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Sep 09, 2019, 12:58 PM
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putput's Avatar
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FSA Exh OD, gains? Impellers or is this too far LOL


So in hte never ending quest for more speed, I have com e to realize that stock planes kinda of stink lol. Dont get me wrong, they are all fun, but after reading a LOT about FSA, exhaust diameters etc... It is quickly coming to a realization that if I want more speed, motor swaps are not enough.

For example, the Eflite Viper. I know there are guys that claim 120mph speeds, but unless you are in a dive, that is hogwash. It is an honest 100-105mph plane bone stock, flat and level. No dive, just fly it. The FSA was measured based on 70mm fan shroud ID, and the hub measures 31mm od. This gives 100% FSA recommended od of 62.76 .. Well folks, the exhaust is 65mm. So, we are arguably giving up speed, and so big we are beyond making max thrust.

Now in mine, I put in a RC lander 2200kv motor (same as Dr Madthrust, and honestly think the DR unit is re-badged) and get a consistent 115mph flat and level now. BUT--- recommended is typically 80-90% FSA exhaust diameter for max speed/compromise of speed/thrust. That would put my recommended exhaust diameter at 56-59.5mm. Way smaller than currently obviously. (I also too measurement of my F-16(eflite) F-18 (fms) and my freewing dual 80mm A-10. All the Eflite and FMS units are well beyond 100% fsa, where the Freewing unit is at approx 92% FSA.. And P.S. it hauls the mail LOL.
So.... How much does an 80-90% fsa/exhaust diameter matter??? Can I gain 2mph?? 10mph?? Guess...

Part 2-- Fans.. More of a rant/complaint than anything. So apparently, which also makes sense as it does in the prop world also.. Less blades, more pitch is more potential speed. Less pitch, more blades, more thrust. Yet NONE of the blade manufaturers help in this area. I mean how hard would it be to give some useable technical data? FSA of a fan. Pitch. I have a couple of powerfun fans, FSA is totally different, with recommended exhaust of same above plane of 58-62.5mm... (I tried it in the F16 which has the same motor and fan combo, lost more speed, like a bunch... and it is closer to recommended exhaust diameter.

So--- I cannot get a thrust tube in any of these foamies without cutting the fuselage in two, so I am looking for ways to build a different diameter exhaust. Also looking at picking up a few more fans just for giggles, maybe a CS-10, maybe a 13 & 14 blade each in XRP for giggles (I know, blade count in theory should make slower but hey.. who knows ) Why are there no gih pitch, 8-10 blade fans anymore?? Would it not make sense that these would make for better speed?? ( and potentially less ams drawn at the same time from less load) Am I the only guy looking to maximize a combo on a budget???

Thoughts?
Last edited by putput; Sep 09, 2019 at 02:05 PM.
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Sep 09, 2019, 04:59 PM
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Kevin Cox's Avatar
I'm not the type that goes for speed but the fastest configuration of days of old were the 4-5 blade units. The modern fans are less tolerant to small intakes and exhaust and can drive the motor amps up and duration down. It would be a hard sell, these days, for a 2 minute flight time ARF for manufactures.

If you want the fastest air-frame it usually isn't a scale one but something that is optimized for speed. So the tweaking an exiting model's exhaust may get you a few more mph but at the expense of duration and an extra load on the components.

Good luck!
Sep 09, 2019, 08:46 PM
Classic jets rule
AIR SALLY's Avatar
Kevin has it right the old HET 6904 fan (4 blade) or wemo 5 blade are the fastest . Use a 4000 ish kv motor and a 4000 or 5000 4s batt. You could roll a mylar tail pipe to 2.200" for best thrust and speed.
Every body wants the sound of the high blade count fans. So mph goes down. Some stock ducting is not the best either, but better than it was 10 years ago.
Sep 10, 2019, 11:44 AM
Big gov never Works
St. Martin's Avatar
And the 9b Evo minifan will work in the same 80-100% FSA ducting, as the 5b pro. About 20% more load. And sounds so much better than the pro or HET.

Don't try to go under 100% FSA on the high blade count chinese fans, it will not run efficiently. The only way to get them to increase efflux or static, is to increase power(more weight). The WeMos/HET are adjustable, in exhaust/intake area, to match the frame.

Fuzz
Sep 11, 2019, 03:54 PM
Registered User
.

Nowadays, the majority of flyers go
With the nice sound fans. Almost all those
Screaming 5 blade fans are things of the past.
They just don't want to ruin the ears of
Their spectators, neighbors for good.
:-)


.
Sep 11, 2019, 05:27 PM
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putput's Avatar
Thread OP
So... Changing the exhaust really won't do anything given the set up. I've already put a2200 kv motor in which improved things a lot.
Guess my options are to either drop kv and try 8s, new ESC etc... Or, play the fan gamble and try something like a cs10, I've got a power fun unit I can try, but it lost speed in my f16 with the same current combo.
Don't want to invest in a250-300 dollar per system in this jet, I've got a habu that is getting that soon lol
Sep 11, 2019, 08:20 PM
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putput's Avatar
Thread OP
So why is it that if I go to say a9 blade wemo fan, changing the exhaust diameter may help pick up speed, but on a12 blade it doesn't??
Honestly trying to understand. I've been doing nothing but reading for days about dynamic thrust, how to get more with smaller exhaust, efflux etc...
Mind numbing lol.
Sep 11, 2019, 11:34 PM
Registered User
Quote:
Originally Posted by putput
So why is it that if I go to say a9 blade wemo fan, changing the exhaust diameter may help pick up speed, but on a12 blade it doesn't??
Honestly trying to understand. I've been doing nothing but reading for days about dynamic thrust, how to get more with smaller exhaust, efflux etc...
Mind numbing lol.
First off, I don’t claim to know. However, when I imagine what is happening, I substitute a wing for an EDF blade, then try to visualize air flow. All things being equal, more blades mean higher cross section, but not higher FSA. If performance is the subject, than higher blade count is inferior. The energy to cut through the air does not go up linearly with RPM. As a result, the higher the RPM, the greater the energy difference in rotation.

This substitution causes another consideration. I will call it “the biplane with upper and lower wings to close together concept”. If you can imagine the air flowing in the vicinity of the wings of a biplane. Now imagine a higher AOA. Do you see the fast moving air above the lower wing “hitting” the slow air under the upper wing? Even if the flow remains laminar, it is still counter productive. If it should detach and become turbulent, in other words stall, than worse yet. Those vortices need to get their energy from somewhere. All things being equal, low blade count fans are like having the wings of a biplane further apart.

About AOA. Lets say your jet goes 100 MPH and the efflux to get it there is 200 MPH. To get the jet to 150 MPH, it will take an efflux greater than 250 MPH. So the difference between outside and ducting air speed is greater at higher speed. That makes for a higher AOA at the blade. This makes the above paragraph relevant. It also explains why a high blade EDF will stall before a low count. That is why they need an exhaust area nearer to 100% FSA than a low count EDF.

I have neither studied nor experimented regarding this phenomenon. So, I hope I am not spinning you in the wrong direction But, I often imagine phenomena. When I imagine this one, it is how I do.

Still waiting for a counter balanced single blade EDF with supersonic blade tip speeds. So the fuel guys can’t hear if their engines are even on

Scott
Sep 12, 2019, 02:48 PM
Big gov never Works
St. Martin's Avatar
Thats a good observation, Scott. The blades on the WEMo Evo, are quite different than that of the chinese high count rotors, for sure. They are designed for higher efflux and low blade stall.

Quote:
So why is it that if I go to say a9 blade wemo fan, changing the exhaust diameter may help pick up speed, but on a12 blade it doesn't??
Honestly trying to understand. I've been doing nothing but reading for days about dynamic thrust, how to get more with smaller exhaust, efflux etc...
Mind numbing lol.
Because with the 12 blade fan, you cannot reduce exhaust area to increase flow rate, without stalling the fan more than it already does. Reducing the exhaust area is like holding your thumb over the end of a water hose. It increases flow rate. Because the Evo rotor is so much more efficient in high speed, it has a large margin of area adjustment to the frames requirements.

Thrust decreases with an increase in forward speed. The frame will try to reach the speed of the efflux flow speed(equilibrium). But even with a very clean air frame, this is impossible, as drag increases with speed. Dynamic thrust is actually a mis-term for effux speed. You can have a slower larger mass of flow(chinese fan) or a much faster smaller mass of flow(Evo). You have to find the happy medium for the size of the outlet, to match the frame drag and the way you want to fly.

The Phase-3 F-16's were a good example of outlet sizing. If I wanted to go fast, with lower static thrust, I ran a 54-56mm exhaust. If I felt like flying close in, with a lot of slow verticals, I switched to a 58mm outlet. Thats the flexability of a fan designed for ducting.

I see all these testings done with the 10-12-14 blade fans. All about static thrust and not in a ducted situation. Efficiency does not make a whooshing sound. It is quiet.

And Flylow is 100% correct. People buy for cost and sound, not efficiency.15-20 years ago, these fans could not have been used, with the round cells and motors we had. Their waste is offset, to some degree, by lighter LiPos and high torque motors. Also add in the air frames inefficiency to have to carry the big batt packs to supply these fans.

Fuzz
Sep 12, 2019, 03:46 PM
Registered User
putput's Avatar
Thread OP
Okay, making more sense now with the blade stalling..
So if I want to try to go faster, stick with same motor, I could try getting something like a9 blade wemo, which in itself will theoretically allow a little more rpm from less load. Now alone it will have less thrust with the same 110% fsa I currently have, but might have a touch more speed.
Now, with a fan that is not choked, I could start playing with exhaust outlet, which I'm guessing around 56-58mm would work. This to increase load again, but also give more efflux, potentially gaining speed..
Sounds close??
Sep 12, 2019, 07:00 PM
Registered User
.

The wemo fan 5 blade or 9 blade, with FSA
lesser or equal 56mm is for speed. 58mm to
60mm is for thrust. I always use my wemo with
58mm. I already placed all of my wemo 5 blade
Rotor with 9 blade. Because You can go fast with 5 blade rotor but The noise is really annoying.


.
Sep 12, 2019, 08:16 PM
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putput's Avatar
Thread OP
Well I grabbed an 8 blade from a friend's to try for giggles. It's alloy, perfect balance. The blades look very similar to the fms blades as far as pitch, depth etc.. going to toss it in and play a little, then see if building a neck down in the tail does any good.
Sep 13, 2019, 04:10 PM
Classic jets rule
AIR SALLY's Avatar
Most rotors work best with the manufacturer's stater blades, switching a diff rotor in someone else's shroud could cause efficiency losses.
Sep 13, 2019, 04:47 PM
Registered User
putput's Avatar
Thread OP
This is a complete unit, just put my motor in it and balanced it
Sep 13, 2019, 09:51 PM
Registered User
Quote:
Originally Posted by air sally
most rotors work best with the manufacturer's stater blades, switching a diff rotor in someone else's shroud could cause efficiency losses.


+1


.


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