Ducter Fans and motors + Watts & thrust

[Texas Buzzard]

I was told to put this question here. I was told it was too theoretical for the Ducted Fan Guys...............I disagree. But here it is.
Texas Buzzard




Join Date: May 2004
Location: McAllen,Texas
Posts: 234 Discussion - Thinking Out Loud DF

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I am looking at the outlet area of my DF unit. It seems that the outlet area is about 90% ( measured) the area of the inlet. From what I have read that is about right - right?

Now, my Jet Screamer motor is rated at 45,00 kv. My fan ( the X-fan ) has 3-blades. I can't measure the thrust but it is significant ( in the 12 to 16 oz range.)

Some Thoughts:

Momentum is mass X velocity or m of the moving fluid x the velocity of the fluid or mass of the air pumped by fan. Is that true? DF pump air giving the mas of air a velocity. The air aquires a momentum - right. So if two different fans are giving a body of air ( inlets are the same) the same momentum then the motor must be using the same number of Watts - right? So ARE WATTS AN GOOD INDICATOR OF THRUST?

TWO FANS but ONE MOTOR , get the same thrust?

I wonder if I get the same thrust with two different fan all other things being equal, i.e., the motor is liminted to 200 Watts input v x amps. The Watt IS A UNIT OF WORK, by the way. 1 joule ( of work)/ sec = !.0 Watt !
In the engineering system ft lb/sec = work , joules are the same or nt m/sec -----------------power = work done /time.

The fan is doing WORK on the AIR since the air is given a velocity. Sure there are loses in changing the Watts IN to motor into momentum if the air being pumped. But for all practical purposes ( like most modelers accept) can the power (Watts) the motor uses be somehow equavalent to the thrust of a DF system?

For instance I am going to test two different fan units but use the same motor on the two fans. I will call the two Fans by a name. The 3-bladed 55mm fan is called Fan "A".
The 5-bladed fan of the same dia. inlet (55mm) is called Fan "B".

IF I find that fan "A" turns it's maximum rpms when my motor ( a 45,000 kv motor) is showing it is using 200 Watts
THEN
If I put the same motor in my fan "B" ( also 55mm dia inlet, and 5 blades) and the motor is shown to be using 200 Watts of power, the same as in Fan "A" - - - - - - CAN I EXPECT THE SAME THRUST FROM BOTH FAN UNITS?

In short, using one motor and then changing from a 3-bladed fan to a 5 bladed fan ( but keeping the Watts the same on both instances) will they, the two fans both produce the same thrust? Somehow I think the 5-bladed fan will draw more current and more Watts at maximum rpm., and my motor will turn fewer rpms with the 5-bladed fan but put out the same thrust.[/B] Comment please.

[BMatthews]

No, it's more than just watts. There's a whole heap of related factors that determine how effectively the fan can operate. Inlet ducting or lips can generate a lot of turbulence and if the outlet is not designed right it can create excessive back pressure which stalls the fan and it'll run without creating good thrust. Turbulence at the face of the intake can also result in the fan not operating cleanly so you again loose efficiency.

Back when glow was king I read a couple of magazine articles on ducted fan design. The author did some testing using various intake lips and it was amazing how much it affected the fan's efficiency. I've seen folks running fan units just tacked onto models with no intake lip. If they knew they were robbing themselves of a goo 1/5 to 1/4 of the units potential they would do a little more work. Similarly there's a significant gain to be had by using a tapered end pipe but it's mostly for higher speeds once flying. And the length of the taper is critical as well. It needs to be quite a gradual taper and the taper section relatively long.

All of which is to say that just because the two units are giving the same thrust it does not mean they are using the same amount of watts. One may be operating more cleanly than the other.

You also get into the exhuast velocity. One may be moving a greater mass at a lower velocity so it'll accelerate a model faster but have a lower flying speed.

Now if you were using the same housing and have an effective intake venturi lip and you play with the tail cone to try to tune it to work the best with each blade unit then I think you have a good basis for a fan to fan comparison because you've locked or controlled the other factors by making them adjustable to suit the fan or fixed and consistent.

[JetPlaneFlyer]

As noted above there are a lot of variables... Fundamentally you are comparing apples with oranges. The 'apples' is Watts, which is a measure of POWER... The 'oranges' is oz thrust, which is a measure of FORCE. Power and force are different things so it's impossible to ever say that one is equal to another or that just because two systems use the same power in Watts they will produce the same thrust in oz.

Having said this if you take two identical systems and you put a high watt motor in one and a low watt motor in the other then you would expect the high Watt system to provide more power, but exactly how much more is difficult to predict. Thrust wont increase linearly with increased input watts because the more power you put through a given fan the less efficient it will become.

[portablevcb]

From what I can tell you are simply trying to test two different sets of blades in an indentical housing and motor.

One has 5 blades, the other has 3. Are they the same pitch? Basically if the motor is drawing the same current you should be getting close to the same 'power' output, but, as in props, that isn't the whole story. If one has different pitch then it will have a different thrust. But, a lower static thrust might actually fly better when at speed.

Also, there is the issue of different efficiencies with the two. One may be 'wasting' some of that power instead of converting it to thrust.

A good use for comparing input powers of setups is when working with duct sizing. You can set one up and compare outputs.

But, why not measure thrust? It is very simple to make a thrust measuring setup with a digital scale, a couple of sticks and a pivot. Beware, thrust on the ground is not the same at speed, especially if your speeds are higher.

charlie

[Texas Buzzard]

Yessir, each of the three posters put quite a lot of knowledge into their posts.

THANK YOU ALL.

BMatthews, JetPlane flier and Portablevcball touched in variable that are known to valid.

I may not have been as clear as I could have been in ASKING THE QUESTION. I reread my post and I put the question at the very end of the post.

Here it is again, I copied it - "In short, using one motor and then changing from a 3-bladed fan to a 5 bladed fan ( but keeping the Watts the same on both instances) will they, the two fans both produce the same thrust?

You see I have the 3-bladed fan. I wanted to know if it is cost productive to buy a new 5-bladed fan ( the impeller) This fan unit as sold gives one the option of either of the two fans.....but you have to shell out $$s for the new 5-bladed fan.

The stator ( non-moving) and the tailpipe + the ring on the front are going to be the original ones.

Thanks all of you, I am new to DF as you can tell so a little help goes a long way.

[portablevcb]

Short answer is no.

They may have different pitch. Efficiencies will be different. Still easier to measure the thrust and compare the two.

In most cases a 5 bladed ducted fan is more efficient, IF things like blade shape are optimized along with some other variables.

For ducted fan work I would tend toward more blades and higher rpm.

charlie