Question about the concept of thrust

[PenguinWaddle]

Ok I gota ask you guys something. What exactly is the concept of thrust? I know it has to do with like... something about accelerating 1m/sec... but i was wondering something.

If i put an engine on a weight that was producing 200g of thrust and put enough weights on it to make the whole thing equal to 200g and have the engine going upwards, does that mean that the whole thing will theoretically hover or will it accelerate upwards at the 1m/sec?

Also, how does this factor into how airplanes work when they roll off a runway?

[Lomcevak]

If you have a 1:1 thrust to weight ratio, the thrust produced is equal to the acceleration of gravity and thus you should be able to hover (this does not imply that this is the only factor in one's ability to hover, just a physical fact). You must have better than 1:1 in order to accelerate against gravity (up). Flying off the end of a runway involves creation of lift by a wing. Lift is generated by moving an airfoil through air with some effective angle of attack. In most airplanes the thrust faces forward creating that forward motion that allows the wing to generate lift. Lift equals gravity in level, unaccelerated flight. Excess lift must be produced (by accelerating the airfoil to a higher speed) in order to gain altitude.

[PenguinWaddle]

Well is there any general equation to figure out if the pressure under the airplane is great enough when given surface area and thrust to create vertical movement (take off)? Or is that just a rather complicated thing to figure out....

[J Hockin]

Someone responded to me in another forum that thrust should be about 60% of the aircraft flying weight. So if it weighs 20 oz. you should have about 12 oz. of thrust. Prop and engine combo charts from GWS will give you the proper drive and prop size to achieve that. I think I was flying my PT17 with about 13 oz. of thrust and it weighs around 24 oz. so I am a bit low on the thrust side. I does climb out OK though. I going to change drives and props to get around 15 oz. of thrust. I think you want more thrust for acrobatic maneuvers, something I am not that good at yet, except I can do loops. Also the more thrust a combination of engine, drive and prop exert, the less endurance the battery has. So...the more power you want, the less time you get to fly. It takes a lot of power to get a model moving fast, so your endurance drops down. That is why I like a lower power setting - so I can cruise around a bit longer. To each his own!

Jim:)

[PenguinWaddle]

haha great... i really dont have much to look at since im using a ducted fan A-10 so no prop-motor combos for me :D

[geoff_s14]

Also the more thrust a combination of engine, drive and prop exert, the less endurance the battery has. So...the more power you want, the less time you get to fly. It takes a lot of power to get a model moving fast, so your endurance drops down. That is why I like a lower power setting - so I can cruise around a bit longer. To each his own!

Jim:)

Jim, this isn't necessarily the case, as certain combinations of prop/drive/motor are more efficient than others. Take for example my Funnypark, with a 9070 prop and EPS300 motor, thrust is 260g for a power consumption of 8W at full throttle, but with the 9047 prop max thrust now becomes 267 for 5.7W of power on full throttle. The thrust has gone up (marginally) but the power consumption has gone down by 30%

[PenguinWaddle]

6 watts? I thought you give motors a hell of a lot more power then that... I mean on my ducted fans, both draw a total of 18 amps...does this mean im pulling like 1 volt or something?

[geoff_s14]

6 watts? I thought you give motors a hell of a lot more power then that... I mean on my ducted fans, both draw a total of 18 amps...does this mean im pulling like 1 volt or something?

Oops, I meant Amps not Watts didn't?
8A with a 7.2v cell = 57W

[Bruce Abbott]

er... I think geoff_s14 may have miscalculated slightly. A 9x7 prop needs about 23W to make 260g thrust, and that's assuming a 100% efficient motor!

In general, the heavier the model, the more power is required to fly it. There are two basic reason for this.

1. To gain height potential energy must be added to overcome gravity. Obviously a heavy model needs more energy to raise it to the same height as a light one.

2. Airflow around the model causes drag, which must be overcome to maintain speed. A heavy model needs to fly faster for the wing to generate more lift, but this also creates more drag. Alternatively the wing's surface area could be increased, or a higher lift profile used. But these both increase drag!

The propellor (or fan) only has to produce enough thrust to overcome the drag that occurs at flying speed. Because of its high rotor pitch, a ducted fan produces almost constant thrust as speed increases, unlike a propellor whose thrust reduces rapidly as airspeed approaches the prop's 'pitch speed'.

Thus, a sleek ducted-fan jet can often get away with less than 50% static thrust to weight ratio. However, it may accelerate slower than a prop-driven model. When taking off it pays to let the model build up speed before trying to climb out.

[PenguinWaddle]

lol good because i was figuring according to GWS's site, i have 320g of thrust and they say its 500g take-off weight and im confident i exceed that 500g thanks to my battery and a small wooden bulkhead inside.

One question though... i connected the engines to a 25amp power supply... well, i connectd teh whole sysetm to the power supply in lue of a battery. When i went to full power, i was drawing 18 amps as i said earlier when the GWS website says that the EDF55/300 at 9.6 V will draw 6.1 amps creating the 158g of thrust each.... is this bad? Im using an 8cell Gold Peak 1100 2/3A pack thats supposedly 25+ amps (with deans ultras) and i dont want to be flying this if the motor is going to be drawing more amps then its suppose to... or is that possible...

[geoff_s14]

er... I think geoff_s14 may have miscalculated slightly. A 9x7 prop needs about 23W to make 260g thrust, and that's assuming a 100% efficient motor!



er Bruce....you are right, see my correction above - I was of course talking about Amps, not Watts in my first post.

Geoff

[Michael Heer]

I am moving this thread to the Modeling Science Forum. Moderator Michael Heer

[Bruce Abbott]

i was drawing 18 amps as i said earlier when the GWS website says that the EDF55/300 at 9.6 V will draw 6.1

In my tests it drew 7.3A at 9v and 8.3A at 10V, so I can only conclude that GWS use a different kind of volts! (or more likely, they used an 8 cell battery that couldn't sustain 9.6V under load).

[Lomcevak]

Well is there any general equation to figure out if the pressure under the airplane is great enough when given surface area and thrust to create vertical movement (take off)? Or is that just a rather complicated thing to figure out....

http://www.grc.nasa.gov/WWW/K-12/airplane/Images/lifteq.gif

If you are really interested in this, there is a great simulator on that website too! BTW, your original question asks about thrust. Thrust figures as it's effect on the airframe (speed) in this equation.

http://www.grc.nasa.gov/WWW/K-12/airplane/lifteq.html