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Is engine power really that big of a deal?


fuzzybabybunny
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Say that you've got a choice between a 130cc motor that's only 1kg lighter than a 200cc motor. The prices are both similar. 

As I understand it, thrust is governed by how fast the prop is spun. A 125cm prop spinning at X RPM will produce less thrust than the same prop spinning at X + Y RPM.

A more powerful motor allows a larger gear ratio to be used so that a big motor at 5,000 RPM can spin a prop at X + Y RPM while a smaller motor is only powerful enough at 5,000 RPM to spin the prop at X RPM.

Horizontal thrust from the prop translates into airspeed across the wing which translates into lift. So you climb faster with more thrust, which may be useful for quickly getting to a safe height after takeoff.

But this is where I stop seeing the advantages. 

A bigger motor means worse fuel economy, so to maintain level flight a 130cc motor will use less petrol (like 33% less) than the 200cc motor when you have the wing, chassis, and pilot constant. I guess a different gear ratio can mean the larger motor can spin slower than the smaller one to maintain the same propeller RPMs for level flight, but it seems that overall the bigger engines simply use more petrol than the smaller ones no matter what.

Maximum fuel economy is getting the smallest engine you can find and driving the biggest propeller you can get, right?

The rest of the flight characteristics are governed by the wing, not the motor.

A more powerful motor wouldn't necessarily even translate into a longer lifespan, right?

So unless you're flying tandem, are heavy, or fly at high altitudes, is there really a reason to go for a 200cc motor? Seems like compared to a Top 80 or something that motor will simply use more petrol.

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in some cases you are over simplifying it, but it is possible to simplify it further, ignoring gear ratios, reduction drives, prop pitch etc, that already been calculated by the manufacturer and they aim to supply the most effected solution they have. So ignoring all that we are down to engine size and thrust.

 

80-100cc giving 50kg vs a 180-200cc giving 75kg thrust.

Lets say you need 40kg thrust for level flight. that put the smaller engine at 80% of its max power where as the bigger engine is only using 53% of its max power or range of power. At max thrust the smaller engine burns 4lph. the bigger engine producing more power burns 6lph at max thrust. at equal thrust they burn similar amounts of fuel.

the small engine will typically rev to 10000rpm sometimes more. the larger engine 8000 as an example.

So at 40kg thrust / 80% power the little engine is 8000rpm

the bigger engine at 40kg thrust / 53% power is only 4200rpm

If one were 100cc at 8000rpm and the other 200cc at 4000rpm the fuel burn is going to be pretty similar.

 

Now for power, given a set pilot weight and wing size, he does not need more power to go faster, it will help him climb faster. If that pilot decides to fly a smaller wing, then he will be able to fly faster and still be able to climb well. If he put the smaller wing with the smaller engine, it would hamper his climb out and his level flight could then be 95% of max power, not a good way to treat your engine. 

Bigger engine will seem more noisy and is often more noisy at max rpm than the smaller engine, but then it is also producing more power. Put both engines at 40kg thrust and the larger engine will probably be the quieter engine. Cooler running and less stressed.

 

Things are never a constant and many top80 owners find it the most economical engine they have ever owned. but then are they winning eco comps still? I think mini2 and thor200 have beaten it, both are 200cc engines. I guess it shows the pilots tuning skills, throttle control skill, piloting, wing choice, trim setting.. all has a part to play.

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Oh wow 200cc engines are actually winning eco challenges compared to the Top 80?

I'm pretty new to this. Can you provide some links to results?

I read somewhere a while ago that the best economy will always just be a small engine with a big prop. And then I figured that I'm only 75kg and a Miniplane would only weigh 19kg so I began wondering why I'm even thinking about machines with bigger engines.

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On 8/26/2016 at 11:39 PM, nforster said:

Icarus uk result- macfly polini 250, mark Morgan , approx 2.5l per hour viper 4 18

That was running standard jetting and standard prop pitch, Uk Open i managed to get 2.14lph and in testing down to 2lph on my 18m.. Looking at comp Eco task results can give you a false figure if you do not understand the task that was set..  

UK open pure eco 2.14lph

Worlds Pure eco 1h 17m on 1.5kg of fuel..

Viper 4 18m with my 250 ave 19/20Km per litre...  at the Worlds at Popham the top eco distance results were Polini 250 and 200's and i think one french pilot with a top 80 tunned to the limit with a PWK style carb...

 

Edited by morgy
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They are seriously impressive ecomomy figures Morgy, and not with kit that i would of first thought of being capable of such frugal flying - a large motor and `tiny` wing..

How do you get such economy, do you spend 90% of the time engine-off in thermals, LOL?

What sort of engine speeds are you seeing in level flight?

Have you experimented with the adjustable-pitch prop on the Macfly?

Do you think one of the reasons they can be so economical is their use of a slide carb which permits a greater degree of fuelling control over the rather crude Walbro diaphragm jobbies?

 

 

 

 

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smaller engines (80-100 cc.) are normally advised to beginners (a powerful engine could be dangerous for novices) and very lightweight users (up to 70 kg.)

As custom-vince has already explained, a bigger bore engine is generally quiter than a smaller one, simply because for a level flight a bigger bore will run to a lower RPM range than a smaller one. For cross-country flights you will keep 80% of the time a level flight, so this a good advantage in case the cross-country is your favourite kind of flight.

I do own a vittorazi fly 100 evo (100 cc.) and I have to run it @ abt. 7500 rpm for a level flight. A 180 cc. engine has to be run @ abt. 1000 RPM less for a level flight.

I'm very satisfied about my fly 100 and honestly I don't feel the need of more power, and the above is the only reason why I will switch to a bigger engine as soon as will be possible. Less noise, less mechanical stress for the engine.

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Had to edit my post.. Lack of sleep during the comp effecting my brain cell.. ;-)

Hann 

The world eco figure were a thermal task IE Pure eco but thermal activity switched off half way through.. The UK Open figure were also a pure eco but with no thermal.. 

I do use an adjustable pitched prop and does make a difference in the fuel burn figures.. We have pilots on Mac fly 250 claiming ave fuel burn figures around 3lph flying small snakes 16-18-20m most are around 80-90kg 

If you really want to squeeze every drop out of your machine you can very very Anal about jetting and pitch, 

In flight speeds are ave 41kph slow fast trims 50-55kph  65+ on bar 

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I would totaly agree with Vince in most of the facts stated.

One more thing to you to consider. When you will start to fly at 40 mph speed with Reflex wing top 80 will not be able to provide ascent rate at all.

Regards,

Ivan

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This thread has focussed mainly on the economy benefits and theory of larger engine. What happens in reality is different.

If you are a light person (ie. 70 to 80 kg) then I don't think there is any real advantage to be had from having a big engine. I can still climb with trimmers out and speedbar fully on, on a 22m reflex wing with a 130 cc engine. I have never once felt I need more power. Even when I was at higher altitudes (10 to 11000 ft). If I had more power I would just be battling with torque effect more and lugging about weight I didn't need, with a bulkier thing.

The efficiency point is not that relevant to me either. I usually get bored or cold and land long before my petrol tank is empty. 2 hours is the max for me really, 1 hour flying is fairly typical. I actually quite like burning up fuel too, cos then I can buy some more and I don't have any sitting around degrading in a fuel tank. I am normally flying about trimmers fully open going as fast as I can. If I really want to fly as far as I can then I will attach my paraglider wing - that makes way more difference than me choosing a bigger motor.

Unless you are competing the reality is that selecting an engine which is appropriate for your size is better, than picking a bigger one. When you first start weight does make a difference as you have failed take offs and get tired out. Also you will end up carrying it about (from car to take off).

For the average paramotorist, the most important factors when choosing your motor must be reliability/user friendliness, how it feels to fly and weight (probably in that order). The rest is irrelevant until you've been flying a while.

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On 01.09.2016 at 22:22, fuzzybabybunny said:

So basically what I'm getting from this thread is that there are no downsides to larger engines compared to the small ones like the Top80, besides a bit of weight?

Yes, basically, but it is a bit more complex.

I depends upon your weight too, indeed. If you are above 80 – 85kg I would not recommend to go for light engine.

Personally I flew 7 different paramotors with Snap Ego (the same thrust as TOP 80), and few larger engines: Moster 185, Simonini and C-Max. A am 92 kg and I will not go for light engine. One of my mates just decided to sell his Apco Force as his TOP 80 is not coping with horizontal flight with accelerator fully pressed. My Moster is giving me only 0.5 – 1 m/s ascent rate with accelerator fully pressed with my 28m Apco Lift at 38 mph speed. And it is minimum of what I consider to be safe.

I would not say you will experience over thrust and torque effect problems if you got the right harness and it is set up properly (not discussing the paramotors which are not safe by definition). Just as an example – I had Nirvana Rodeo and it was perfect, but when I got Nirvana Instinct (the same engine and moto frame), it was really bad in terms of torque effect – the harness was too big for me and whatever I tried there was a tendency to have a twist-prone position at full throttle.

The same situation with starts and take off – I would say it is much more dependable on your skills and harness settings than 2 kg of extra engine weight. And the other thing is the prospective. If you would ever think about pylon racing, just forget about the light engine – you will need all the thrust you can get. The same rule is if you will have later on to fly the fast wing with about 40 or more mph speed. But if you are of 60kg weight and will fly slow wing with no accelerator ever attached – yes, you can go for light engine. Generally, the engine with more thrust is giving you more flexibility in your future wing choice and paramoto “career”. It might have happened than in one year you will stop doing this hobby, but at the same time it might come to the point when you will start considering to do the tandem flight and light engine will not cope again. Your choice at the end of the day. And of course the choice of the right paramotor (frame, harness, etc.)  is not less but may be even more important that the engine.

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Ok, after reading some more the issue of fuel economy seems to be a really complicated one and something that's not easy to maximize.

Say that for a certain weight and prop level flight requires 40kg of thrust.

40kg of thrust is obtained by spinning a certain prop at 2000 RPM. 

Thus, the motor needs to spin the prop at 2000 RPM, and this is where it get complex.

The RPM that the motor needs to spin at to obtain this propeller RPM is determined by the reduction drive, so it's not an easy comparison. If a 200cc motor and a 125cc motor where fitted with the same reduction drive, so that the 200cc had to run at the same RPMs as the 125cc motor, the 125cc motor would get better fuel economy. But if the 200cc motor were fitted with a redrive that allowed it to spin considerably slower than the 125cc motor, while both achieving that 2000 RPM propeller speed, then the 200cc motor could theoretically achieve better economy. 

What I find interesting is that the MacFly Thor 250 and Top 80 seem to do very well on the eco challenges, despite being literally polar opposites in terms of power. And the new Air Conception Nitro 200 seems to do awful, as well as the Moster 185s. 

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On 26-8-2016 at 2:31 PM, fuzzybabybunny said:

Say that you've got a choice between a 130cc motor that's only 1kg lighter than a 200cc motor. The prices are both similar. 

As I understand it, thrust is governed by how fast the prop is spun. A 125cm prop spinning at X RPM will produce less thrust than the same prop spinning at X + Y RPM.

A more powerful motor allows a larger gear ratio to be used so that a big motor at 5,000 RPM can spin a prop at X + Y RPM while a smaller motor is only powerful enough at 5,000 RPM to spin the prop at X RPM.

Horizontal thrust from the prop translates into airspeed across the wing which translates into lift. So you climb faster with more thrust, which may be useful for quickly getting to a safe height after takeoff.

But this is where I stop seeing the advantages. 

A bigger motor means worse fuel economy, so to maintain level flight a 130cc motor will use less petrol (like 33% less) than the 200cc motor when you have the wing, chassis, and pilot constant. I guess a different gear ratio can mean the larger motor can spin slower than the smaller one to maintain the same propeller RPMs for level flight, but it seems that overall the bigger engines simply use more petrol than the smaller ones no matter what.

Maximum fuel economy is getting the smallest engine you can find and driving the biggest propeller you can get, right?

The rest of the flight characteristics are governed by the wing, not the motor.

A more powerful motor wouldn't necessarily even translate into a longer lifespan, right?

So unless you're flying tandem, are heavy, or fly at high altitudes, is there really a reason to go for a 200cc motor? Seems like compared to a Top 80 or something that motor will simply use more petrol.

Hello,

I fly a miniplane top80 , 1.25 prop, with a dudek universal 28 reflex glider for almost two years now.
My age is 58, my naked weight is 85 kg and the total flying weight 120 kg. My flying area is in the Netherlands
0-wind starts are no problem even when I fly in reflex-mode (trims complete open), than it is still possible to climb 1m/s. (must use full power) Level flying 80% from full power
Top80 Spare parts are very cheap (New cilinder and piston around Euro 200)

Greeting,
René

 


 

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The size of an engine alone is no guide to fuel economy, neither is its maximum power. Any engine may be tuned for maximum power, torque or efficiency at a particular RPM, but there will always be a compromise. To win an Eco Challenge, an engine should be running at its maximum efficiency at cruise. This could equally be an 80cc engine at 8000rpm or a 200cc engine at 4000rpm.

A 2 stroke engine is most efficient at its resonant frequency, when the airbox, porting and exhaust pulses combine to get almost all the exhaust gas out of the cylinder and almost all of the intake charge in with minimal losses. If that frequency (i.e. rpm) coincides with the cruise power setting, then you will be getting the best efficiency from that engine. However, tuning for maximum efficiency at cruise will not give maximum power at full throttle.

There is also the complication of torque/power curves. If the engine can make 20HP at 10,000 rpm, but only makes 10HP at 8,000 rpm, it won't have enough torque to get the prop past 8,000, so it will never achieve it's full power. It quickly becomes apparent that it is necessary to balance the engine characteristics, drive ratio and prop to make sure that both the engine and the prop are used effectively.

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