Low wing trainer dihedral - suggestions please

[hayofstacks]

I had 240 watts on my slow stick, and that used to weigh 19oz .

[JetPlaneFlyer]

Originally Posted by npowell28

The motors only rated for 3s. I'm pretty happy that it is going to fly. 75w per lb should be more than enough.

But consider that if you have 75W on a fresh battery you will be down to maybe 55W when your battery is nearing the end of the run. Sure it will probably still fly but i just dont see the point in putting in a tiny motor, probably having to add lead noseweight, then having to fly the motor flat out just to stay up?
There is hardly any more to pay for a larger motor and even if you dont really want the extra power it's always there to get you out of trouble and when running at part throttle a large electric motor is much more efficiant than a small one that's being thrashed.

But each to their own, i guess there is some challenge in seeing just how big a plane can be flown on a tiny motor, but the actual flying part wont be a whole load of fun.

[JetPlaneFlyer]

PS.. the motor will run on 4s just fine. The only consideration is prop size will need to come down and there comes a point where the prop is impractically small for the plane in question. If the motor is very high Kv you might need to watch the RPM doesnt get too high for the prop.

[npowell28]

Originally Posted by JetPlaneFlyer

PS.. the motor will run on 4s just fine. The only consideration is prop size will need to come down and there comes a point where the prop is impractically small for the plane in question. If the motor is very high Kv you might need to watch the RPM doesnt get too high for the prop.

Just trying to get my head around the suggestion of using a 4s battery. Hope this doesn't sound like a stupid question but here goes. Based on the fact that watts = volts x amps, if i use the same motor and prop combination, will it draw the same amount of amps on a 4s as it does on 3s??? Logic, well my logic at least, would suggest that using more volts would mean less amps but then i'm still very new to this game?? Hope that question makes sense.

[hayofstacks]

Originally Posted by npowell28

Just trying to get my head around the suggestion of using a 4s battery. Hope this doesn't sound like a stupid question but here goes. Based on the fact that watts = volts x amps, if i use the same motor and prop combination, will it draw the same amount of amps on a 4s as it does on 3s??? Logic, well my logic at least, would suggest that using more volts would mean less amps but then i'm still very new to this game?? Hope that question makes sense.

Yes, and no. 400w on 3 cells and 400w on 4, the 4 cell would use a smaller amount of amprage for the "same" power output.

BBBUUUUTTTT!

Power is achieved by moving air over the prop. The speed is basically based on the kv rating of the motor. For example, a 1000kv motor would spin 10,000rpm's at 10v's(just to make the math easy...) now increase that by 1/3 voltage (3-4 cell), and it now tries to spin that same sized prop 13,333 rpm's. This will greatly increase load on the prop as it is spinning that much faster, and your wattage and amperage will go through the roof.

Got to www.headsuprc.com and look at some of the motors that are rated at 3-6 cells. On 3 cells,you can get close to the same power (wattage) and thrust with maybe a 17" prop, but on c cells,it might only be a 11" prop. The 3cell setup will also use quite a bit more amoerage for the same thrust numbers then a 6 cell, although power output is esentally the same.

[pizzano]

Hmmmmm......

"Power is achieved by moving air over the prop. The speed is basically based on the kv rating of the motor."

Completely lost me on that tid-bit of info ........."power" as in thrust, "power" as in torque or "power" as in air speed...?

I thought the "prop" provides the "air" flow moving over/under the control surfaces..and the amount of air flow is dictated by prop rpm, diameter, pitch......?

Also thought the "power" to the prop was initiated by the torque and rpm's the motor provides and decreased or increased by the prop characteristics, environmental conditions, plane weight/wingspan, voltage and wattage the motor can reasonably maintain under the aformentioned conditions....?

Guess I've just been confused all this time.....since it is a bit confusing after all....lol

[npowell28]

Just when you thought you were reaching a point where it was sorted in your mind, it all goes fuzzy again. Hmmmmmmm.

[Rockin Robbins]

The reason there is some dihedral in just about all low wing airplanes is that the low position of the wing, interaction with the fuselage gives an effective negative dihedral, which makes the plane negatively stable in the roll axis. In other words, the plane's default response to an outside force is to roll even more in the wrong direction. It's a plane with a death wish and very difficult to fly, especially by remote control.

For an aerobatic plane, designers put in enough dihedral to cancel the negative dihedral effect of the low wing to make either a neutrally stable plane or one with a slight positive stability. In a trainer you might want to go to a medium positive stability.

Because the amount of negative dihedral effect is dependent on the shape of the fuselage it isn't possible to do much more than copy an existing design and be prepared to change the dihedral for fine tuning. The one to one and a half inch per panel recommendations earlier seem like a good place to start.

When designing your own plane, it pays to build so that you can change things around as you tweak. Some people rebuild planes many times before they feel they've perfected the design.

When you're buying RTFs you can tell the companies that took the time to do the tweaking process to make a great flying plane and the companies who just built something and marketed it. It's the difference between the Parkzone T-28 and some of the Banana Hobbies junk. The Horizon plane, having been built and rebuilt to make it fly right won't have the bad habits of most of the BH planes. That's one of the reasons people pay more with a smile.

[npowell28]

Originally Posted by JetPlaneFlyer

Aren't you guys going a very small on the motor? A 74g motor is a bit weak for a 4lb trainer type plane, no? Consider that if you get 300W on a fresh pack you will be down in the low 200's by the time the pack is run down. That's going to leave you pretty marginally powered.

I just don't see the point in being stingy on the motor when you could put a bigger one in for near enough the same price. The bigger motor will also be more efficient when run at part power rather than thrashing a small motor at it's limit the whole flight.

I haven't test flown this build with the smaller 35-36 motor yet but i have decided to fit a bigger one. Bought a XYH 48-50 800kv from Giantcod.co.uk. I'll fit it with a 13inch prop. Looking at the figures online should be good for 370 watts giving 88 watts per pound. I tested my batterys the other day on a 6 minute full throttle run and they didn't drop below 10v so this motor should be fine. I bought a load of 3s batterys so really want to use them as standard for my builds. I can't see me building anything bigger than 60 inch so they should be ok. However as i've said before i'm still learning all this stuff so i may well be buying some 4s in the future.