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Forces, Thurst, Torque, Power.....


Guest Cralis
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G'day.

 

I am reading my BAK manual, and got to the section on Forces, Moment, Work, Energy and Power. (Pg 172 in the colour version). Note, as I type, I'm thinking aloud, as I am also trying to work it out for myself.

 

Yes, I did science at school all 15 years back, and did OK, but forgoten a lot, and just want clarrification.

 

Firstly, am I right in saying that 'A 10 tonne truck is more powerful than a F1 car' is incorrect. In normal english, that would sound correct, as you need more power to pull the 10 tonnes, and a f1 car would not be powerful enough to do so.

 

However, in aviation (and scientific) talk, that statement is false. The F1 car is more powerful than the truck, as it can go faster and travel further in the same amount of time. This is confusing me.

 

Power = the rate at which you do the work. Therefore, how fast you can do the work. So, power = speed? No. Because Power = thurst X speed. The book says that Power is a combination of force, distance and time. Which then means that there is no such think as a powerful kettle (which heats water faster than a less powerful one). As there is no force... no distance.. just time.

 

Man, this is confusing. :)

 

I guess then, it's not the KETTLE that's powerful, it's the filliment. But that doesn't move either, and has no force. So, down a level. It's the force of the 'electrons' causing the heat, and their movement over time?

 

The same then should be said about the cars, no? The car has no power. The engine does. And then this has nothing to do with movement (Power is a combination of force, distance and time), as the engine, reletive to the car, is stationary. :ah_oh: So, is the power now how fast the engine can move the propeller?

 

WAIT... I remember reading that! I'm on to something here. OK, so powe is related to how fast the propellor spins (Same as with a car and how fast the engin moves the wheels). So it's NOT anything to do with how fast the car goes... reletive to the ground. (Lets not go near gearing! 031_loopy.gif.e6c12871a67563904dadc7a0d20945bf.gif).

 

Ok, so I may be answering my own question here. Right, so power is more to do with the speed of the prop. Then.. torque? Is torque = Power? This section doesn't define torque. So I'm unsure how that fits in.

 

Just read that 'The tractor has more torque, but it moves slower'. So, that indicates Torque NOT = Power. Torque = Thust.

 

So, can I safely drop 'Torque' and replace it with the word 'Thust'? If so, then 'Thurst = how hard you push'. This is what confuses me. A more powerful man can push me a lot harder than a less powerful man'. So it seems Thurst and Power are related. Who's more powerful? A 100m sprinter, or a weight lifter? To me, it's the weight lifter, but the correct answer would be the sprinter. The sprinter has more power (As he runs further in less time), while the weight lifter has more Thurst, as he can..... yeah, stuck now.

 

Back to Thurst * Speed = Power. The spinter has low thrust, but high speed. The weight lifter may have low speed, but higher thrust. (Both running... Comparing Weight Lifting and Sprinting isn't a good plan). So, they could have the same power?

 

It's confusing! 087_sorry.gif.8f9ce404ad3aa941b2729edb25b7c714.gif

 

 

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Guest weekendwarrior

Cralis,

 

I guess your interpretation is in a way too simplistic, or too complicated, or maybe both.... I am going to try and keep it simple, I apologize in advance if I fail in the attempt (which is likely)...

 

Before going into torque/thrust/power etc..., start with energy.

 

Energy can be of many forms, mostly here we are interested with chemical, thermal and kinetic. Electric is another form. I guess what you are calling "force" is kinetic energy.

 

An engine is transforming the chemical energy contained in the gasoline in thermal energy (heat) and kinetic energy thru the motion of the crankshaft, thereby spinning the prop.

 

The energy required depend of the work required.

 

For exemple if you want to move 1 ton over 10km or 10 tons over 1km, it is the same work.

 

Now the rate of work is how fast you do this, e.g if you do the 10km in 1h or 1minute, you will use the same energy but over a much shorter period of time the faster you go, so you need more power.

 

There is electric energy which is transformed in thermal energy dissipated in your kettle, and the more powerful it is the more water it can boil at a faster rate. And for a given power it's faster to boil half a kettle than a full one.

 

(Ok now deep breath and a look back at your original post to make sure I actually answer your questions)

 

A 10 ton truck might be exerting the same power as a F1 but under vastly different conditions, the truck hauling a heavy work slow, the F1 a light one fast.

 

A force is longitudinal (push or pull), whereby a torque is rotational (twist). And since engines deliver energy in a rotational fashion by spinning a crankshaft.

 

A torque is defined as the product of a force and the length of the lever arm. Think about lifting something heavy at arms length and you'll get the idea.

 

The power is generated by the engine.

 

The thrust is generated by the propeller.

 

The thrust is proportional to the mass of the airstream multiplied by its velocity :

 

How much air you move and how fast (pollies therefore create thrust if not trust, but I digress)

 

You will need a given amount of power to spin a given propeller at a given speed.

 

Yes the engine is mated to the propeller, but since different propellers will give different thrust when spinning at different rotational speed, depending on number of blades, pitch, diameter, etc... and different engines will deliver different power at different RPM as well, and that you also have to factor in efficiency, e.g how much power is lost and how much translate into thrust, you cannot say that thrust = power, what you can say thrust is the result of power.

 

Regarding your tractor statement, think about your car instead. The reason it has a gearbox (sorry but yes had to go there) is to adapt the engine load to the conditions. You are likely using less power doing 110kph on the freeway in 5th gear yet are going faster than you are accelerating uphill towing a caravan in 1st gear, because you are then doing a lot more work.

 

Similarly you need more power when climbing than when cruising, even though it's faster to cruise. Make sense ?

 

Hope it helped to clarify things more than confuse them, but I am not sure about that: Now I need a beer. Or an aspirin. Actually, both.

 

 

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Thanks WeekendWarrior. That has cleared up a lot. Before you hit the drugs and alchohol ... :) ... while I was trying to work this out, before your post, I wrote down, 'You need POWER to obtain TORQUE which is then translated into THRUST' on a notepad. Is that a correct statement? You saidm 'what you can say thrust is the result of power'. So it looks like I was close to clicking.

 

So, when you mention the 'Power' of an aircraft - you're speaking specifically of the engine. Nothing to do with any gearing or type of prop. If you wanted to speak about the... strength... you would be talking about the thust of the aeroplane?

 

 

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Craig mate its called BASIC Aeronautical Knowledge for a good reason. At the rate you are going you will be baffled by science and we will bamboozle you with bull excrement.

 

The length of debate about a flight manoeuvre is always inversely proportional to the complexity of manoeuvre. Thus, if the flight manoeuvre is simple enough, debate approaches infinity.

 

-- Robert Livingston, ‘Flying The Aeronca.



 

 

 

 

 

 

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Guest weekendwarrior

Yes this is the power of the engine not the aircraft - to get more power you get a more powerful engine, or more engines.

 

The power is the result of torque multiplied by revs, so you need both torque and revs to get power. If you have revheads mates they might be able to explain it to you. I'll give it a shot :

 

To get more torque out of an engine, you typically add more displacement (engine capacity) by bigger bore/stroke, or by adding more cylinders (V8 anyone ?). And to get more power out of that engine, you spin it faster, more revs. So you can actually have a similar power output with different engine capacities:

 

For example a superbike with a 1000cc engine (one liter) will crank out 180 horsepower at say 12,000 rpm.

 

A two liters car engine might crank out the same 180hp at say 6,000 rpm. And a Lycoming O-360 (an engine typically powering Cessna 172 or Piper Archer), which is a 5.9l, will produce the same 180 hp but at a lot more sedate 2,700 rpm (which is matching the propeller speed without the use of a gearbox. Some aircraft engines like the Rotax spins faster but do have a gearbox to reduce revs from crank to propeller).

 

So there you have very different designs for apparently the same output, but very different requirements. The motorbike has much less torque, but then it has a much lighter load as well. The acceleration of a superbike is extremely enjoyable (revhead talking here), but that power is only used a few seconds at a time, whereby the aircraft engine will deliver continuously 55% to 75% of its rated power. In that respect the aircraft engine is a lot like a truck, hence the big capacity engine.

 

I guess you'd still be talking about the power to express performance, because that's the easiest thing to compare against and people are familiar with this notion.

 

But theoretically yes I guess you would get better thrust (and therefore performance) from say a 160hp engine mated to a very performant propeller in mint condition, than you'd get from a 180hp engine mated to the wrong propeller for the application and if it inefficient e.g chipped with stones. Now I am not a subject matter expert, so I would be happy for one to comment further !

 

 

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Guest weekendwarrior
Craig mate its called BASIC Aeronautical Knowledge for a good reason. At the rate you are going you will be baffled by science and we will bamboozle you with bull excrement.

 

 

Good point too lol

 

 

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Terms.

 

Cralis, I used to teach this stuff and you are blowing my mind, Keep it simple and understand the meanings of the terms you are using fully. Learn a simple (but correct) definition of each term and understanding will unfold. Get a good basic grip of Newtons 3 Laws of motion, (actually he was not the first but history has given him the credit) because most of what happens around aeroplanes can be explained using them. Nev..

 

 

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Guest weekendwarrior
whereas what happens around helicopters can only be explained by ------ ------- well it cant really because its's a mystery

I beg to differ, it's fairly simple:

 

Helicopters fly because they are so ugly, the ground repels them.

 

 

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Thanks Frank.

 

I'm trying to learn the BAK. It's possible I am asking too many 'Whys'. But I prefer to understand things, than just read stuff to regurgatate at a later stage. Different strokes for different folks, I guess. I'd love to be able to have a vast knowledge of flight - not average.

 

My goals have changed a bit in the past few weeks. I'd like to instruct on weekends in maybe 10 years time. And when I start that, I'd like to have all the answers for my students. It would give me a lot of confidence in myself, but also I could join in in some of the discussions on these forums with confidence that I understand what I am talking about. As a basic example, I don't want to land in the situation of, 'The wing gives lift'. Then get asked, 'How?', and answer 'Because the BAK says it does'. I'd like to understand why, instead. (Please note, the BAK does explain how... It's an fake example).

 

So, yeah, maybe I ask too many 'Whys' and 'Hows', which are better left to the more advanced guys, but.. I am keen to know these things. I may post my thoughts with the antisipation of a more experienced guy saying, 'No Cralis, it's more like this....' and explain it. 'Weekendwarrior' did that at the start of this thread. And thanks for that.

 

This thread though may be a bad example, as I agree that there is a chanin reaction that can continue in science. 'This happens.', 'Why?', 'Because this happens.', 'Why?', 'Because this happens...'.... endless loop.

 

I was just trying to figure out how the forces, Thust, Torque and Power all fit together.

 

I understand that most of my question doesn't have to be known... but... it would be nice to understand. :)

 

 

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Thurst

 

So it seems Thurst and Power are related. Who's more powerful? A 100m sprinter, or a weight lifter? To me, it's the weight lifter, but the correct answer would be the sprinter. The sprinter has more power (As he runs further in less time), while the weight lifter has more Thurst, as he can..... yeah, stuck now.

 

Sorry to be so late on this one, but I guess it depends on the drink i.e if it where Gatorade I reckon the sprinters thirst would be greater but if it were beer the weightlifter would certainly prevail 087_sorry.gif.8f9ce404ad3aa941b2729edb25b7c714.gif 114_ban_me_please.gif.0d7635a5d304fa7bdaef6367a02d1a75.gif

 

 

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Guest weekendwarrior
So it seems Thurst and Power are related. Who's more powerful? A 100m sprinter, or a weight lifter? To me, it's the weight lifter, but the correct answer would be the sprinter. The sprinter has more power (As he runs further in less time), while the weight lifter has more Thurst, as he can..... yeah, stuck now.Sorry to be so late on this one, but I guess it depends on the drink i.e if it where Gatorade I reckon the sprinters thirst would be greater but if it were beer the weightlifter would certainly prevail 087_sorry.gif.8f9ce404ad3aa941b2729edb25b7c714.gif 114_ban_me_please.gif.0d7635a5d304fa7bdaef6367a02d1a75.gif

It's spelled thrust not thurst ! Or you really meant thirst ? 006_laugh.gif.0f7b82c13a0ec29502c5fb56c616f069.gif

 

 

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Thanks Frank.I'm trying to learn the BAK. It's possible I am asking too many 'Whys'. But I prefer to understand things, than just read stuff to regurgatate at a later stage. Different strokes for different folks, I guess. I'd love to be able to have a vast knowledge of flight - not average.

 

My goals have changed a bit in the past few weeks. I'd like to instruct on weekends in maybe 10 years time. And when I start that, I'd like to have all the answers for my students. :)[/quote}

 

Hi Craig,

 

"Different strokes for different folks", ABSOLUTELY,and no,we can never ask too many questions so don`t be afraid to ask as many as you feel you need to,just be carefull of the answers you get.

 

In my opinion,when learning BAK,the best way to learn it correctly,is,to focus only on what is writen in the BAK manual and not to ask too many,"Whys" as incorrect answers can cause confusion,making it harder to learn.

 

I would recomend a very good book,titled, "Mechanics Of Flight" by A.C.Kermode,this book will answer your questions,correctly.

 

Maintain your current attitude and you should make a good instructor.

 

All the best,

 

Frank.

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