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Help! Pun Jet

BigD

Junior Member
#1
I'm building a mini Pun jet and I have a question about the prop with the motor mounted aft, do I need a tractor prop(CCW) or pusher prop(CW) for it to fly properly? Also, with the prop at the tail end, is there some precaution needed when hand launching to avoid a prop strike to the hand?
 

jaredstrees

Well-known member
#2
You generally would use a pusher prop, but really the tractor prop will work. just make sure that the numbers are facing the front of the plane. I've used tractor props on pusher planes often. As far as launch goes, haven't tried a pujet yet. I hear they are difficult to fly. I would probably hold a wing and launch it side arm (like you're skipping a rock), slightly up at about 50% power. You should be able to feel when there is enough throttle to push the plane. Good luck!
 

kdobson83

Well-known member
#3
Good luck with the pun jet. I built one last summer and it was the squirrellyest plane I've ever flown. Had just over a battery in it and blew it up in a full nose first explosion. Lol
 

Merv

Well-known member
#4
do I need a tractor prop(CCW) or pusher prop(CW)
It only matters if you are using a glow engine. They don’t run very well backwards. With an electric motor, it does not mattet which prop you use or where you place it, as a tractor or pusher. An electric motor is happy running forward or backwards. The only potential problem is with the prop adapter, if you run a motor backwards it’s possible to unscrew the prop nut. It’s an easy fix, just snug them up.

I agree with @jaredstrees, make sure the numbers on the prop are towards the front, the direction of travel.
 
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mrjdstewart

Well-known member
#5
Pun jet is a hand full as mentioned but with enough expo and throws set properly it is manageable. when i launch i use an underhand toss up in the air. just make sure your arm is still swinging up and back as you release. your hand will be clear of the prop if you do. i tried over hand launching it but on my last attempt got a painful reminder of what happens if you don't get you hand out of the way quick enough.

good luck,

me :cool:
 

BigD

Junior Member
#6
Thank you all for your fast response, I'm really impressed with so much helpful information! It's good to know that I can use either tractor or pusher props, but what I didn't think of would be using a tractor prop, which I have an abundance of, and simply switching two wire leads to the ESC and have the motor run backwards. If I understand what Merv has to say, that should result in a "pusher" prop effect. Is that correct Merv? Also, I would assume the tractor prop should be orientated with the numbers facing forward, even if it's being used in the "pusher" mode. Is that correct?

About it being "squrrelly", the flite test building vid kind of mentions as much, saying to tone the the initial throws to 12 degrees(or maybe even less) and using lots of expo, maybe 50-60%. I'll do that for my first flight and hope to get it flying long enough to be it trimmed out and mannerly before moving onto anything more aggressive. I also hope to get help from a club member to hand launch it for that first flight, but I would want to be sure they didn't hurt themselves.( It's hard to maintain cordial relationships with your flying buds on the way to the hospital to get their fingers sewed back up! Not to mention having them call you "colorful" names for the rest of the flying season.) I'm familiar with both the underhand toss and the side arm toss( I had the flite test F22 and would launch it either way) I asked the question because in the building video Josh over hand launches the Pun jet on it's maiden and I was surprised at that. The FT guys are usually pretty careful about safety so I was wondering if there was something about this tiny jet's configuration that made it safe to overhand launch.
 

kdobson83

Well-known member
#7
Props are designed to be efficient in one direction. And they usually put the size markings on the front of the prop. So the size numbers should pretty much always face the direction your plane travels. I say pretty much because there is always an odd ball/rebel out there who doesn't follow the "rules". Lol Now, you can run the prop backwards, and it'll still produce thrust, but it'll be louder, less efficient, and produce less thrust.

So, the moral of this story is to make sure the size markings on your prop faces the direction your plane flys, and make sure your prop nuts are good and snugged up. Use nylon locking nuts or if you have one of those mini spinner style nuts, use a scrap piece of landing gear wire or something to stick through the tiny hole and snug it up nice and good. I do this and have never had a prop come off. And I'm sure I run a number of my motors the "wrong" way according to it's thread pattern.

Keep note tho, on tractor style planes, especially on minis, the motor pod/mount might have thrust angle build into them to counter act the torque roll the motor produces. If you use a motor going clockwise on a pod that has right hand thrust angle built into it, you'll have torque roll issues.
 

Merv

Well-known member
#8
If I understand what Merv has to say, that should result in a "pusher" prop effect. Is that correct Merv? Also, I would assume the tractor prop should be orientated with the numbers facing forward, even if it's being used in the "pusher" mode. Is that correct?
Yes, with electric, I don't care if the prop is CW or CCW. It will work equally well as a tractor or pusher. For all props, the numbers should ALWAYS face the direction of travel, towards the front of the plane. Make the motor spin which ever way you need to by reversing any 2 of the 3 wires.

If you use a prop with the numbers facing backwards, you will substantially less thrust than you should get. Something like 1/4 of the thrust you should get.
 
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BigD

Junior Member
#9
Okay, thanks again for the info. So, my take away here is:

Using a tractor prop, one designed to rotate CCW as viewed from in front of the airplane,( with the motor turning the prop CCW) with the numbers facing forward in the direction of travel, will give me the better efficiency/thrust arrangement.

Or, if choosing to use a "pusher/reverse" prop, one designed to rotate CW as viewed from the front of the airplane, with the numbers facing forward in the direction of travel, will give me the better efficiency/thrust.

But, NOT to use a tractor prop/designed to rotate CCW, with the motor rotating CW to emulate a "pusher" effect.

Thanks again to Merv and kdobson83, you both have been a great help.
 

Merv

Well-known member
#10
But, NOT to use a tractor prop/designed to rotate CCW, with the motor rotating CW to emulate a "pusher" effect.
No, with electric motors, I don’t care if the prop is CW or CCW. BOTH can be used as a tractor or pusher. If your prop is on the front (a tractor) it could be either CW or CCW, you’ll have the numbers pointing away from the motor, toward direction of travel. Then take the same setup and make a pusher out of it. You MUST take the prop off the motor and flip it, now with the number pointing towards the motor and the front of the plane. Also MUST reverse the direction the motor spins. This is the case with both CW and CCW props.

With electric motors the terms “tractor and pusher” ONLY applies to where the prop sits, on the front or on the back. These terms DO NOT apply to the rotation of the prop CW or CCW. The prop MUST rotate in the direction it is designed for either CW or CCW, no mater where is sits on the plane, tractor or pusher. It works this way because you can reverse the spin of an electric motor. You could not easily do that with a glow engine.
 
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BigD

Junior Member
#12
Okay, sounds like some experimentation is in order here to help confirm I got things right. So, all my props are CCW and I've always thought of them as "tractor" and almost always my experience has been with the motor mounted on the front of the airplane with the numbers pointed away from the motor and towards the front of the airplane and direction of travel.. I did have the flite test FT22 and it had a tail mounted motor. In the building video they were explicit about mounting a CCW prop with the numbers pointing to the front of the plane so that's what I did and it worked well. That was a few years ago. With the Pun jet building vid they say little about the prop. Sorry, Merv, for being so slow on the uptake with this concept, but if I now FINALLY get it, my CCW prop, mounted on the back end of the Pun jet, with the motor rotating in a CCW direction, and with the numbers pointing toward the motor and the front of the airplane (kind of like the prop were on backwards), I will have a thrust force that "pushes" the airplane forward sort of like it would do if it were mounted on the front and "pulling" the airplane forward as with all my other planes.
I usually put all my new builds on a watt meter to test for the best prop selection for my motor/ESC/battery set up. While doing so I can easily test for what effect reversing the motor rotation will have, thanks for that idea jaredstress.
Once again, you guys have been GREAT and I can't thank you enough. If I knew you personally I'd take you out for a beer.
 

Merv

Well-known member
#14
I will have a thrust force that "pushes" the airplane forward sort of like it would do if it were mounted on the front and "pulling" the airplane forward as with all my other planes.
Yes, this is correct.
The problem happens when you flip the motor, this reverses the rotation relative to the front of the plane. In the glow days, you could not easily fix this, that is reverse the spin of the engine. This gave rise to the terms "pusher prop" & "tractor prop". Today these terms are just obsolete to describe the prop.