors, among them being:
- Engine size and type used (more powerful may be heavier)
- Engine max RPM
- Engine max Horsepower
- Engine max Torque
- The type of exhaust (on a 2-stroke engine) makes a HUGE difference!
- A resonant, tuned exhaust system acts like a "super-charger", effectively increasing the capacity of the engine, the hp, and the torque.
- Reduction Ratio used (to balance available power and propeller)
- Some manufacturers re-port the engine, to modify its power-curve.
- Propeller size (meaning diameter, pitch, mean chord, active area,
profile, airfoil, etc) However, propeller diameter has the greatest affect -
larger, slower-turning props are more effecient, but more cumbersome.
(Larger props also require larger frames and prop-gaurds)
Unfortunately, more power usually comes at the expense of weight and bulk.
We all want more power, but we wish to carry the lightest, smallest motor possible.
These are two conflicting requirements, so a compromise is usually reached.
As a rough guide, I beleive that a pilot should get as powerful a motor as he can comfortably carry.
Smaller, lighter pilots do not need as much power, and would be burdened with the larger motors.
Likewise, larger, heavier pilots will battle to get off the ground with the smaller and lighter motors.
If in doubt, ask a reputable instructor or dealer for advice.
Address comment to Keith Pickersgill at firstname.lastname@example.org