Hi, i am sure this has been asked a thousand times before, but i ahd a look back over the last 10 pages and couldn't see anything on it so i thought i would ask.

I have no flight experience at all, so please bear with me, i am getting my first plane in a few days (or whenever it turns up) so i am making a start, but i was wondering about the feasability of a Harrier.

I was thinking about having 4 small edfs. one connected to each thrust tube. you could then use 2 gyros to controll the 4 motors to keep the plane level in hover mode. to move left and right you could vary the speed of the motors to tilt the plane and move it. as for forwards and backwards you could use a combination of motor speed and duct angles.

would that even be possible, or would it be just too heavy and overly complex to build and setup??

thanks

Lewis

Originally Posted by Lewist

would that even be possible, or would it be just too heavy and overly complex to build and setup??

In theory its a piece of cake, in real life no one have made it happen yet. Weight is your main enemy/challenge.

ok, what sort of thrust would a edf for a 300 motor have running a really hot brushless on 11.6 volt Lipos.

it guess the main construction would have to be foam to save weight!

Thanks

Lewis

thinking Minifans with Mega /2 should be able to burst to 400watts/fan giveing the power to lift... say 7lb

with two 3s3p TP6000 it could be done

300fans are a bit too small I think... (kind of the same weight for the motor but in a smaller fan)

I was thinking 300 sized motos to keep the fan units small, to keep the overall size of the model smallish, bearing in mind there would be 4 fan units! also need to have large enough intake to feed them whilst still maintaining scale!

i think it is possible, but as has been said weight is the enemy! i think i might have a crack at this but it will be a long haul project,

i think using 4 fans would be better than 1 huge one because of the ability to alter the speed of each unit to keep it stable in a hover. if one were used i guess either thrusters would be needed to stablise it or flow controll on each nozzle would be required!

Originally Posted by Haldor

In theory its a piece of cake, in real life no one have made it happen yet. Weight is your main enemy/challenge.

What about an all depron airframe, cdrom (or small brushless) motors and lipos..

If you were willing to put up with small batteries/short flights, I reckon control in hover may be the hard part...

It's not a big problem to build something that hovers (for a short time) using ducted fans. All you need are some clever mixers and a few gyros, once you have the power to weight issue resolved.

The difficult bit is building a Harrier (or anything else) that will transition from hover to forward flight and back again. This has been done a couple of times, but only with very non-scale models using big vanes in the prop/fan wash for control (and usually IC motors rather than electric).

Why can't the model use the same idea as the real one, rotate the nozzles from facing rearwards to facing down (or slightly forwards) and let the gyros hold it steady while it slows!

The 50Hz update rate from an Rx is too slow to consider making a closed loop control system for something as complex as hovering an aircraft. It has been done, but the aircraft are always on the verge of tipping over, unless you can make something that has a minute moment of inertia in all three axes. It needs a micro controller to accept inputs from the gyros and send modified outputs to the stability control devices, and the update rate would ideally be in the kHz range to allow a high proportional gain without instability. With the advent of lipos it will be possible to do it thrust/weightwise, but it's not a weekend project to sort out the control system.

Stu.

Stumax.. sorry you will have to bear with me on this... 50Hz update rate? does the gyro not deal with the stability controll, what is the refesh rate of those?

also would this end up being a complete pig to hover.. i can fly helicopters, but i guess this is a totaly different ball game!

this would be a long term project, i would plan to make a test rig first to test the gyros and controll systems before i even consisdered making the model

Originally Posted by Lewist

Why can't the model use the same idea as the real one

The real one uses compressed air bleeds to both wingtips, nose, and tail for control in the hover.

i had heard that (you can tell i have done a huge ammount of research!)

i am sure however that the same sort of effect could be gained by varying the speed of each EDF on each nozzle.

The one area of concern that i do have, is will i need to turn the gyros off in fight mode. now in hover mode if you push the aerlon (spelling!) stick to the right it would speed up the EDFs on the left of the plane, causing it to bank slightly and move to the right. in flight if this were to happen it would cause the left EDFs to increase speed thus pushing it towards the deck in a hard turn. now the difference in trust between left and right would only have to be fairly small becuase in hover mode you would not need a lot. but in filght mode would this cause odd or bad handling charactoristics??

thanks

Lewis

Originally Posted by Lewist

... this would be a long term project ...

Indeed it is ... Sorting out the control system takes quite a bit of time and patience, with LiPos power is not so much an issue anymore, provided you are willing to have fairly short flights. A test rig is of course the way to go, but I am pretty sure you can do it with standard off-the-shelf electronic components .

http://www.rcgroups.com/forums/showt...&highlight=X35

50 hz.... ??? AAAHHH.... why not control hovering stability on its own (no user input)... you simply tell it to go up down or left, and then hook up a PIC controller w/ a PID loop (u probably only need prop and integral....deriv is not so important) so basically you flip a switch to go in hover mode... moving elevator up would increase the EDF power, down would decrease, moving it left or right would increase power on one side and decrease on the other then once you let go of the stick the PID kicks in to correct the plane to stable hovering positioN!
I think making the plane fly forward will be your next big challenge...you probably have to rotate the 4 fans 90 deg.. unless you are using nozzles (but then i think u'd lose efficiency becasue of the air having to make a 90 degree turn while hovering...and u still need to turn the nozzles!!!!)
So you will need at least 1 motor to control that!
Pretty complex i'd say but probably possible

Originally Posted by Lewist

i am sure however that the same sort of effect could be gained by varying the speed of each EDF on each nozzle.

So how would you get yaw control? Apart from that I don't think the nozzles would be far enough apart to get positive pitch or roll control anyway, and I doubt the fans would be able to speed up and slow down fast enough to maintain control.

Originally Posted by Lewist

now in hover mode if you push the aerlon (spelling!) stick to the right it would speed up the EDFs on the left of the plane, causing it to bank slightly and move to the right. in flight if this were to happen it would cause the left EDFs to increase speed.

Assuming you have now rotated the fans aft (if not, what is making the plane move forward?) speeding up the left fans will give right YAW, not right roll.