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RC ramjet

lobstermash

Propaganda machine
Mentor
#1
I've been thinking about whether there's any merit to sticking EDF units in ramjet configuration... One of the issues with a lot of EDFs is that the motor is fast, but sacrifices significant torque to achieve its thrust, making the motor run at low efficiency and consequently produces excessive heat. If two EDF units are mounted end to end, would they be more efficient?

In my mind, the output from the front one would act like a supercharger, giving the rear EDF higher efficiency at low speed. As long as there's enough airflow around/through the front EDF, it shouldn't affect the rear EDF's airflow.

What do you think? Am I nuts?
 

jetpackninja

More combat please...
Mentor
#3
For it to act like a supercharger, The front EDF would have to be bigger and/or faster than the one in the back.
Would it increase the efficiency of the fan in the back? Probably, but most likely less than the result you would have got from putting the bigger/faster fan in the back in the first place.

Two fans of the same rating would most likely lower your performance.
More weight
Higher power consumption
The air now available for intake by your rear fan will also be more turbulent, the fan blades will not move the air as efficiently.

Ya never know. On any given day there's a good chance that I'm full of crap...
 

teflyer

Full Circle
#4
I think it has some merit. However, I am not saying it would work.

Contra-rotating props generally produce more thrust than a single prop. If I remember correctly about perhaps 10% more thrust. However, the reason why co-axial helicopters are usually more efficient than a single rotor is because there is not much weight added with the coaxial compared to the single rotor. However, with EDFs, there is quite a bit of bulk being added for a smaller increase in thrust, lowering the efficiency of the setup.

so, if EDFs were somehow massless, then yeah, it probably will be more efficient. But in a real world situation, probably not.
 

jetpackninja

More combat please...
Mentor
#5
Right- I think you have reinforced my thinking.
If- you did get a 10% gain, you have doubled the weight of your poser system and likely doubled your power consumption.
Smasher- Just build one and let us know!
 

teflyer

Full Circle
#6
yes I did!:)
Just clarifying the idea that a contrarotating propeller does increase thrust, but not necessarily efficiency.
 

lobstermash

Propaganda machine
Mentor
#7
Might do... Got a very powerful 55mm and an average 50mm unit to play with. I'll make up some thrust tubes to incorporate the two and see what the results are...
 

teflyer

Full Circle
#8
post design sketches and pics if possible please!
What would be cool is just having an experimental setup and sort of a wind tunnel setup with the large intake and then the two fans and then some flow straighteners and then the tunnel and a place to put an anemometer and then the diffuser. But that's just me dreaming.
 

lobstermash

Propaganda machine
Mentor
#9
Yeah, it's not going to be a very complex setup. I don't have any fancy equipment, so I'm going to get thrust measurements with my kitchen scales and use power consumption from the battery charger to calculate Watts etc. I've got a busy few days though, so I won't be able to do this for a couple of weeks. I'll update when it happens.
 

colorex

Rotor Riot!
Mentor
#10
Keep in mind that a ramjet is different from an EDF - the ramjet uses fuel which expands (as the space is limited it increases presure instead of volume), whereas an EDF just compresses air using a smaller exit for the air. An EDF is just a prop in a tube.
 

lobstermash

Propaganda machine
Mentor
#11
Yeah, I know. Ramjet isn't really the right word for it, but it's the closest thing I could think of.

EDFs may be a prop in a tube, but they can't run at the same efficiency as a larger prop because they have to spin so fast. As the plane speeds up, the efficiency increases as air is forced into the intakes and the motor spins more easily.
 

colorex

Rotor Riot!
Mentor
#12
So in this case, you'd just be forwarding the zero airspeed inefficiency from one motor to another. What I think might work, is a big slow EDF, funneled to a fast small one. But again, with air you can't put out more than you take in.
 

teflyer

Full Circle
#13
so far the best word to describe the setup that comes to my mind is an air compressor in a turbine or jet engine.
I think the results should be quite interesting. You will need to connect two EDFs so no air can escape perpendicular to the flow.

Can someone explain the reasoning behind why choosing to set it up as a large EDF and then a small one?
From what I am seeing, the only advantage in this setup is using the idea of compression of air from a large EDF to a smaller one to have high velocity air escaping

EDIT: Never mind, figured it out
If anyone wants to read my reasoning, not very good because I assume a lot.

The thrust, in an ideal situation, is dependent on the velocity x cross sectional area x velocity x the density of air
so an increase in velocity by a factor of some amount will result in an increase in thrust by that amount ideally but in reality it is probably less.
 
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earthsciteach

Moderator
Moderator
#14
For this to be truly effective, you'd have to inject fuel into the fan train and ignite. POOF! Hard to do with foamys...;)Oh, and the two turbines would HAVE to be on a common shaft to do that.

While I do think there may be merit in placing a larger, slower fan inline with a smaller, faster one in terms of efficiency of some sort, you have to look at it from a mass flow basis. The mass flow through the larger fan will be equal to the smaller fan. The only way to increase speed is to increase thrust. In order to increase thrust, you have to increase mass flow. The only way to increase mass flow is to turn the motors faster. Sure there may be some increase in mass flow, but probably not worth the effort. Certainly nowhere near close to double.