Prop questions

[Beemerider]

I am learning to fly again and have questions concerning the right size prop to use on my Explorer 2M sailplane.

This is the motor I installed:
http://www.ebay.com/itm/160764528391...84.m1497.l2649

I was told to use a 11.1v 2200mAh 20C li-po (which I just bought 2)

On the first flight I had a borrowed prop which was a 9 by 7. It flew very well according to my friend who has been flying for 40 yrs.
Yesterday we flew it again this time using an 8 by 4. It porpoised all over the sky acting like it was tail heavy. The only change was the prop. We brought it down, and added some down thrust which helped but it was still a tad tail heavy. My mentor tells me the motor is happier with the new prop--lower current draw, efficiency and so on--and I understand those dynamics. I'm trying to understand how a plane can fly so much better with the wrong (?) prop. And so much worse with the right (?) one.

To be clear I had no down thrust with the original flight. We just put a battery in it, checked balance (it was slightly nose heavy) fired it up and tossed it in the air. Actually it pretty much flew out of my hand.

I'm going out in the morning in search of some props but not sure what to look for. Advice?

regards
Wayne

[kyleservicetech]

Originally Posted by Beemerider

I am learning to fly again and have questions concerning the right size prop to use on my Explorer 2M sailplane.

This is the motor I installed:
http://www.ebay.com/itm/160764528391...84.m1497.l2649

I was told to use a 11.1v 2200mAh 20C li-po (which I just bought 2)

On the first flight I had a borrowed prop which was a 9 by 7. It flew very well according to my friend who has been flying for 40 yrs.
Yesterday we flew it again this time using an 8 by 4. It porpoised all over the sky acting like it was tail heavy. The only change was the prop. We brought it down, and added some down thrust which helped but it was still a tad tail heavy. My mentor tells me the motor is happier with the new prop--lower current draw, efficiency and so on--and I understand those dynamics. I'm trying to understand how a plane can fly so much better with the wrong (?) prop. And so much worse with the right (?) one.

To be clear I had no down thrust with the original flight. We just put a battery in it, checked balance (it was slightly nose heavy) fired it up and tossed it in the air. Actually it pretty much flew out of my hand.

I'm going out in the morning in search of some props but not sure what to look for. Advice?

regards
Wayne

Do you have access to an DC ammeter, or wattmeter? One thing that must be done is to make absolutely certain you're not overloading your motor/ESC/Battery.

[Beemerider]

I personally don't but my mentor has said the next thing we need to do is check that very thing. He has one. I will have one soon. On our last flight yesterday we broke the 8/4 on landing (had a prop saver on the motor) Hit just the wrong way I guess. I'm also looking for a folding prop.

[mclarkson]

I'm not sure that answers the original question, though. Why does the change in prop so radically affect the plane's flight characteristics?

That said, the 9x7 prop is doing more than two times the 'work' - in terms of moving air - of the 8x4 prop. (An 8x4 prop carves out a cylinder of air 8" wide by 4" deep per revolution - about 200 cubic inches. A 9x7 prop, on the other hand, carves out about 450 cubic inches per revolution. More or less. In theory.)

In theory, it is giving you over twice the thrust. To the extent that that bears out in the real world, that amount of thrust might to some extent overcome deficiencies in the plane's setup - dragging it through the sky by the nose, rather than letting it fly.

[Beemerider]

On the first flight it seemed the plane flew and even did well with the motor off (to my untrained eyes) and even my coach said it was seemingly set up right. We had maybe 5 to 6 mph winds. 2nd flight w/smaller prop and no wind it was difficult to control. I have much to learn. But at least each day I carried plane home intact. I had no such luck 20 yrs ago.

[pmullen503]

I have the essentially the same motor, different brand. The 9x7 was too much prop for 1600 KV on 3s. The 8x4 is about the largest prop you can safely run. If fact you probably were getting more thrust with the 8x4 than the 9x7 because that small motor can't turn that prop efficiently.

I run a 7x3 on that motor which gives excellent efficiency and long flight times on a powered glider. Set the brake on your ESC to stop freewheeling of the prop for less drag while gliding.

You could use a prop saver instead of a folding prop to give you a broader choice of props. If you do get a folder, get a few different blade sets to find which works best on your plane.

[kyleservicetech]

Originally Posted by Beemerider

I personally don't but my mentor has said the next thing we need to do is check that very thing. He has one. I will have one soon. On our last flight yesterday we broke the 8/4 on landing (had a prop saver on the motor) Hit just the wrong way I guess. I'm also looking for a folding prop.

A windmilling propeller on a sailplane has a LOT of drag. One place to look for folders is Sonictronics. They've been making them for decades. I've still got a few of them, they are near 30 years old. Take a look:
http://www.sonictronics.com/xcart/se...=search&page=1

[tom1968]

If it was a little nose heavy with the 9x7 and a tad tail heavy with the 8x4, that's probably the root of the problem. Thought it's hard to see how the props' weight would be THAT different.

I wonder if pmullen isn't onto something: "you probably were getting more thrust with the 8x4 than the 9x7 because that small motor can't turn that prop efficiently." Maybe the plane was actually flying faster with the 8x4. Did you try just gliding with the 8x4, and did it calm down when you did?

[Beemerider]

I added several degrees of down thrust and a tad right. Have an 8x6 on it now and it's flying pretty well.

I really appreciate all the replies and advice given. I am learning a lot from you guys.

[hayofstacks]

To an extent, the size of the prop may change with the plane too. A very draggy plane will fly at a higher throttle, and might need more prop to get the same speed as a sleeker model. A smaller prop will spin slightly faster under load. In the air it changes with the pitch of the plane. A lot of 3d planes spin a very larg prop so they have maximum thrus at all directions, but a smaller prop will be more efficient, and possibly fly the plane faster. There are actually quite a few tings to consider.

What I have done, is I have a watt meter, and a old tach. With the tach I can tell how fast itsspinning and how many amps it pulls. I want to set up either a test stand to mesure thrust, or a fish scale to pull against the plane to get a better idea on thrust. I would als like to find a program I could use on the phone to tell me prop speed, but that really isn't that big of a deal.