Tandem EDF

[Simon Chaddock]

A concept of putting 2 EDFs in tandem has been mooted many times over the years. In general the idea it has been shown not to work or to be a very inefficient way of increasing the thrust.

Quite a while ago I built a 'vertical riser' with a single EDF and aprt form the huge power and light weight required I was amazed at the effect of the motor torque until the air speed had built up to allow for the aerodynamic control to take over.

With the reasy supply of racing drone motors and multi blade props available ni CW and CCW I wondered a tandem counter rotating EDP (props rather than fans) would be able to supple the required thrust and solve the torque issue at low air speed.

I had already 3D printed an airliner nacelle using a drone motor and 3" prop so I decided to see what happened with 2 in tandem in a bespoke printed cowling.

Both props (3x3x4 blade) and motors (2205 kV2300) are identical. The exhaust nozzle is about 90% of the FSA.

On a test stand with the EDF pointing up wards so the thrust can be directly measured on a set of scales.

This test rather rather confirmed the expected result. For twice the watts it generated just under 50% more thrust. This was actually rather more than I expected but there is virtually no torque. It would obviously be a much more efficient use of the power with the fans mounted side by side!.

The next step will be to create a tapered cowling with a bigger diameter front prop with the duct reducing by say 85% area to the second with a further 85% FSA nozzle.

Something like this CAD view

Such an arrangement might raise the thrust level to a bit nearer to twice that of a single fan but there are of course a serious number of variables at work.

Edited By Simon Chaddock on 30/11/2020 20:27:08

[J D 8]

I might be talking through my hat but would it not be better if the second prop would be of greater pitch as being in the air stream of the front prop it would be running in an unwound state much like an aircraft does when flying compared to being static ?

There have been many contra rotating prop types, the set up being mainly used due to the need to eliminate torque and reduce the blade length for clearance. I don't know if they ran with ran with different pitch, blade section or rpm though.

  For pull push set ups like Cessna 02 it is said to have some 85% efficient of a standard twin set up but the pilot did not need to be trained in engine out situations like a normal twin.

Edited By J D 8 on 30/11/2020 21:46:33

[Nick Cripps]

Simon, is there a big difference in current consumption on the 2 motors?

[Colin Leighfield]

It is true that in model contra-prop applications the rear one usually has a coarser pitch than the front, so it might be interesting to make the comparison.

[Simon Chaddock]

In the first constant diameter equal prop version it was more a trial to achieve a bench mark. The current consumption was about 1/3 to 2/3 front and back.

For the tapered duct I would expect the the rear prop will need a coarser pitch (or more rpm) to further speed up the airflow from the front prop. I expect there will have to be quite a bit of trial and error testing.

Given the overall inefficiency of small diameter ducted props (or fans) I doubt any tandem arrangement will ever reach doubling the thrust of a single motor and prop or fan however if I can get to say175% the the lack of torque may make a reduced thrust to weight ratio acceptable and a 'slow' vertical riser.

[Robin Colbourne]

The front fan can benefit from a venturi intake, or at least a rounded entry on the duct. Maybe the rear fan would benefit from additional air being drawn in after the front fan? I'm imagining something like a Jetex augmenter tube with the front fan ahead of it, and the rear fan in the tube.

I agree with the above suggestions about having a greater pitch on the rearmost fan.

How about using opposite rotation fans on each motor to cancel out most of the torque issues?

[Max Z]

Posted by Simon Chaddock on 01/12/2020 00:24:21:

In the first constant diameter equal prop version it was more a trial to achieve a bench mark. The current consumption was about 1/3 to 2/3 front and back.

?? I would expect exactly the opposite, with the rear motor running in already accelerated airflow?

[Nigel R]

max, would that not be true if they rotated the same direction?

I would expect the tandem arrangement to work better if the flow was straightened in between the fans?

[alan p]

would it improve as a bypass if the second fan also drew in fresh airflow from vents in the cowl aka harrier inlets?

[Max Z]

Posted by Nigel R on 01/12/2020 10:31:16:

max, would that not be true if they rotated the same direction?

Well, I would expect some effect of the rear prop having to battle the swirl of the front prop, but the net transit airspeed generated by the front prop would definitely reduce the effective pitch of the rear one. How the power balance works out, only Simon can tell I guess, but it would surprise me if the rear prop consumes 2/3 of the total.

[Piers Bowlan]

I think there are three things at play here to my mind (at least). 1). The second prop is in an accelerated airflow compared to the first. 2). The airflow striking the second prop is rotating in opposition to it's rotation, so therefore has a lower A of A than the first prop. 3). The second prop is operating in turbulent airflow so will be less efficient. The staters which hold the forward motor should be curved in an attempt to straighten the airflow, as Nigel pointed out. Being flat they will just cause more turbulence in the rotating airstream from the forward prop. Generally speaking pusher arrangements are generally less efficient than tractor, the Cessna 377 being a case in point where the rear prop is operating in turbulent airflow.  

Personally I don't think that the tandem arrangement is optimised for reducing torque and if you want more thrust from two fans without the torque. You would do better to place two counter rotating ducted props (or EDFs) side by side, which would double the thrust compared with one fan. Obviously you would still need a bigger battery

just my 2p worth.

 

Edited By Piers Bowlan on 01/12/2020 12:05:50

[John Lee]

I'd suggest experimenting with a row of stator vanes to straighten the airflow between the two props. The second prop at present must have a very odd angle of attack.

[Brian Sweeting 1]

I'm no expert but back in my working days involved with ventilation it was not uncommon to utilise contra-rotating axial fans to increase airflows and pressures.

This text book, of old, covers a lot of things but is the bible as far as fans go. It's a big pdf file but may help EDF users.

Woods Fan Book

Start with axial fan in series around page 115.

[Simon Chaddock]

The reason for considering axial fans although in this case ducted props was to see if I could equal the thrust of an EDF with no torque. An important feature or problem with my EDF V-2 with wings.

At take off the motor torque caused it to roll despite the efforts of the 3 axis gyro and with a portion of the control surface actually in the EDF air stream.

My experiments suggested the tandem arrangement was never going to equal the thrust/weight/diameter ratio of the 70 mm EDF, however the performance of the RS2205 drone motors I had used on my jet airliners was such that with a 4" 4 blade prop it would provide a suitable thrust to weight performance particularly as it would use less than half the amps doing it.

My V-2 needed to be completely rebuilt anyway having suffered a spectacular and realistic crash when its EDF disintegrated as it lifted off.

I will resurrect the V-2 thread to cover the rebuild.