4 Edf Vtol

[spuddly]

Hi All,

I'm trying to construct a VTOL similar to Likais (http://www.rcgroups.com/forums/showthread.php?p=8050377#post8050377) however I'm running into some problems. I wanted extra thrust so the model could carry more weight so I am using 4 x HET-6904 fans + 900 Watt HET-RC Typhoon EDF 2W-23 Brushless Motor + 72-Amp ESCs (i.e. this set: http://www.rc-warbirds.com/product_info.php/cPath/112/products_id/574), each running from a 5S LiPo. I also have 3 gyros for stability.

However, even with all 4 motors at full throttle, it seems unable to lift off. The motor/ESC/battery plus the receiver and gyros are mounted on a light plastic base, so there's no unnecessary weight (although the whole lot together weighs around 3kg which is pretty heavy). Since each of the EDFs is "suitable for a model of 1600 grams" though, I would have thought this would have tons of thrust to spare.

Is there something I'm totally missing? A setup like this should obviously be able to lift it's own motor/esc/battery weight right?

Any suggestions would be welcome!!

[Brandano]

you can't take the "suitable for a model weight" figures as valid for a VTOL model. a plane can fly with as little as 1/4 of its weight in thrust, even less for clean design (ever seen the two stroke engines on self-launched gliders?). The static thrust figures is what you really should look for. Try putting the lot upside-down on a stand and put everything on a scale, then measure the weight with the motors ooff and then on. The difference in weight is the thrust you have available.

[techspy]

I would try cutting out some of the batteries. Try setting it up to all run off one battery and test. Then try with 2. I am running a tri-copter with all 3 running off one battery. Yeah you will get longer run times with a battery for each, but the weight gets to be too high, especially if you go for a higher mah rating.

[PLATINUM]

I would note that even if the 1600g's is off by half he would still have better than a 1:1 ratio of thrust to weight. With motor efficiency at 87% of each edf's rated 900watts..he has nearly 3200watts of combined thrust available.

Does your design raise the edfs off the ground enough.. i.e. are the fans far enough away from the ground so it isn't creating ground effect?

[spuddly]

Thanks for the suggestions!

I was thinking of taking out two of the batteries, but would running two of the Typhoon EDF 2W-23 Brushless Motors (which according to their specs have a max current of 54 Amps) be too much for a single 5S 25C 3300mAh LiPo (which have a constant discharge rate of 82.5 amps)? Drawing too much current damages the battery right?

Platinum - I've tried with the fans between 5cm to 20cm off the ground with no success. But many EDF VTOLS can take off with there fans within only a few cm from the ground I think?

Would adding conical thrust tubes to the bottom of the fans to focus the thrust help? Or putting the EDFs nearer one another? It's strange to me that the fan which requires 18.5V can't even lift itself and a medium capacity 18.5V battery.

[PLATINUM]

Certianly seems odd.. I would think that thing would just jump off the ground. The 4 fan vtol in that vid didn't seem to have any conical tubes.. nor where his edfs close together.

You could parallel 2 or 3 of your lipos so you would have a 6600mah pr 9900 mah battery. This would allow you to easily draw the amps you require. Balancing the bats is a must as well..

[moparren]

Hi All,

Is there something I'm totally missing? A setup like this should obviously be able to lift it's own motor/esc/battery weight right?

Any suggestions would be welcome!!

Have you done any duct work? Intake and exhaust is critical for a duckted fan. Particularly on the intake. A square intake can kill any thrust you might get. For static thrust, you want a nice smmooottthhhhh bell mouth. At the very least, you need a round intake, I'm not sure if the fans you are using came with intake rings but if the did, make sure you use them. Size is also important. Your intake should be 100% of the fan swept area. FSA (area of the fan minus the area of the center pod where the motor is.)

On the other end, smooth is again the word of the day. Keep the same diameter or if you want to do some math, make your exhaust 80% of your FSA for the most thrust.

Years ago when my dad and I first started playing with fans we found that we picked up around 5 mph just by making duct improvements. It makes a big difference

[PLATINUM]

If you look at the vid he posted.. there is 0 ducting on the 4 x edf vtol.

Do you have any pics of this bird Spudd?

[Brandano]

I tend to agree that a bellmouth would improve static thrust, on some full size plane this can make a 10% to 25% difference in overall thrust from a ducted fan. This is actually mainly an effect of low pressure on the lip of the bellmouth, so it's suction rather than thrust. Reducing the area behind the fan will increase the speed of the airflow, but will actually reduce the static thrust. You can gain speed, but it's akin to changing a prop for a coarser one, it won't work well at low speeds. Here the task is hovering, so static thrust is everything

[erbium6]

Hi Spudd,

Here are my configurations for my VTOL. The AUW is 2.4kg. Throttle at slightly above half would keep the aircraft afloat. However, right now nothing seems to qork correctly. I can't throttle up all 4 fans with 1 throttle speed. its been 2 weeks and I still have no luck after changing receivers and re-programming everything on my transmitter.

I am using a LiPo battery ;
- 11.1V
- 2200mAh
- 25C

and my motor (Align) ;
- max continous current, 30A
- 4050KV
- 330W

and ESC Governor (Align) ;
- 60A

[yflint]

I agree with Moparren that the shape of the intake lips is critical.
I purchased in the past few Het 6904 systems and they come without any intake lips.
It will be good if we can have some pictures of the setup.