most efficient speed for level flight

[chrisscammell]

I would like your opinions on which level of trim on a glider will give the most efficiency i.e mpg. This assuming no wind.

I am into getting maximum distance per litre of fuel and need to know which speed to fly at. I am flying a high performance free flying wing with a glide of 9:1. I have fitted it with paramotor risers with trimmers.

Should I be flying hands up or trimmers released to get maximum efficiency?

My thinking is that the glider is 'cleanest' at hands up trim and that releasing the trimmers, although giving more speed, will create proportionally more drag and therefore reduce efficiency.

Would appreciate your thoughts?

[learner_driver]

Hi Chris

Trimmers make it both fast and inefficient.

so hands down 10 inches is Minimum sink

hands up is best glide.

Here comes the complex bit.

if you want to go as far as you can down wind you want to fly at minimum sink = less throttle. And the wind will carry you

If you fly into wind you will no doubt need trimmers out to maximize distance as, flying into wind you may be sat still over the ground, at best glide or min sink

try to google speed to fly.

have fun

[chrisscammell]

Thanks but I understand 'speed to fly' theory for unpowered flight (I have over 1000hrs of free flight on hang gliders and paragliders). What I need is data about speed to fly under power, assuming no wind and level flight.

Does reducing the angle of attack with the trimmers produce more or less efficient flight under these circumstances?

[learner_driver]

Hi Chris,

using the trimmers turns the sink-rate from about 1.1M/s (Trimmers in), to about 3 M/s down (Trimmers out) - depending on the wing

Kind Regards

Simon

[chrisscammell]

Thanks but again, this only tells me what I already know, that trimmers off in un-powered flight is less efficient but faster.

What I need is information about efficiency in powered flight. When the comp' boys are trying to go as far as possible on a litre of fuel, do they fly with trimmers on or off?

Max L/D on an unpowered wing is hands up usually. As you add power into the mix it raises the angle of attack. Releasing the trimmers lowers it again. But does trimmers off create more drag, as it changes the wings profile beyond the optimum.

Then there is speed. Does cruising at 40km/h with trimmers on (clean profile) burn less fuel per mile than crusing at 50km/h trimmers off (reflexed profile)?

Flying faster creates less induced drag but more parasitic drag. Add that into the mix

It's not as simple as saying that it's the same as efficiency in unpowered flight. That power changes the angle of attack and makes a difference. The question is WHAT difference?

[morgy]

Hi Chris

I have been thinking about the same question as you for the past few weeks now as i am planning on doing some long XC flights down wind. I can not find out any info on this on a google search. I have found loads on the mc creedy (speed to fly) subject but nothing on powered speed to fly.

I am a bit of an numpty when it comes down to the mathamatics about FLIGHT,LOD,SPEED TO FLY ETC

I am sure there is someone on here that would be able to come up with a link or explain what your asking..

I see your fairly new to this forum.. where do you fly???

[alan]

I must admit I have pondered this when flying xc but i always put it into full reflex because you are covering the ground more quickly.

On Saturday I flew to Flamborough on the east coast when I took off i stayed on the Paramotor circuit at the airfield until the other two paramotorists were airborne approximately 25 minutes on the way back i ran out of fuel and out landed and just before i did i put it back to full slow and i came to what seemed like a standstill.

I was at about 2000ft when the fuel ran out and managed to catch a nice thermal which gave me plenty of time to work out the wind direction and made for a nice landing.

So my theory is although you burn more fuel on reflex you are also covering the ground a lot faster so this maybe compensates for the loss of glide ratio although i would love to know the proper workings for it.

Incidentally on 10.5 to11 litres of fuel i covered just over 50 miles with an average speed of 27mph with no speedbar.

[alan_k]

Incidentally on 10.5 to11 litres of fuel i covered just over 50 miles with an average speed of 27mph with no speedbar.

Seems a high fuel burn of over 5.5 litres per hour with no speedbar

Mine is 4 l/h without bar.

Cheers,

Alan

[alan]

Hello Alan,

Yes my motor had been adjusted on a visit to Parajet for an alteration on a pulley and when i flew it i was not getting full throttle in the air so i put it back to factory settings on that flight which makes it run slightly rich, and as i have got today off i will be tinkering with it.

[pete_b]

It will depend on your fuel burn at differing trim settings at your all up flying weight

SO

Measure your fuel exactly (start a stop watch on take off and the gps for your speed) take off and climb slowly to you cruising hight fly for how ever long you want (the longer the better) shut off motor (stop stop watch) and land.

Measure your fuel exactly and work out how much you have used in millilitres is best, then divide the fuel used by the minutes in the air will give you your usage per min multiply by 60 = per hour.

Now do the same on nutral trim, fast trim and full speed bar.

You can see from your gps which settings give you the best speed over ground.

Then do it at differing wind speeds and you have all the data you need to fly at the best setting for

Fuel economy

Get there quick never mind the fuel

Stay up here as long as I can

how much longer can I fly on the fuel I have left

ect

ect

ect

Yes it is a pain but it is the only way if you really want to know

It differs due to all up weight

Air pressure higher or lower

ect

ect

I know I did it for the comps one year