Articles of Interest
Fuel Consumption of Powered Paragliders by Keith Pickersgill
When flying powered paragliders, there are two primary aspects to fuel consumption:
1) Litres (or gallons) per hour (i.e. Fuel-Endurance), and
2) Litres (or gallons) per distance covered (i.e. Fuel-Reach)
The two require very different tactics.
OK, now for the tactics:
In the first example of Fuel-Endurance, a typical scenario would be one PPG pilot gets airborne before his buddy, and wishes to orbit overhead waiting for his buddy to get airborne before embarking on the route for the day. He wishes to consume minimal fuel while waiting aloft.
Another example would be a PPG pilot whom is just cruising around one area and wants to enjoy a nice flight for as long as possible. He has no intentions of actually going anywhere.
2) Distance Covered:
The second aspect of Fuel-Reach, a typical scenario might be a group of PPG pilots flying down the coast to a popular lunchtime venue. They expect to return much later and expecting a headwind to pick up for their return leg, so wish to use as little fuel as possible to get to the restaurant and back again.
First, it must be remembered that all aerobatics, woga-woga, spirals, extended climbs, etc all will increase your fuel consumption detrimentally. We are going to assume that the pilot has reached his cruising altitude and is maintaining level flight with only minor turns.
Fuel-Endurance is usually just a matter of flying the wing at the correct trim for minimal fuel required to maintain level flight. This is easy to assess. What RPM are you cruising at (level flight)... any increase in level flight RPM will reduce your endurance.
On most wings, best fuel endurance is usually at trimspeed. This is off the brakes and off the speedbar.
If your wing has trimtabs, then this is usually with the trimmers pulled in all the way to the SLOW position.
A few wings such as our Xplorer ParaWing have Neutral-Trimtabs, i.e. the trimmers are in the neutral position at 50% and can be either slowed down or sped up from there. In this case, trimspeed is at the 50% neutral position.
Flying at trimspeed should give you the best Fuel-Endurance.
Flying any slower than trimspeed increases the drag you need to overcome with the engine and will require higher RPM to prevent descent.
Flying any faster than trimspeed (by dropping trim or pushing speedbar) reduces the AOA, reducing the lift, inducing a descent, so requires more thrust to restore the equilibrium.
This is the general principle, which you could easily check and fine-tune by checking your level flight RPM at different wing-speed settings.
For example, I have found that on certain wings while flying at trimspeed, the AOA increases very much under power, due to these being essentially designed as gliders, not powered wings. In this case, flying slightly faster than trimspeed may improve your fuel endurance.
To determine your own wing's best airspeed for best Fuel-Endurance, you do not need an airspeed indicator.
What you are actually changing is the trim angle of the wing, which ultimately affects its airspeed.
You will need a sensitive altimeter, or better yet, a paragliding Variometer which is a very sensitive Vertical Speed Indicator (VSI).
How to DIY:
Fly at trimspeed (no brakes, no speedbar, trimtabs on slowest -or neutral setting). Modify your engine RPM until you have no vertical movement, i.e. no climb or descent. Record this RPM setting, which is your reference point.
Next, apply about 15% brake and re-adjust your engine RPM until flying level. Note your new RPM, which should be higher than your reference reading. You could add more brake, but BEWARE of the STALL under power!
Next, ease off the brake and return to trimspeed, then add a touch more speed by either dropping a bit of trimtab, or pushing slightly on your speedbar. Re-establish level flight by adjusting the engine speed and note your new engine RPM.
You could repeat this procedure for fastest trimtabs setting, then for fastest speedbar setting, then for both fast trimmers and full speedbar.
Eventually you will see a picture emerging, indicating your engine RPM required for level flight at varying wing trim.
You could plot this on a graph with RPM on the vertical axis and airspeed or wing angle on the bottom horizontal axis.
Congratulations, you have just mapped your wing's power-curve! From this, it is easy to see which airspeed or wing-trim has best fuel endurance.
My guess is yours will be either at trimspeed, or just a touch faster.
How far can you go?
Let us look at Fuel-Reach, i.e. how far can you fly on limited fuel. This is similar to figuring, "how little fuel can I use to get from here to there".
Let us first assume no wind at all.
Flying at trimspeed as above for best Fuel-Endurance is actually flying with the wing's Angle of Attack (AOA) somewhat greater than the designed cruising AOA due to the thrust being several meters below the centre of drag (which is usually a few inches below the wing).
A paraglider is an extremely inefficient wing due to its very high drag. This high drag causes the wing to "drag" slightly behind the pilot/motor under power. As it drags behind, it increases in AOA which further increases its drag, so it falls further behind, etc until equilibrium is reached.
At this point of equilibrium, much of the motor's thrust is battling just to overcome excessive drag.
You can reduce the excessive drag by reducing the AOA, by pushing on the speedbar or dropping some trimtabs.
This causes the wing to fly faster and will require more thrust to prevent height loss.
Basically as you speed the wing up, you will need to open the throttle more to maintain level flight.
Though you are covering more ground (flying faster), you are using more fuel per second, due to the higher power level required to avoid height loss.
In zero wind, you will find the best Fuel-Reach is usually with the wing accelerated about 60% between trimspeed and fully accelerated.
Now to fly with your speedbar at 60% can be tiring, as you knees are bent and taking a lot of strain.
This is why many experienced PPG pilots have a double-step speedbar arrangement, somewhat like two rungs on a rope ladder.
The two rungs are usually arranged so with feet fully extended (and knees locked down) on the lower rung, the wing is accelerated approx. 60% of maximum, i.e. flying at best Fuel-Reach in calm air.
The second rung, higher up on the speedbar lines, is used to reach 100% accelerated flight.
Trimtabs are also popular, as they can be set to any level without any effort to keep them there. They also allow asymmetric adjustment to counter propeller-torque effect.
I usually have both speedbar and trimtabs fitted for maximum flexibility. Some wings are certified with both (such as the ParaWing, Reflex and Silex), but experienced pilots may modify standard wings by adding trimtabs or replacing the risers with a compatible set containing both.
A word of caution here: If you fly a wing accelerated beyond its certified level because you have modified the risers, be EXTREMELY cautious in rowdy air especially when the motor is at idle or off in flight - your AOA may be dangerously low, leading to frequent deflations and/or technical recovery from asymmetric deflations.
Use the extra acceleration only under power in level or climbing flight, never in glide!
What happens to our Fuel-Reach under the effect of wind?
Penetration into wind:
Well, it is easy to visualise flying into a fresh headwind...
your groundspeed may be so slow, you may use up all your fuel before reaching your destination.
The stronger the headwind, the faster you need to fly to make headway, the further you will reach on your existing fuel. You will need much more power to maintain your height, but you really need the penetration of extra airspeed.
A headwind of about the 60% the strength of your airspeed at trimspeed needs full speedbar and/or fastest trim setting for best Fuel-Reach into wind.
e.g. if your glider's trimspeed is around 40km/h, then a headwind of 24km/h (60% of 40km/h) will require you set your wing for fully accelerated flight (approx. 50km/h?) to make best Fuel-Reach into wind. This may require almost full power, which really drains your tank quickly, but it is the only way to penetrate upwind.
With limited fuel supply, there is a limit to how much wind is worth battling against, as you may simply not reach your goal even when flying fully accelerated.
Also remember that stronger wind is usually more turbulent, so be careful with flying your wing in accelerated mode and be ready to back off at the first sign of pending deflations. Consider the option of abandoning the flight if the headwind is too strong or the turbulence too great for your comfort and experience level.
Often wind increases with altitude, so you may be tempted to descend to lower levels to find less headwind, but this might place you right in the layer of mechanical turbulence nearer the ground.
Free Energy from the wind gods:
Flying downwind on the other hand in a total pleasure, as you use the free energy of the wind to carry you closer to your goal. The longer time you remain airborne and facing downwind, the further the wind will carry you.
How do we remain aloft as long as possible? Its easy, we adopt the best Fuel-Endurance approach outlined above!
Back off the speedbar, reel in the trimtabs, keep off the brakes, back off the power as required to just maintain level flight, and relax!
Lower airspeed is more comfortable on your body, its quieter, there is less propeller-torque effect to compensate for, so its very relaxing flying unless the air is rowdy.
You may even decide to climb gradually to seek a stronger tailwind... a GPS is really handy here to assess your groundspeed which will help indicate your tailwind-component. As you climb, the ground appears to move more slowly beneath you so do not rely on your senses - it is just an optical illusion. Looking at your shadow below may help determine your groundspeed if you have no GPS.
An airspeed indicator is useless in this instance.
Cross Country Aerial Tours:
When covering vast distances downwind, consider how you are going to get back! Do you plan to fly back into a fresh headwind? Will you have enough daytime and/or fuel? Can you refuel en-route? Did you bring spare oil along? Can you land near fuel-stations and safely re-launch again in this wind speed and direction? Is the wind likely to increase any further? What about increasing turbulence and low level rotors?
Perhaps you plan to return by road. Do you have radio contact with a recovery driver? Do you have your cellphone along and a willing driver on standby?
Do you have a map or GPS to assist identifying your location after landing? Is your driver familiar with the region?
All these aspects should be considered before embarking on that glorious downwind aerial cross-country tour. Still, it may be well worth planning such tours beforehand and waiting for the right weather.
So far I have touched on only speed-to-fly regarding fuel economy. There are many other aspects to consider and a few other tricks to extend your flying time or distance.
Make like a bird!
Experienced paraglider pilots will readily seek out lift of all sorts and use this to gain height, sparing the motor and saving fuel.
At different altitudes you may find wind moving in differing directions. Balloon pilots use only this to steer and guide their craft to their destination. PPG pilots could do the same, but you need some help from a GPS to indicate your ground speed.
Gliding from thermal to thermal with the motor just ticking over or switched off, could greatly extend your flying time, though they seldom drift in your planned direction.
Finding Ridge-lift en-route may prove a major bonus over thermals, as you need not waste time climbing in a vertical column going nowhere fast.
A good pilot could assess the weather and local meteorology to plan a route that will utilise known thermic areas, thermic times of day, and ridge-lift along the chosen path.
Choosing a route that runs mostly downhill, i.e. the destination is at a lower altitude than the take-off area, will require much less fuel to cover, whereas climbing to a higher destination will consume more fuel than expected.
These are all flying tactics the PPG pilot can employ to extend his flying time or distance.
Motorheads unite! Bring your toolbox ...
What about mechanical tricks? Yes there are many of these too.
Variable pitch propellers allow you to increase the prop-pitch to enhance low RPM cruise at the cost of maximum thrust. So if you can take off with less than full power, consider increasing the pitch of your propeller or fitting a different fixed-pitch propeller which allows maximum RPM towards the bottom end of your power-curve.
On my own motor, my best multi-purpose prop allows maximum 6700 RPM. It has good take-off power, reasonable cruise at 5400 RPM and acceptable response for manoeuvring and aerobatics.
For long cross-country flights, I fit a cruising propeller that only gets to 6200 RPM at wide-open throttle. Take-off might be a bit trickier in light conditions with noticeably less maximum thrust, but at trimspeed I can cruise at a nice low 4900 RPM which is economical and quiet for extra comfort.
Likewise, for display flying, aerobatics and manoeuvring, I fit a prop which gets all the way up to 7200 RPM at max.
Again take-off is trickier as the motor is over-revving above its powerband, but the acceleration and response is fantastic with quick power on demand. However, this prop chews up fuel at cruise and especially at climb power!
Bigger is better? so they say...
Larger propeller diameters are more efficient, especially in a pusher arrangement as on a paramotor.
The pilot, engine and fuel tank all blocks the flow of air to the propeller. A longer propeller has more surface reaching out into cleaner air.
To generate enough thrust to maintain level flight at any specific wing speed setting, a longer propeller need not spin as fast, so a lower throttle may be used, saving considerable fuel.
Smaller is good too?
Smaller engines are sometimes more economical than larger engines, though they may lack some power in the top-end for take-off. If fuel economy is more important to you than maximum power or noise levels, consider a smaller engine. In the recent World Air Games, the smallest engines with HUGE propellers won all the economy based tasks.
Tickling your carb?
When I am seriously low on fuel and need to push a bit further, I lean out my carb's high adjuster to the limit without the motor actually overheating and seizing.
I keep a sharp eye on my CHT (Cylinder Head Temperature Gauge), and a sharp ear out for the sound of engine pinging. Any sign of overheating or pinging, I richen a bit to cool the motor and re-adjust after a few minutes again.
Fortunately all Xplorer paramotors have the carb within easy reach in flight and both adjusters facing the pilot with T-bars which can be easily adjusted by hand. The popular Tillotson carbs are jet-free and dynamically adjustable.
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