Some thoughts on efflux vs thrust. - RC Groups
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Aug 16, 2003, 06:41 AM
Registered User

Some thoughts on efflux vs thrust.

Sorry I know I always like to start these technical info threads, but I get odd ideas and like to throw them around.
Anyway I'm constantly reading/looking at EDF's, DF's, and turbines. I only fly EDF's but I like all R/C jets.

Anyway it seems to me that many turbine models don't have much better than a 1:2 thrust to weight ratio. Like a 12lbs turbine in a 20+ lb airframe. Yet they still ROG, and fly around extremely fast.
I've also seen some gas DF's that'll get 160+ mph and some 180mph. Now I know a turbine has like a 600mph efflux velocity so I won't go on about them.

Yet when looking at gas DF's most of the faster high performance one's would never even get off the ground without a thrust tube. The little ones too like the Kyosho F-86 GDF. A thrust tube on it make a 30mph difference.

I know everyone has told me that watts is what matters, but that makes no sense to me. I know watts is = to motor power but the fan and ducting it seems has much more to do with thrust and velocity.
Take a GWS Zero for example. On the stock geared power system it has lots of thrust but never gets much above stall speed. It hits a point where the motor setup simply won't pull it any faster. Yet if you prop it down with a different gearing it doubles it's speed, and will still perform all the same manuevers.

Ok I'll get to the point before I confuse myself lol.
Anyway so what happens if we reduce the size of an electrics thrust tube? First I know amp draw increases due to higher pressure inside of the duct. However according to all the papers on DF's I've read thrust in a DF is = to both the pressure on the disk, and the pressure on the duct wall.
I know static thrust wise it drops, but thats sitting on a workbench. If we're losing static thrust then it must be going somewhere which my guess would be efflux.

Yet whats better in flight? 6lbs of static thrust, and 100mph efflux or 5lbs of static thrust, and 140mph efflux? Those aren't exact numbers I know.
I also know that ducting is a huge source of drag in our models. Which is also why I think a intake that hurts static thrust in-flight will result in less drag. Like 90% fsa on the intake.
Few gas DF's especially the faster ones will ROG without 400 feet of runway. Most need a good bit of runway on pavement I think.
Given though most of our wasted run time trying to ROG is in the first 15 seconds when the aircraft is going from a stop. Once it gets rolling it accelerates much faster. So there's little time difference between a 200 feet rollout and a 250feet rollout.

Or to jump further through the chase it seems our EDF's should be capable of 160+mph in flight. I mean a 90mm gas DF only puts out 3lbs of static thrust. Without a thrust tube most will fly just fast enough to tip stall on the first turn after running way down the strip.

Even a big gas DF unit is only putting out 10-11lbs of thrust. We can do that with 2 midi fans and actually get more like closer to 14lbs. What about speed though? How does a 10-11lbs thrust model get 160+mph, and we get 120mph out of a dive?

Most of the smaller stuff we bungy launch anyway. So I was wondering if anyone has experimented with reduced thrust cones or maybe they have and they've just already found the sweet spot. I dunno just seems we should be getting more for the power some of these EDF's have.
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Aug 16, 2003, 03:01 PM
EDF rules... :)
AirX's Avatar
The fan intake can be taken lower to about 90% FSA and still perform, someone tested and confirmed this but handlaunching would suffer because at low airpeeds the intakes are not in a surrounding airflow that allows completely efficient flow into them but at speed the intakes performed well with less parasitic drag of air spilling out arround the intake lip. As far as the tail cone adding to efflux speed this is a resounding yes because fluid flowing through a cylinder that is constant flows in a constant velocity, fluid that flows from a large diameter swaging to a smaller diameter experiences a positive change in velocity that is directly proportional to the percentage change in area and negative when going larger on outlet size. So one can draw a conclusion from this that by going smaller on the outlet size the efllux velocity will rise, when going larger the efflux velocity will drop. The downside to this is the modifying number that represents the fans ability to compress the air it is pumping(fan efficiency) comes into play, the fan itself must be able to push the air out the constricted exhaust tube otherwise there is a point where the fan will not have better perormance by going smaller and amp draw will show that. Load will increase on the motor till that point is reached and load will probably stay the same(theorising and have not had a chance to test).

Eric B.
Aug 16, 2003, 03:52 PM
Registered User
Yeah I'm going to have to do some experimentation.
Aug 16, 2003, 04:24 PM
shut up & fly
hoser's Avatar
I have pondered those same exact questions also.

first I think we can match .45 IC DF performance allready.

.91 performance (5 hp) is here but a t a price $$$$ cause as its been beaten in my head watts and horse power are the same thing.

to achive 5 HP you will need 3,730 watts !!!!. the last time I checked planecrazy was the only person that achieved this.

you are right on speed and that is based on the blade design, many IC DF's take off like dogs and then when on step just go. cause the fan is designed for that.

in electrics the power drops though the flight as oposed to a gas engine which remains constant throughthe flight and as fuel runs out a IC plane becomes lighter so EDF impellers have to be a little more versitile in my opinion.

experiment with the exhust tube but don't bother going smaller than 80%

for these small edf's (2.5"-3.5") 100mph is more than fine otherwise you loose them quick.
Aug 16, 2003, 07:22 PM
Carbon fiber is our friend
Steve C's Avatar
There are some of us who have been flying EDF in the 160+mph area for a while. It does take a bit of power, but it's not tough to do.

The ducting is pretty important. Get that wrong and it won't make much difference how many watts you throw at it.

Steve C
Aug 16, 2003, 07:41 PM
shut up & fly
hoser's Avatar
that's true Steve.....but most of those came of a rubber band

I heard a verctor broke 200ph too from a very high dive, I also used to brake 100 with my f-18 on a dive too.
Aug 16, 2003, 09:07 PM
Registered User
Yeah I was only talking about matching performance speed wise, and not trying to match the power of a os .91
Diving in my book doesn't really count though as my GWS ME-109 gets pretty fast in a dive lol. Lol

Really just talking about flat out speed. Also means more thrust at higher speeds than if it's setup for maximum static thrust.
True you wouldn't want most mini fan jets doing over 100mph. They are pretty small.
Aug 17, 2003, 12:27 AM
Registered User

Mass flow

I don't think that efflux is as meaningful as you're thinking. For example, a given efflux speed through a small tailcone will be less effective than a lower efflux through a larger tailcone.

"Mass flow" might be a more meaningful factor to consider, if you can get it. Shrinking the tailcone increases efflux speed, but mass flow is constant at best, and more likely to decrease due to the extra work the motor has to do to accelerate the air to get it all through the smaller exhaust.

It would be really interesting to have a graph of thrust vs. speed, rather than just two numbers for static thrust and efflux speed. But still, there's never enough hard specs, and always too many variables.
Aug 17, 2003, 09:39 AM
EDF rules... :)
AirX's Avatar
HI Nate,

I agree the mass flow(cfm) is constant otherwise the medium would not increase in acceleration through the smaller opening.
The smaller exhaust opening does not produce as much reactive force because of the smaller area acted upon by the airstream but it is effective in leting the aircraft accelerate to the airstreams speed or until the drag of the airframe equals thrust being produced.

Eric B.
Aug 17, 2003, 11:08 AM
Registered User
I had read that reducing the exhaust diameter was like increasing the fan pitch as long as the intake stayed the same. Due to the angle of the air entering the fan.
Like if you have 100% FSA intake, and 100%FSA extake then lets say the fan has a pitch of 10. Reduce the exhaust FSA to 90% and you've effectively increased the pitch (I dunno the exact numbers) lets say to 12. Thus the fan is now stalled at lower speeds while the pitch speed is increased.
Yet if you reduce the intake diameter also to 90% the fan pitch isn't supposed to change even if the extake is 90%.

Which does and doesn't make sense. There just seems to be a lot of info on fans which conflicts.

On a note though it seems most gas DF's are designed around a 90% intake, and a 85% extake. Somewhere in that range. According the above mentioned rule thats the same as having 100% intake, and a 95% extake.
Which makes sense as a fan is not a rocket so it's sucking itself through the air just as much as it's pushing against the air.

The difference though that from what I've read the drag created by the ducting is higher both the aircrafts frontal area drag, surface drag, and parasitic drag. Because as the aircraft accelerates to high speeds the mass of the aircraft is making speed want to remain almost constant while the fan is unloading. So the reason the fan is unloading is due to pressure in front of the fan increasing.
So by increasing the intake diameter it means the fan will unload at higher speeds. When the pressure in front of the fan the fan unloads, and is no longer able to move air fast enough to generate any thrust.

Although I think all of that has been figured out already by other modellers. But it does still make sense that if you could vary the extake diameter in flight then we would be able to increase the in flight thrust at higher speeds which would give us higher speeds without hurting our static thrust.
Still if you look at the thrust to weight ratio of many real jets, we have exceeded their ratio's in the models.
But we seem to look a lot at thrust to weight ratio's. When whats important is thrust vs drag. Wingloading simply gives us the flight speed range. We shoot for light wingloading so landings aren't too hot. Thrust to weight only determines acceleration.
But we want lots of that. That's what makes them fun to fly lol.
Like the Kyosho T-33 stock flies pretty scale although maybe a little slow for scale.
I'll bet though the T-33 could be made to go pretty fast without having to increase the power. But to do that would render it non handlaucnhable.

Anyway I'll stop I just woke up, and sometimes I tend to prattle on. I just find the science of it all pretty fun, and interesting to discuss.
Sep 08, 2003, 05:25 PM
Registered User

Optimum Exhaust Area

There is an optimum exit area. What happens is, as you make the exit smaller, the exhaust velocity increases (and thrust also), but at some point the fan can't push enough air through (and thrust falls off). Since mass flow multiplied by the difference between free stream air velocity (zero in the static thrust case) and exhaust velocity equals thrust, it is a matter of finding the optimum area.

I have been experimenting with the Wattage fan, and have found that a 1.98 inch diameter exit has a little less static thrust than a 2.07 inch opening. Thrust fell off with a 2.12 inch exit diameter.

In the F-86 days, they trimmed a jet engine by riveting a small piece of 1" X 1" angle in the exhaust at the exit (these were called mice). They were ugly, but they cut the exit area down by a small increment. This caused the exhaust velocity (and thrust) to increase, but also the exhaust temperature to rise, so when the temp was on the limit, no more mice. In Korea, pilots got the maintenance guys to give it more than the limit, called "ratting" up the tail pipe (big mice). We don't have EGT to tell us when we have cut the exit down too far on an EDF model.

I tried "ratting" up the 2.12 size opening, and was able to get it equal to the static thrust of the 2.07 inch opening.
Sep 08, 2003, 09:07 PM
EDF rules... :)
AirX's Avatar
Velocity multiplied by the difference of change in diameter is correct assuming that the exit area is the same as the fans swept area. when the area is left to the same area as the ID of the fan shroud the exit area is greater than the fan swept area and the velocity slows down. It has nothing to do with the pitch of the fan, it is simple physics called the conservation of mass. In other words the air entering the fan must equal the air leaving the fan. Because the fan swept area is smaller than the area the shroud exhibits the air will slow down as it leaves the fan entering a constant shaped tube becasue the FSA is smaller than the Tube area. When the exit diameter is swaged down to smaller than FSA the air velocity will be accelerated becasue the air entering the fan must equal the air leaving the fan(conservation of mass) until the fan cannot compress the air through the smaller opening. The trick is finding the correct ammount of area(sweet spot). Now the force produced by the air is the same in theory as the force that the fan produces at that flow rate even though it is applied to a smaller area.

Eric B.
Sep 09, 2003, 09:44 AM
Registered User
portablevcb's Avatar
The analysis is fairly accurate on thrust vs exhaust dia. Besides supersonic considerations, subsonic flight also requires the variable exhaust nozzles found on modern jets for optimum thrust in all flight regimes. When you get up to speed a higher nozzle velocity works best, when taking off a lower velocity is best (given constant fan parameters, as we have).

If you design for flat high speed flight, then you might lose a lot for static, ie, ROG, conditions. THe pressure entering the fan will change with the increas in flight speed. At the low speeds our models fly this might not be as noticable, but, should at least be considered.

I have always wanted to try out a variable nozzle, but, have not come up with a way to do it that doesn't weigh very much (extra servo, plastic, etc) especially when the extra weight is in the tail (bad for CG). Any ideas?

Short answer is, yes, thrust tubes work, but, you pay for it somewhere.
Sep 09, 2003, 11:46 AM
Registered User
You are right that the optimum exit area is not the same for all conditions. We are going to put the Wattage test article in a wind tunnel later this year to see what the optimum is at high speed (maybe 50 mph, this is a homemade tunnel). Also intend to work the inlet side of the problem. A preliminary static test with and without the cheater hole, showed about 9-10 oz of thrust with the cheater, 6-7 oz without. All the early jets (F-80, F9F, even the later F4) had cheater holes, either suck in doors or actuated doors, which opened for the takeoff situation. There may be more to be gained there, than at the exit.

One way to do the variable exit would be with a plug nozzle like the Me262, which translates in and out of the exit.
Sep 09, 2003, 01:41 PM
EDF rules... :)
AirX's Avatar
Cheater holes in our fan situation is just added drag in the inlet, The differences in turbines is you are trying to give the turbine enough air so that it can burn the fuel efficiently ergo small inlets for high speed flight and blow in doors for takeoff power when airflow is at its slowest. Adding a cheater to these model systems is just designing for low end performance when designing a correct intake system is the best solution ie we arent burning fuel to move the aircraft but moving air and it tends to flow better when there is the least amount of turbulence. We are talking about cats and dogs when we address this subject, just look at BVM and his jets intakes and for that matter flow thru the fan and exhaust, it is maximized for the least turbulent flow with no cheaters.

Eric B.