Is their an easy rule of thumb for engine thrust requirements vs weight of model?

I'm building a SPAD weighing 600 grams, and my motor is giving me around 450 grams of thrust. Whilst I realiise that there will be other considerations such as wing size/configuration and drag, can one easily say "Yeah, that should be enough", or "Hmm a bit underpowered" simply from the thrust and weight numbers?

Cheers

Chris

what kind of SPAD is it (roughly) and which particular engine are you planning to use??

Kingx,
It's a delta wing made of correx (Idea from www.mugi.co.uk ) the engine is a brushless electric(Sorry can't read the number at the moment, it's inside the body) Does this help?

Thanks

Chris

450 grams of thrust should be fine for a 600 gram model, if that thrust is delivered with a propeller of sufficient pitch to match the flying speed of the airplane. Some motor manufacturers like to rate their motors' thrust using very low pitch props that run out of thrust at far too low an airspeed, resulting in a model that struggles to get much above its stall speed; no fun to fly.

Rotaryphile,
A subsidiary question please.

I measured the trust myself using kitchen scales(Put plane nose down on scale, measure weight, turn on engine, measure new "weight", and much to my surprise, it didn't seem to matter what prop. I put on. Your answer has given me the clue - I was measuring the thrust whilst the plane was static. So, given that the trust was the same with all props, should I go for the one with the biggest pitch to give me highest thrust when at flying speed?

Cheers

Chris

to be honest a good Speed 400 motor would be sufficient. But since this Delta model looks "special" ,going for brushless power is ok provided you use Lithium-Polymer batteries. The thrust figure that you provided is more than enough to propell this Delta to high speeds.

Ps. "engine" usually refers to internal combustion units and not electric motors.

Thanks kingx - and I'll try and keep my engines in my helis and my motors in my electric SPAD's in the future!

Measuring thrust in that manner can result in gross errors, unless the scale pan was very tiny indeed. The force on the scale pan will be heavily influenced inflow to the prop. I rigged up a little mechanism that I tie my electric models to, while they sit horizontally on the floor, and the mechanism pushes down on the scale pan in relation to the thrust. If props with different pitch produce the same static thrust, a highly unusual situation, I would use the highest pitched prop for flying.

You've got the right idea about using the higher pitches. But it'll depend on the model as well. In the end you'll need to play with them a little. Certainly the higher the pitch the faster it'll fly but probably your current is higher too. In my testing I found that rising pitch numbers and current go hand in hand until the prop is high enough in pitch that it stalls the blades. At that point the thrust seemed to remain constant for a given diameter but the current still continued to climb with increasing pitch. But it wasn't linear, just a trend I noted. So ideally you should start to fly a prop that's one or two pitch sizes too large for static and it should link up quite well in the air. Then play with a couple to either side to see what works best to provide your model with the speed that works best for it while providing the best compromise of run time.

Chris,

Here is what I do...yes it is kind of lengthy but pretty simple..might help. Thru a large part of the flight envelop(from the stall point to about 2 times the speed at stall) you can figure that the glide slope of a plane is going to be roughly the same as the Aspect ratio(AR) or a little better. So a plane with a AR of 1 might have a glide slope of around 2. A plane with a AR=8 might have a glide slope of about 10. So, if you want to find the power need for a plane to maintain level flight.... first find the flight envelop then you divide this speed by the glide slope (which gives you the sink rate) and mutiplied the sink rate by the weight which will give you the basic power nessasary for level flight. But you want to be able to climb so from the min power with climb ... double the min power this will give you a plane that will fly but not be a real performer, from double to triple it if want a moderate performer to sport, from triple to quadrupale if you want something in the sport fly to 3d range.

EX: Find the rough flight envelop

Plane area .25 m^2
AR=1 so lets assume glide slope of 2 to 1
Weight=.6*9.8=5.88 N
wingload=5.88/.25=23.5 N/m^2
Max Cl assumed to be 1
q=1.225

Velocity=sqrt((2*wingload/(q*cl))
Velocity=sqrt((47/1.225)
Velocity=6.2 m/sec
So our flight envelope is 6.2 to 12.4 m/sec

6.2/2=3.1 or 12.4/2=6.2 (speed/glideslope=sinkrate) glideslope=about 2
18 to 37 watts for level flight (3.1*5.88= or 6.2*5.88) (sinkrate*weight=power for level)
so and average of 55/2=27.5 just to maintain level flight but at the very lest we double this so 55 watts of deleverd power is what we need, but lets say we get about 60% from input power, so we would need a minimum of 92 watts (55/.6) of input power just to have a poor performer.

You said that you are getting 450 g of thrust
.45*9.8=4.41 N thrust
and Thrust*speed=power
assume that the dynamic thrust stays the same as static(it usually drops with foward speed)
so at 6.2 m/sec * 4.41=27.3 watts delivered
and at 12.4=54.6 watts delivered. (Just barely what we need 55)

or another way to look at it we would need static thrust minimum of:

18/3.1=5.8 N or 600g thrust will provide min power.....I would not build this plane but it would fly.... I would want something with 900 to 1200 g of thrust static.

If you know the Diameter, Pitch and RPM of the prop you can get a better idea about what the dynamic thrust will be...maybe this help a little.

Happy Flying

Steve

I also fly electics, and I have found a pretty simple method to tune the thrust. First you need a better way to measure your current thrust. I bought a digital postal scale, and then made a trianglar bracket on a stand that will transfer horizontal force into a downward force. I simply tie the model to this with fishing line, apply the power and see how much thrust I get.

The next thing to measure is the amps, this is basically going to tell you how much flight time you are going to get at certain throttle settings, but more importantly it will tell you if you are going to burn out the esc or motor. Most will take surges over their ratings. It is not easy to find a cheap amp meter that will measure over 10 amps. (Mine only goes to 10)

Given this setup I then try a bunch of different props to tune the thrust verses the amps being pulled. Putting on a larger prop with more pitch is going to increase the thrust and amps but of course lower the flight time. If after this you are still not getting the thrust you want, increase the battery.

My rule of thumb for a plane that will do good arobatics is 1:1. Same thrust as weight. 3D you need 1.5 : 1.

Good Luck,
Larry

Larry and Steve et al,

thanks so much for your concise and learned explanations. I was going to try the plane this weekend, but it seems that either my method of testing thrust is flawed or my motor is very underpowered, so I need to make one of the "triangular thrust measurement mechanism" devices, and re-look at the thrust/weight/power consumption issues and then determine if I need to buy a bigger motor/esc combo. I'll let you know how it all goes on Monday.

Thanks again

Chris

Chris,

If you can't get this motor to fly the mugi you might want to check out something like what Dick's building. Say something about 900 by 240 for the wing. Just a peice of corro with a wire at about 25% chord. Just an idea.

[link]http://www.rcuniverse.com/forum/m_2204578/mpage_3/key_/tm.htm[/link]

Steve

The weekend came and went - score? Mugi 0-gravity 9 !

I made the plane way too heavy, and much too underpowered. Whilst I was amazed at how well the plane took the crashes(Correx is a lot stronger than I'd ever imagined) Basically nothing else worked:- [&o]
There wasn't enough forward speed to get any control surface authority
Whilst the CoG was right, it didn't glide at all, just mushed(quickly) towards the ground. The wing loading must have been something like twice where it should have been
The motor, even on full power didn't provide anything near the right level of thrust.

BTW - before anyone thinks that this is a Mugi problem, may I say that I had to play with the published design because I didn't have the right materials (Correx in two halves 'cos I didn't have a single sheet big enough - which required extra strengthening, wrong grade of Correx etc) I knew that the plane was going to be heavy as a result - I didn't appreciate quite how badly a few grams here and there would affect everything

So what's next?

I'm going to build, bigger(1.2m rather than 800mm), lighter(2.5mm Corex, single sheet),faster(bigger motor) version - and see what happens then. If that doesn't work, I'll try Steve's idea, and go for a different design. This has been a real learning experience, thanks for all the input along the way! I'll let you all know when I get the score up to model 9 gravity 0

Cheers

Chris

Hey that plane looks like the pibros, I stuck an os .10 onto the pibros set up as a pusher
when i tried to hand toss it for trimming it flew like a brick so then i used a bungee launch setup to launch it an man what a fun plane that was. Maybe you should try to bungee that mugi into the air, just make sure you have some up elevon dialed in, cause if it takes a nose dive during launch that thing will cartwheel into next week. I know that from experience. Just an idea.