A low power ducted fan

[Simon Chaddock]

I have flown the DuctFan twice more, first moving the CofG 5mm further forward to just about on 1/3 chord and then with a little less tailplane incidence. Better each time so the tail incidence is being reduced a bit more.

 

What does surprise me is how well it flies on reduced power (as low as 50%) or to put it another way the extra thrust from the last 25% throttle is relatively small.

 

It almost as if the fan was being "over driven" in fact in the above video I am sure the fan actually  slows down a bit as it picks up speed which suggests to me that when stationary the fan airflow is restricted in some way.

 

It is a very coarse pitch (approx 3x9!) so I wondered if the blades are actually partly stalled at full power with zero forward air flow.

 

Just a thought.

[Simon Chaddock]

A short edited video of a DuctFan 'duration' flight. Rather windy (note the steam plume from the power station) and turbulent which made this already twitchy model hard to fly smoothly. 

9 minutes 30 secs from a cheap 1500 mAh 3s is not too bad & when I get it flying properly it should stay up even longer.

[Simon Chaddock]

I cant explain it but the way the fan behaves leads me to think that it might perform better as a simple 7 bladed prop.

I know the duct acts as an end plate to the broad chord blades so reducing tip losses  thus increasing their effective aspect ratio and it does indeed add a bit to the static thrust but at these very low Reynold numbers air is pretty "sticky" so when actually in the air..........

So no duct.

It certainly flies like this but rain stopped play before I had a chance to do very much.

We shall see.

 

[Simon Chaddock]

Well it flies just about the same without the duct!

This suggests that the improved blade efficiency from the end plate effect of the duct is more or less balanced by the reduction in drag without it, at least in this low power application.

One interesting feature you can hear in the video is that even as an open fan it does not speed up with air speed as a conventional prop does. I think this means that at the speeds this plane flies the inlet air is always at an insignificant speed to the fan, which with its 9" pitch it probably is.

 

[Simon Chaddock]

The next modification is to move the booms out wide to allow space for a conventional prop for comparison purposes.

I decided that if I was going to all the trouble to move them at all I might as well move them out to the span of the tail plane.

As the elevator servo wire went up one boom and the aerial up the other it required quite a bit of surgery.

The elevator servo connection had to be completely extracted and extended.

Hopefully it will all go back together.

With the elevator servo now much further outboard it might require a small balance weight on the opposite wing tip.

 

 

[Simon Chaddock]

The wide booms completed.

Moving the booms so far apart (12") was probably a mistake as the tailplane now flexes rather easily at its centre.

Perhaps of more interest is the close fitting duct..

A layer of 1mm Depron was glued to the inside of the duct. This seriously fouled the blade tips but as Depron is soft the fan was just gently run up to speed until it was free!

There is now just about 0.1mm tip clearance which should improve the performance of the fan a bit.

[Erfolg]

As long as the tailplane is structurally sound no problem.

If concerned at all, why not a thin piece of spruce, or similar piece of wood stuck onto the leading edge.

I am more interested in the fan performance duct on and off though.

I think from convention, that a propeller beats a shrouded fan hands down.

I guess the other interest would be the effect on thrust from a long duct either or, on the inlet and outlet, to the thrust.

I guess the other interest is that you appear to get good performance from a low power. low revs? set up.

[Simon Chaddock]

Erflog

For an EDF it is indeed low revs about 12000.

Duct losses are recognised and a figure of up to 20% for a 'fuselage' length type is considered acceptable which is why I kept mine down to just a shroud.

Fitting a prop, probably a 7x6, will simply demonstrate just how much more efficient it is.

The black leading edge of the tailplane is a strip of hard 3mm balsa.

My other concern is that the horn is now right at the LH end of the elevator so it will twist a bit producing a rolling input.

In hindsight it would have been better to have put the elevator servo on the other side as then its weight and any elevator twist would tend to act against the motor torque rather than with it as at present.

[Erfolg]

Simon

You are worrying to much.

 

I seem to find that all the fine tweaking to improve things seem to make very little difference.

 

I am interested in your results, when you can put set of data together with conclusions.

 

We have had a number of DF models within our club. Yet it is not obvious why some work better than others. There is a Kysho F16, which hand launches with apparently no issues. A Mig 15, which needs bungee launches, and frequently does not get away. There have been a number of others, which all fly well once airborne and away, yet some really need that bungee to get going. A quick look offers no obvious reason for the differences. Could it be the Fan, as the ducts all seem much the same. In the odd case it is obvious the ducts are much shorter, these seem more tolerant of launch than those with long ducts.

 

We have no runway, so hand or bungee launch.

Edited By Erfolg on 20/01/2011 16:00:15

[Simon Chaddock]

Erflog

I fear my Ductfan is so far from the normal EDF that comparisons would be of little benefit.

The usual figures to ascertain likely take off performance are the wing loading and the static thrust to weight ratio..

The Ductfan wing loading is 7.3oz/sqft and thrust ratio of 0.55. These figures would be quite respectable for a prop light weight park flyer so not surprisingly hand launching is not a problem.

In a conventional EDF the thrust ratio may be similar or better but the wing loading will be far higher.

[Simon Chaddock]

With the close fitting duct the Ductfan seems to fly on even less power than before but without some rather expensive on board kit the question is how much current is it really taking?

I created a ground video with the various low power current levels recorded.

By simultaneously comparing the motor pitch in this and a flight video the actual flight current draw can be obtained reasonably accurately particularly as the fan off loads so little in the air..

The answer is the 3.5A setting. Wow!

Edited By Simon Chaddock on 25/01/2011 16:14:19

[Simon Chaddock]

Calm but cold (-2) this morning, good for an endurance test.

 

If my assessment that it will fly on 3.5A is correct then a flight of at least 20 minutes should be possible on its1500mAh LiPo.

 

It flew for 21 - and I did not run to the LVC but just to the point where another 2 "clicks" on the throttle were required to maintain height..

 

It carried the Key Chain cam but the cold got to its battery after just ten minutes despite the Cam Cosy - doh!

 

Bring on the summer.