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Member postings for Dizz

Here is a list of all the postings Dizz has made in our forums. Click on a thread name to jump to the thread.

Thread: EDF thrust tube design
01/08/2012 12:20:52

Hi Grasshopper,

i do build and fly EDF and given your criteria personally I would trade off some speed for static thrust (makes hand launches easier) and go for a 90-95% FSA exhaust area. That said, you ask 10 people an EDF related question and you will get 10 different answers!wink 2

Good luck with your project

Pete

Thread: F9F Foamy Set-Up
27/07/2012 15:42:00

Nice one, always a big confidence boost when the first flight goes well.

Pete

Thread: E-Flite Mig 15 UMX with AS3X
18/01/2012 23:10:04
Posted by David Ashby - RCME Admin on 17/01/2012 08:48:27:
Looks interesting, a micro EDF eh?
 
Do you think they really mean 11750Kv?
 
 
 

Edited By David Ashby - RCME Admin on 17/01/2012 08:51:16

Having heard it in the video I also don't think that 11750kV unreasonable, particularly given the diameter of the fan is only 28mm.
 
Very rough calculation:
11750kV x 7.4v x 80% = approx 69,000 rpm
 
Pete
Thread: Spare fan for Red Velocity/Lander 64mm metal fan
14/01/2012 00:15:11
Good to know you can actually get the spares.........good luck getting the old one off!
I gave up on balancing the Sky rotor and bought a wemo min pro from Puffin.
Pete
27/12/2011 17:53:11
OK, will keep an eye open for a 64mm Lander one in that case.
Actually the Stumax test went very well, it is simply the smoothest fan ever and the sound is awesome! Everything held up under full power this time without collapsing, blowing up or catching fire, so I can crack on with the Viggen again now.
That Sky fan on the other hand........................ It is still not running well so if it doesn't come in after the next balancing session I'll take all the tape off and start again. I have to persevere though because it will will look good down the intake of the P1154 I'm building next.
Happy New Year
 
Pete
26/12/2011 10:17:38
It is rather - I have one in my Typhoon and 4 going in a Vulcan (when ever I get around to finishing the design), so I'll probably be in the same boat at some time.
The more I think about it, the more I would be inclined to use a Wemo mini fan rotor. With a jig it would be a pretty esay job to take 2mm off each blade and the re-balance. Advantage over those other 64mm fans is that the material is pucker and the blade count/pitch/area seem closer to the original Lander item.
I'm having an EDF morning today, currently on a break from balancing a 12 blade 70mm Sky fan then I have a Stumax 110-52 to run up on 12S .
Pete

Edited By Dizz on 26/12/2011 10:19:52

23/12/2011 20:54:18
Hi Stuey
I checked the vendors I use and afraid no joy - in the UK or abroad.
Found a potential option at Hobby King:
Or what about getting a 69mm fan and cutting it down?
Merry Christmas
Pete
 

Edited By Dizz on 23/12/2011 20:57:53

Thread: Ducted fan theory and practice
04/11/2011 10:56:46
My FD2 fuz based closely on the P1091 fuz which is basically a West Wings hunter. I have been lazy and used the triangular Hunter intakes (because they work and I had the vac-formings) rather than go through the process of making the FD2 rounded intakes. Not sure what you mean by "additional factor", but the intake area is just about 105% FSA.
 
I have quite a lot of thrust/power/velocity data from different EDF tests done over the last few years and will collate into a single spreadsheet this w/e, but I don't think this forum will allow me to post the file to share.
 
wrt the collapsing duct: I have used 2 different fixes in the past.   On the P1121 I simply laminated some glass cloth to the outside of the plastic vac-forming (keyed plastic first with coarse wire wool).  The second technique on my Viggen was to use Hysol areopoxy over carbon fibre tows wrapped around the GRP duct and additional balsa supports.  Looks a bit messy (it was! ), but does the job.
 
 

Pete 

 

Edited By Dizz on 04/11/2011 11:06:11

30/10/2011 19:17:06
And so with that we are back to the point I tried to make on 19 Oct: without a wind tunnel and some decent insturmentation we are not going to be able to improve on what we can already achieve using basic testing and well know rules of thumb.
 
Regardless, I hope you can come up with something solid relevant to the higher power levels available now.
 
Afraid the weather put paid to any hope of flying the P1091 and getting a S&L speed today. However I'm moving along with a FD2 I'm building and should be able to provide a range of figures from that over the next couple of months.

23/10/2011 21:15:26
WM400 with ARC 28-58-1, CC120HV ICE and Zippy 3000 40C 6S.
FSA= 30.4cm^2 Exhaust area = 23.7cm^2 (78% FSA)
 
30 seconds into run: 1709W, 53,728rpm, Ve=189.5mph 84.7m/s (from How Fast system).
 
Then I stopped the fan, reset the How Fast and ran it up to full power again, but this time I held the pitot tube against the side of the shroud with the opening about 3mm ahead of the edge (I didn't use a rounded intake lip).
 
vi=109.4mph 48.9 m/s......................not exactly "very low"
 

22/10/2011 20:50:18
But Richard it is you who makes the assumption vi=0 (in your 18/10/2011 18:57 post calculations). I agree with the other guys and think the static intake velocity is actually quite high. I am going to bed in a WM400 fan on the test stand tomorrow so will measure the intake velocity. It will not provide an accurate figure though because the static probe will not be facing into the flow, however it will give an indication..
 
Not sure about your mph to m/s conversions. I make 135mph=60.35m/s and that will obviously affect your calculated results. I'll still hold back on the P1091 Doppler speed I have and see what is predicted.
 
I could get the EF-16 intkae area from the replacement that I'm slowly progressing, but I don't have the exhaust area or efflux velocity so there is no point.
Pete
 
PS
Planning on an EDF session next Sunday, so (weather permitting) I should be able to get decent airspeeds for my  P1091, M52 and Typhoon - may be a few others too - to feed the verification process.

Edited By Dizz on 22/10/2011 20:56:08

21/10/2011 23:51:48
Posted by Richard Sharman on 21/10/2011 21:51:28:
Posted by Keith Simmons on 21/10/2011 21:26:20:
I think the moving EDF has less work to suck the air in the intake so the fan was able to spin a bit faster and so a more thrust as a result.

I have heard this "explanation" before, but admit to being a bit skeptical about it. Some people even say they can hear the motor note increasing (because it is running faster) in the air. But when challenged to tell the difference between that and the Doppler effect of an approaching model they usually can't. I can't hear anything that could be interpreted this way. And I've never seen the phenomenon of "on the step" which I suspect is a myth. Certainly my planes speed up as they get airborne and climb to cruising height, but so does my biplane, and my pattern ships. What we need is some observations of rpm, current draw and velocity in flight, which I want to do some day.

Can't say I have heard any change in rpm - the Doppler analysis doesn't show it either.
Stu Maxwell of Stumax EDFs fame is a member of the "the higher pressure in the duct ahead of the fan improves efficiency" school..........having followed his recommend to keep the intake duct sealed and seen positive results (in my opinion) I am too.
Would an increase in pressure not mean that the fan is shifting a greater mass for the same rpm?  It is a fact that EDFs do not work so well at altitude (just read the US forum threats).
Am asking Santa for a DX8 for Christmas - the telemetry options should be useful for in-flight EDF.

Edited By Dizz on 21/10/2011 23:55:39

21/10/2011 23:01:26
Posted by Richard Sharman on 21/10/2011 21:14:12:
Can you say what the entry area, fan swept area, and exit area values were? We could then use the theory to compare the static and dynamic cases.
 

P1091
Wemo mini fan HET 2W-20 on 2650mAh 4S 40C pack.
FSA=29.4cm^2 Exhaust =25.5cm^2 (87% FSA) Total Intake area 29.8cm^2
 
30s WOT figures:
 
Installed Static thrust = 1064g (10.43N); 800W on the Wattmeter; efflux velocity measured with the How Fast = 135mph
Static thrust on the test stand=1140g ( 11.2N); 765W; efflux 146mph (no ducting)
 
I do have a Doppler speed for the P1091, but only one slightly into wind pass so I can't really declare an airspeed.
Haven't got any fan figures for the dead EF-16, but I have several Doppler 'grams - max speed straight and level I have recorded is 122.8mph +/-1.8mph
 


Edited By Dizz on 21/10/2011 23:29:34

21/10/2011 13:58:50
"For example, no one has commented yet on the relationship between static thrust and dynamic thrust -- what is it? Because it's not obvious."
 
2 of my models have definitely generated more thurust once moving than when stationary (Modified Phase3 EF-16 and P1091): static thrust measured nose down on the scales indicated a T:W approx 0.9-0.95, but once in the air and "on the step" they will maintain a 90 degree climb without slowing. Both are(were) fast (120mph+ stright and level), have(had) bends in the intakes and sealed ducts (no openings for ESC cooling): I have a feeling that there is a gain from a dynamic pressure increase in the duct ahead of the fan - but lots of facors/variables involved and I have no way of finding out what is actually happening in flight..............especially the EF-16 because it exceeded Vne and is no-more
19/10/2011 22:16:32

Err..., well.....,no, actually! Because when designing a plane we need to work out what the exact geometry of duct is to build it, and when flying a plane we need to select the right battery for the desired duration/performance. So how would you do that? trial and error ? or theory and practice ? As our friends across the water would say, it's a no-brainer !

Third way...............experience ("rule of thumb").
 
Fourth way................use/adapt a proven design (use the experience gained by another).
 

19/10/2011 22:07:33
Richard
I appreciate you are looking at it as a system, but where are the system boundaries? I was taking the plane of the intake and exhaust faces. If you extend the boundary "a long way" from there both velocities will tend towards zero. However you were questioning where the missing Watts in your example calculation were going - I'm saying that without considering what is happening inside the system you wont be able to improve the Ploss figure.
 
wrt airframe characteristics: consider 2 aircraft with the same ducted fan units/motors/intakes/exhaust such that static thrust is the same - one a very large and draggy (but lightly loaded aircraft) that can only achieve an airspeed of x m/s where thrust=drag, then a second highly streamlined dart where thrust = drag at 5x m/s.
How do the equations handle that?
 
Yep, reckon my money is pretty safe. If the static vent for the unit is inside a enclosed compartment with no openings it will relate to the pressure inside the enclosure when it was closed, not the current static atmospheric. The air inside the fuselage will tend towards the air pressure at the openings to the fuselage eg the gap around the canopy hatch. Where the local airflow accelerates (like over that lovely aerofoil shaped cockpit) it will be less than static, where it slows down it will be greater than static. Of course everything may balance out for a certain set of conditions, but that will change the next day.
What is the % error between the "almost exactly" in agreement airspeed and averaged GPS ground speed? I have the How Fast pitot static system from BRC and that is claimed to have an accuracy (instrument error) of 2%, but as I said, small errors are magnified when a square appears in the equation.
Regards
Pete
19/10/2011 21:20:06
Posted by Erfolg on 19/10/2011 11:39:30:
I was a little surprised "fly Navy", with respect to air pressure and its effects. I did not think anyone suggested that the pressure was constant. I would expect in accelerating of the air through the duct would cause a reduction in pressure as predicted by Bernoulli. Any expansion and contraction of the duct will vary the pressure, relative to atmospheric acting on the duct walls.
 
As for the comments with respect to the fan, I do not know, yet suspect the fan is important. What is important I do not know and would be interested in a better understanding at a theoretical level, so as to be able to relate to what can be engineered at a practical level.
 
I was using the example of the collapsing duct to illustrate that there is obviously a difference in pressures in the duct from that outside. For a unit mass of gas at constant temperature, if the pressure changes then so does the volume (hence density too).
 
I have test stand power and efflux velocity figures for several different EDF and motor combinations which show that fan design definitely does have a bearing on the power consumed(lost) and efflux velocity.
 
IMHO, without a wind tunnel and some pretty sophisticated instrumentation we are not going to be able to improve on what we can already achieve using basic static testing and fairly well known rules of thumb.
Pete
19/10/2011 01:53:02

I have been following this thread since it started and as you have asked what others think here goes: I don't think many of the assumptions are valid when considering modern brushless EDF power trains and this will impair your estimation process:

1. Air pressure does change through the ducting as indicated by collapsing intake ducts and exhaust ducts that press outwards.

2. In a 69mm fan at 30k rpm the blade tips have a velocity of approximately 220 mph - at 50k rpm (a fairly common figure these days with 6S EDFs) the tips are doing 368mph (M=~0.5). Drag (hence power required to over come it) is proportional to the velocity squared, so the prediction equations need to incorporate a rpm factor.

3. No allowance in the losses for how many blades, wetted area or pitch - all absorb power apparent as a "loss".

4. It is generally accepted that compressibility effects start to be noticed around 350mph - see (2).

5. Prediction results seem to be independent of airframe characteristics (drag) and intake duct geometry/length - in reality this is obviously not so.

6. In the static case intake air has to be moving into the duct: the mass of air being expelled has to be the same as the air entering the duct otherwise air propelled from ahead of the fan and out of the exhaust wouldn’t be replaced resulting in a vacuum ahead of the impeller…………….or given your measured low (tending to zero) intake velocity and high efflux velocity, the efflux flow has to be at a lower pressure than the intake (pV=constant or p1V1=p2V2– Boyle’s Law).

7. Finally………I would put money on the fact that the pressure inside your Hawk isn’t true static atmospheric pressure (probably lower) purely because of the shape of the fuselage and its relationship to the openings in it – static vents on full size aircraft are very carefully placed and carefully calibrated. Because velocities are squared, the effect of any measurement errors will be increased and lead to even greater inaccuracy.

Regards

Pete

Edited By Dizz on 19/10/2011 01:54:14

Edited By Dizz on 19/10/2011 01:55:39

Thread: motor cutting out
25/07/2011 22:38:46
What is the C rating of the packs you are using? If they aren't up to the job a high current draw will drop the volage - possibly below the LVC. The pack voltage recovers when the load is removed. On many ESC these days you can set the cut-off point voltage, also what happens when LVC is reached eg hard stop, soft stop or "bliping the throttle". NB the BEC still operate regardless to provide power to the Rx and servos. Another feature sometimes associated with a hard cut is if you move the throttle stick all the way back then open again the ESC will give you enough power to achieve a landing.
Pete
25/07/2011 21:05:38
Could be the low voltage cut off on the speed controller. Was the second flight on a new pack? Or maybe the ESC has an over temp protection feature.
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