Got a question about air molecules.

[xxChrisxx]

I don't really know at what point I should start with a worked example, I did a couple but they didn't explain much or were overly complicated.

Do you have any idea where you would like to start with this?

 

[sophiecentaur]

@jakksincorpse
I have a feeling that the idea of a 350hp motor seems to be implying that it is delivering that amount of power all the time. At low speeds, that power will be overcoming friction forces and provide a lot of acceleration too. Only at maximum speed is the 350hp just keeping you moving.

http://www.atmosphere.mpg.de/enid/Information_ss/Velocity___air_drag_507.html
If you look at the graph here of resistive forces vs speed then work out the power needed at a few of different speeds you can see that (in this example),
1. to go at a steady 40km/hr you need 1.6kW
2. to go at a steady 80 km/hr you need 7.8kW
3. to go at a steady 120km/hr you need 23kW

The power goes up steeply because you are multiplying an increasing speed by a steeply increasing force.

Sorry it's in SI units but they are the only way to go, really. (Think 760W/hp and 1km = 5/8miles)

If your engine could provide just 23kW (a tiddler, compared with your monster), it would have 21.4kW to spare for acceleration at 40 km/hr - that would give an available thrust of about 2000N. At 120km/hr, it's go no more thrust left for acceleration.
That's some basics. Can you see that in the context of your confusion about power and speed etc.?

 

[jakksincorpse]

@jakksincorpse
I have a feeling that the idea of a 350hp motor seems to be implying that it is delivering that amount of power all the time. At low speeds, that power will be overcoming friction forces and provide a lot of acceleration too. Only at maximum speed is the 350hp just keeping you moving.

http://www.atmosphere.mpg.de/enid/Information_ss/Velocity___air_drag_507.html
If you look at the graph here of resistive forces vs speed then work out the power needed at a few of different speeds you can see that (in this example),
1. to go at a steady 40km/hr you need 1.6kW
2. to go at a steady 80 km/hr you need 7.8kW
3. to go at a steady 120km/hr you need 23kW

The power goes up steeply because you are multiplying an increasing speed by a steeply increasing force.

Sorry it's in SI units but they are the only way to go, really. (Think 760W/hp and 1km = 5/8miles)

If your engine could provide just 23kW (a tiddler, compared with your monster), it would have 21.4kW to spare for acceleration at 40 km/hr - that would give an available thrust of about 2000N. At 120km/hr, it's go no more thrust left for acceleration.
That's some basics. Can you see that in the context of your confusion about power and speed etc.?

but its not 310 hp. its 1,550hp. and idc about this much anymore. i understand what you were saying now about how friction is better than airflow.

i have another topic to talk about if you two are interested. it'll be in a different thread.

 

[xxChrisxx]

but its not 310 hp. its 1,550hp. and idc about this much anymore. i understand what you were saying now about how friction is better than airflow.

i have another topic to talk about if you two are interested. it'll be in a different thread.

OK I am going to cap this for effect.

YOU DON'T HAVE 1550HP YOU HAVE 310 UNLESS YOU BURN MORE FUEL. YOU AREN'T USING IT AS A JET ENGINE SO WON'T GET JET POWER.

There is no debating this, it's a fact. You cannot use gears to increase power output.I really don't know why you think you do have 1550 hp. Are you aware that energy can't be created or destroyed (1st law of thermo).

You are burning 310HP worth of fuel in your V8. Thats 231 kW of power. Assuming an engine thermal efficiency of 0.3. This requires 691kW of fuel input.

Power = Energy/time. A watt is a joule of energy per second.

So the maximum amount of work your engine can possibly do is 231000 J. In this application you are using that 231000 J to compress air. Compressed air has no power output (it just takes work to compress).

So you are taking 691kJ of chemical energy and converting that to 231kJ of work every second. You are then using that work to compress air (which is probably about 90ish % efficiecnt). That comrpessed air them simply flows out the back. At no point have you added any energy.

All you are doing by gearing the blades is compressing the air faster but not as much. As it's compressed the air pushes back on the blades, and requires a torque to spin the compresser. If you geared it so it spun very quickly you would lose the torque necessary to drive the blades round causing compressor surge.

 

[jakksincorpse]

okay. forget the jet car. I have a better insight.

i was chillin in class today and i thought about vertical lift and propulsion, like how the military tried working on in the 50's but failed due to the uprising of rockets and turbofans

so i have a 3 prop, 6 fan configuration. that has NO gear ratios, just straight power. uses 3 suzuki hayabusa motors with very little modification. all together the 3 engines will produc 520hp.

In theory, this vehicle should weigh 2000lbs

limiting the rpms at 10,000 for longer lasting motor life, this isn't exactly perfect since powerbands fluctuate with torque but at 1000rpm the combined motors are making 52hp, i couldn't do much more calculations without a calculator so i put my desired vertical thrust at 11 feet per second (7.5mph) and i ended up with a 2,600lbf total. so when i start the motor and in reality use very little acceleration, i should be able to climb 11 feet per second correct? we'll use the step by step thing this time because last time we had wrong ideas.

 

[xxChrisxx]

I'm afraid I have no clue where to start with this. Cars are my thing, and I am ok with gas turbines. Propellors i'd need to read up on.

http://www.mh-aerotools.de/airfoils/propuls4.htm

Seems to deal with basics of prop power and thrust quite welll. I've only skimmed it, not read it thoroughly.

 

[jakksincorpse]

it seems as though it would work if i have the right propellor pitch size and rotation. man i want to make a schematic and show you what this thing will look like haha. I'm psyched about it.

that link you just gave me basically proves that i can fly with very little acceleration. that's all I've been looking for. why? because ICE's can idle for freakin ever. and 11 feet per second is pretty fast in my book. that's like a basketball hoop ever second...

using this thing daily, i'd honestly probably only fill up once every 3 weeks. haaha.

top speed seems like it would be around 80mph ascending upwards from my calcs. (including the 15 to 20% engine loss)