Fan testing

[Tim Mackey]

I have now done quite a lot of tests with some 90mm EDF units, in preparation for fitting out my new Hawk.

Results are shown in summary here in a picture of an Excel s/sheet. Should be clear enough.....amps watts voltage held and thrust obtained etc.

Results were taken from video filming on the test rig ( details in another thread somewhere ) and show the difference in performance of the original fly-fly fan Versus the Wemotec midifan rotor. Clearly the midifan 6blade rotor produces better static thrust for lower wattage input, and the results of fitting the intake lip ( which will not be in the model of course ) also show this produces an extra 25% or so thrust compared to the "dirty" air intake of having no lip. Last columns show averages across the whole 6 minute runs - note the Lbs of thrust obtained in the last set, an average of 4.2lb for a 5.5lb model should be OK.....

not ballistic....but OK.

I am posting a short video clip in a few minutes to see / hear the beasty in action !

[Tim Mackey]

video link

try this

[Erfolg]

The gain of 25% by using a shaped inlet is a very significant value.

It suggests to me that the intake on the aircraft, can easily destroy any hope of sucess or may be assure sucess. I would guess that the shaped inlet supplied increases the effective cross sectional area of the duct. If this is so, it suggests any reduction of cross sectional area at the inlet to the fan, say at the models air intake could undermine the performance of the fan unit.

Thinking of the top of my head, if the inlet duct was 1/2 the area of the fan duct, this would require the air entering the duct to increase by a factor of 2. Accelerating air in this way is almost certainly inefficient. Without getting into Bernoulis therom (conservation of energy), momentum etc. it would seem that the duct and inlet shape and size design will generally generate more potential for performance increases (or controlling losses) than than the fan (although important).

Seems to me that straight through type ducts such as the Meteor, Me 262, Volksjaeger, modern civil transporter/passenger aircraft should perform generally better than small side air intake aircraft. Although opening the duct and inlet appreciably would help.

I have no experience of ducted fan models. Although as an engineer I have overseen the design of fluidic systems using computational methods for stress and flow analysis. I could not be bothered now.

Erfolg 

[Tim Mackey]

It is basically just down to dirty disturbed air flow at the front of the intake. In the model itself of course, the intake "lips" as such are formed as part of the oversize air intakes at the fus sides. Some work needs to done to clean up the whole intake path, with lumps of moulding pips and the like along the route. I just used the lip on the last test....because it arrived with the new fan housing  - and it gives me a better idea of what the actual thrust will be like when installed in the fus. These of course are only static thrust results, all will be different in the air, but the idea was to compare the two fan units in a side by side exact same rig. I think its a fair assumption that whichever performs better in the static test should also do better in the air. I am hoping to be able to modify the fitting of the fan in order to be able to change blades afterwards without too much hassle. That way, I can try both fan units in the real flying world

[Erfolg]

I have briefly thought a little more about duct systems,as they are generally just modelled using spread sheet type models, as these are considered simple systems. If you look at IHV book on low pressure ventillation systems (it is many years since even glanced at it), it provides many tables of factors for frictional losses and means of assessing pressure at points within the system.

From memory, losses at entry can be significant, from a sharp lipped duct to much improved with a rounded entry. Also convergant (narrowing by significant amounts) ducts are shown as points of significant losses, as are any bends. The engineers approach to this type of problem is to consider the mass flow and momentum.

I do seem to remember an article in the RCM&E by a German modeller (engineer/scientist?) on the physics of duct systems. Even if you are not interest in the maths, the scientific approach can highlight where lossess are occuring and the relative value of the elements. I must admit I can no longer be bothered, as is the case I  guess with most modellers. But thank God there are modellers out there who do, as they solve the problems giving us fans, brushless motors, ESC that work! Where tinkers like me have fun, or become frustrated.

Erfolg

[Myron Beaumont]

Appreciate the vid but not as action packed as my 1st run up yesterday (the cat disappeared as per cartoons dont worry animal rights people there was nothing dangerous happened)  Anyway a big thank you to moderating Timbo for giving me all advise /help/encouragement  through the initiation .As a consequence here are the results for the AP 700 edf on 4s lipo (sounds like the F------L results  -sorry)

initial off load voltage   16.7 V     after 10 min run  (4 max to min cycles) 16.2 V   No significant heating detected

Max speed  voltage  14.2 V     Max amperage   29A    Wattage reading 415  VA

Grumpy Myron after the first decent nights sleep since I learned of the HAWK

[Tim Mackey]

Morning grumpy..... see... I told you it wouldnt self destruct or spontaneously combust !!

415Watts is about right for the voltage held and the current consumed. Your battery is holding reasonable levels during load, so all the figures seem to stack up. Without a thrust reading, it is iimpossible to tell how well it fly of course....but at 400 Watts, a 3lb model should be fine I reckon. Obviously, the goal now is light airframes to maximise the potential. The70mm fan is nowhere near maximum at those figuures, and depending on your motor specs.....you could probably go to a 5s pack and see the amps and watts increase quite alot   Later !

[Tim Mackey]

The link below shows some typical test results from a 70mm AP fan unit on 4S battery like yours ( different motor though )  and the top one seems quite close to your figures ( amps and watts ) so therefore would seem to be a similar motor.  Figures indicate thrust of around 2lb nearly, and RPM of 30000.

fan thrust

[Myron Beaumont]

Limiting factor these days is reflexes methinks !  Interesting subject - inlet shapes etc .I find it fascinating havi ng worked on RR test beds yonks ago .Of course the main factor is the airspeed of the A/C ,which determines to a great extent the shape of everything .eg Concorde .Your thrust measurements are may I say more or less the same as would be dynamically as our airspeeds /flows are miniscule compared with the real thing .One thought occures to me & that is that its important not to let inlet air mix with outlet high speed flow or should I more correctly say keep the pressure differential right .EPR (engine pressure ratio) was & probably still is the way the pilot can measure/set up thrust in the office.This "leakage" is bound to be more significant on our short EDF's than on a 30 foot  configuration & must be prevented for max efficiency .I expect there are plenty of folks know a lot more about this than myself & would love to hear comments

[Erfolg]

Takes me back also. As a young engineer I worked on the last of the "Metro Vic" jet engines which were being used to develop "AEI Gas Turbines" both stand-by and ship (County Class I believe). I remember when they (not me of course) blew the exhast stack apart on a mis-start (gas turbine) the jets just discharged across the field.

Ducted fans are different from Jet Engines in that the air is required for a combustion process, where there is obviously an expansion of gases because of combustion. Also the inlet to the jet engine needs to be at a velocity which enables combustion to take place. The differences are many, as is much of the physics.

Ducted fans are low pressure, systems. In this instance the mass flow analaysis does reveal all. It is a pity that I cannot find the article that I vaugley remember (I am sure that it was a German), as I am now certain that the system inlet is very important to overall performance. I am now convinced (till I change my mind) that the inlet cross-sectional area benefits from being greater than the fan area. How you achieve this is another matter. 

I note that although most commercial jets are by-pass systems these days, the interest with ducted propellers/fans has stopped after the fantrainer/liner. It would be of interest to no why.

Erfolg

[Tim Mackey]

Jetsome I will post some more details of its design and construction sometime later.

[Myron Beaumont]

Does it play pink floyd on that turntable ?

[Erfolg]

Jet some

That is my point

• Jet engine = open cycle heat engine (often modelled as a closed system)
• Ducted Fan = a mass flow system, where Bournoli, Boyle and others obseravtions/formula are more appropriate (although not enough compression for Boyle to have significant effect).

What I am saying, there are some out there who have done real work on the duct system. Combine their work with the Fan testing (that Timbo is doing) and you will have sucess. Ignore either and a new design may well not necessarily be optimal or sucessful.

I am convinced that the sucess of the Germans in engineering is the full system appraoch. Even in in DF theirs (Schuebal?) seems to amongst the better.

Erfolg

Moderator edit - Schübeler

[Tim Mackey]

Ah yes..."welcome to the machine" ....not exactly a good turntable for the old vinyl stufff,  but it does get a little loud and could well cause a little "brain damage" and of course you must keep your battery "poles apart" and if it all lets go, you better "run like hell" !

Damn.. I promised i wouldnt get suckered into this type of floydesque retort again

[Myron Beaumont]

Jetsome & Erfolg   I appreciate of course that no combustion process is involved  .The turbine is replaced by lektrikkery in a way .Actually the pressure diference on a gas turbine is in the order of 2 at TO power if I remember correctly .I was involved with the making of EPR charts in the performance dept for Spey engines in the mid 60's so my memory is somewhat hazy

[Tim Mackey]

Myron... combustion COULD be involved if you overpush your LiPos

[Myron Beaumont]

So do I divert air from the battery compartment into the inlet for a 1/2 second burst of power before disintegration  Now we're into rockets

[Former Member]

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[Erfolg]

The Metro-Vic jet was an axial flow with a compression ratio of about 4:1. The design was sold to another company. For me it was interesting that Donald Cambels Bluebird had a metro-vic engine at one stage.

There were other development engines which achieved higher compression ratios. At the time we were told that our engines had greater thrust and were more fuel efficient than Rolls Royce engines, but the Government had decided that they were to be a preffered supplier to the MOD projects and we were to get ships propulsion. How much of what was said was true I do not really know. But there were a few centrifugal engines, which I understood to be Rolls engines, knocking about. I did some number of calculations of the theoretical cycle "Bray" in the days before electronic calculators or computers (other than the Main Frame we had, which was busy doing calculations on Steam Turbine blade design). I know wonder if it mattered, and what they did with the calcs, other than pour over them before filing them with the rest that we guys did.

However ducted fans I guess come no way near the compression ratios of the inlet stages (although I have never seen a figure quoted).  The principle laws that need considering are the 2nd and 3rd, F=ma and for every action there is an equal and opposite action. The other consideration are the laws of "conservation of energy".

At the end of the day I guess it is a question of moving as much air as possible (therfore highest revs possible, relative to motor and fan limitations/efficencies), to genrate the force (F=Ma), and conserving energy by good inlet to outlet design. The down side is the aerodynamic forces on the airframe, ie drag, which is related to the cross sectional area and co-efficent of drag etc. I can almost feel myself becoming excited, enough to go into the loft for my text books and notes. However it soon passed, and I am now looking forword to finishing my kitchen roof. So that I can start modelling again

Erfolg, Bsc(hon), CEng, MiMechE (long time since I used that lot)  

[Tim Mackey]

Eric... the picture is not of the rig in use, that just shows the general construction of it.

In use, it is in the middle of my large garage floor, and has clear open space all around to approx 2 metres.

Besides.... the purpose of the test was really to compare the performance between two particular rotors, and all conditions were identical for all tests so it was still "apples with apples".... as it were

[Myron Beaumont]

Timbo  I think we would all like to see some "smoke tests "done in & around an EDF in action _ Give us an idea about airflow _ .Possible or not ? Just a thought ! Of couse I wouldn't dare to suggest a smoke source

[Tim Mackey]

yeh should be do-able leave it with me.....

[Erfolg]

Timbo

Now you have your test rig, it would be quite (relatively) to use

• different size inlets (both idealised and repicating areas used on some models)
• differing contoured inlets (bell inlets which effectively increase duct entrance area , against sharp edged convergent types)
• different shaped sized outlets (reducing conical against parallel)
• flow straightening vanes

to establish what matters in practice.

My prejudice is for well rounded bell, with the same effective area as fan (allowing a little for the bell), parallel duct. It would be neat to  know what really matters, and what is significant, as the record force on the scale is that all that matters. 

[Myron Beaumont]

Good   Maybe 'er indoors can burn your dinner /lunch/supper/buns etc on purpose as a smoke supply on the pretext that you're trying out a "fuel "proof diet   

[Former Member]

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