How much thrust (/weight) is necessary

[meatbomber]

I'm wondering... are there any rules of thumb on how much excess thrust is available to actually achieve stable hover?

i'm currently in the design phase for a VTOL and don't know if my excess thrust will be sufficient as i'm looking at quite a slim margin of only 1.15-1.2 Thrust to weight.

It will be an indoor/slow flyer kind of type so gust and turbulenze should be no factor when flown inside (well maybe the own prop wash).

MB

[FHHOBBIES]

I would tend to think you need more thrust, it should hover at near full throttle but if you get into a bad situation you will not have the power to pull out, my machines hover at around 1/2 to 3/4 throttle. But heck, give it a shot and learn from the design, it can only get better!

[TMorita]

I think you're underestimating the P:W ratio required.

If you have only a 1.2:1 P:W ratio, then when you vector the thrust, the model is gonna plummet.

Model helicopters generally need around 2:1 to fly around reasonably. I would guest you need at least 1.5:1 on a VTOL to achieve the same.

Toshi

[meatbomber]

back to the drawingboard it is then... Well i have already identified a better Motor (Was planning on 4 X GWS EP50s (IPS Motors) And now have 2 AXI Brushless in mind with a front and a Back shaft... now the problem is to get the right size Props (Was planning on staying quite small but might need to increase size almost by 2...

Any one know where to get CCW Props ?

[Lewist]

i am working on an edf vtol project, and my target is at minimum 1.5:1 if i can't get that or more i am not going to bother.

My current hovering test rig has 2:1 and that feels good. it has plenty of control and excess thrust!

[meatbomber]

your rig is flying with a cable and not on it's on power right ? What Kind of batteries are you going to use ?

[FHHOBBIES]

Only props I found so far are the GWS 10x4.5's, if anyone knows where to get smaller ones that would be great!

[Lewist]

Meatbomber - yes, the test rig gets power from 12 GP3300 via the teather. (i am currently only using cheap brushed motors just to make sure it will work).

IN the end i want to use a high voltage low current setup probably running 6-8s or more (I want to try and keep the current at about 60 amps (20 per motor) so that would mean an 8s lipo setup)

At the moment i am puttingh out at full throttle about 500 watts, and the final setup will be looking like 1.5kw. I may chop down to a smaller fan, say a microfan rather than the 70 mm fans i am running now to try and save on cost, weight and power needs.

[TMorita]

...
I may chop down to a smaller fan, say a microfan rather than the 70 mm fans i am running now to try and save on cost, weight and power needs.

I'm not a prop expert, but...

I've heard in general, larger props are more efficient than smaller props, in terms of static thrust.

Toshi

[Lewist]

indeed they are! but cost is also a very large aspect in the completion of this project!

[meatbomber]

plus the weight... the bigger the weight the biger the wattage required to get the thing flying... that's my dilemma right now ;)

[Lewist]

indeed.... lots of power required,

my current brushed setup will drain the gp3300 pack in 5 mintues flat! I am going to want a little more runtime on the real thing! - plus the final finished plane is going to transission to conventional flight!

[Paranoia]

As a rule it's ~85watts per lb. min to takeoff; but ++120watts per lb. is the way to go.

And 1.2x static thrust, to weight.




This makes it hard to have smaller EDF's doing the job.



look at this eg

740 20c 3cell pack ~2.8oz
esc Rx and servo's x2 ~2oz
EDF and motor ~2oz
Air Frame ~3oz

9.8oz = 0.6125lb

1.1 x 0.6125 x 120watts = 80watts so at 10volts that's 8amps

BUT! the fan needs to put out 11.76oz of thrust! and only weight 2oz!

[meatbomber]

well 120W/lb = 120W/454g that's just about what my propulsion system was planned for..
The question remains on how many watts it can put in the air ;)

Well i'll see i'll start building when i get home on the 12th

[Paranoia]

look at this thread http://www.rcgroups.com/forums/showthread.php?t=435760&highlight=edf+thrust

[Lewist]

not fully followed what was above about watts/lb


1 lb = 454 grams (as stated above.. don't know if this is correct!)

i have an auw of 552 grams.
if i use a 12 volt sealed lead acid battery (i usually use 12 gp330 but i don't have them here but it does fly on this power) i get a voltage of 11.3 volts and a current of 10.2 amps per motor. this gives total power output at 115.26 watts per fan so x3 = 346 watts. this gives a total thrust of 660 grams. and a thrust to weight ration of 1.2:1. This is a bare minimum. the craft is only just controllable using this battery.

this roughly means you need 284 watts/lb. if i have my summs are correct.

i ususally fly it with the 12 gp330 which give more like 500-600 watts with a total thrust of about 1000grams. which is MUCH better. it flys with much more authority and controll.

i think if i am still on the correct track to get a really controllable craft you will need in the reagion of 400 watts/lb or 400 watts/454grams.

I guess 1 watt per 1 gram would be an idea target to get a really good power to weight ration!

[meatbomber]

the 120 watts/lb is coming from helicopters and does apparantly not directly convert from low spped high mass airflow from a rotor or propeller to high speed low mass airflow of a ducted fan...

VTOLmans craft seems to fly perfectly at a 1.2 Thrust to weight ratio and he's using slow turning (relatively speaking ;)) but relatively large props.

the 1 watt for each gramm would be cool bu i just don't see power system that can provide that energy density... at least not in a small scale..

[mnfiero]

1 pound = 453.59237 grams
1 ounce = 28.34952312 grams

Is that close enough... ;)

[Lewist]

meatbomber - there is a huge difference (as i am sure you are aware) between a large prop and an edf. although my rig does fly at 1.2:1 thrust to weight, if it gets really 'out of shape' there is barely enough power to get it back level. And this is inside. If it were outside and there was a little wind to contend with, i personaly think that 1.2:1 is too low. I know when i run the 12 gp3300 there is so much more control, the fans can respond faster (a big problem with edf vtol).

I think with modern 20C lipo cells and the use of a high voltage, low current setup 1 watt per gram would just about be possible.

I have a few preliminary numbers that i have been thinking about. I want to shoot for an AUW of 1500grams (52 ounces). from what i have seen i can get an 8s2p 20c lipo pack that should supply about 1800 watts (theoretical max output) and weighs about 650 grams. that leaves about 850 grams for the airframe and electronics. my current test rig weighs in at 550 grams. so that gives me 300 grams for the airframe!

[Paranoia]

I looked at starting a VTOL project sum time ago but the numbers just didn't work out. Even will a 20C 4cell pack and a large fan with a brushless motor your be pushing it.

But I’ve seen one or two projects on rcgroups get off the ground, but there performance is very limited.



but good luck.....

[meatbomber]

have a look at VTOLmans here:

http://www.rcgroups.com/forums/showthread.php?t=440470

[Stephen Weller]

But I’ve seen one or two projects on rcgroups get off the ground, but there performance is very limited.

Perhaps we're defining VTOL differently, but there's some pretty interesting stuff going on here:

http://www.rcgroups.com/forums/showthread.php?t=414145

The tricopter will {sort of} do loops and rolls. Check out the videos.

Then there's this thing with its single BL motor:

http://www.rcgroups.com/forums/showthread.php?t=424087&highlight=hover+foam

And there are several others. I just started a three-rotor proto based on what I've learned from these and other threads.

I suppose it just goes toprove that enough power behind a brick, and it will fly. ;)

[v22chap]

No ,,enough power behind fanfold and it will fly .... :p :) :D

[FHHOBBIES]

It's easy to get enough thrust with a 3 rotor design even with cheaper motors, I've designed a couple already and I'm almost done my 4 rotor one with 4 GWS brushed motors, pics will be up soon!

[yb2normal]

I just thought I'd throw in with my own version of the 'Tricopter' made popular elsewhere on the forum:

http://www.yb2normal.com/tricopter.html

Enjoy :)

Bill