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Mar 29, 2010, 09:23 PM
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Thread OP
St. Martin, Thank you for posting picture

Wow, I was lucky asking this question. I almost use the nose gear as a tow hook.
Last edited by tracytown; Mar 30, 2010 at 08:15 AM.
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Mar 29, 2010, 09:27 PM
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Quote:
Originally Posted by LuvEvolution7
Fuzz, I totally agree with you. what I disagree with is the fact that that is where I used to put the hook for bungee launching a glider.
A tow hook position for a glider is to gain altitude.

Bob
Mar 29, 2010, 09:39 PM
Registered User
LuvEvolution7's Avatar
Bob and Stu....................agreed. but we're still talking about moment arms here and, no matter how long the arm is, 50% is putting too much weight forward. at least that's how my puny little brain sees it anyway. too far forward and it's gonna nose dive into the ground. too far back and it's gonna stall and spin.

also, when we're slope soaring, we're not going for height at all, just the best possible speed with a heavily ballasted glider. I've always used the roughly 2/3 position and have never had a problem with a bungee launch, no matter what aircraft I've launched with a bungee.............and I've had a few other than gliders. I also like dolly launching if you've got the room. it's another safe option.

Rich
Mar 29, 2010, 09:49 PM
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Quote:
Originally Posted by Flip Flop
Hey guys, I am new to bungee launching and am going to be trying it soon. I was wondering how you determined the amount of pull on the cord? I've heard before that it was about 5lbs of pull for every 1lb the plane weighs. Is this correct?
Thanks
Yes... 5lbs pull to 1lbs weight is very good... less is interesting...not enough forward speed for good control.

The other thing I learned the hard way, thick bungee is better than thin stretched over a longer distance. The pull weight is the same but the bungee losses energy very quick.

Bob
Mar 29, 2010, 11:53 PM
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Prof100's Avatar
Quote:
Originally Posted by tracytown
I had a e-flite Bae Hawk 15. I just purchase a Jetapult Launcher at fanjetusa. so my question is it ok to use nose gear as a hook to the bungee silicon tubing wire?
Here you go, make a hook and position as per the drawing.
Mar 30, 2010, 12:55 AM
The blade numbers go up to 11
stumax's Avatar
Quote:
Originally Posted by rcjetpilot
The other thing I learned the hard way, thick bungee is better than thin stretched over a longer distance. The pull weight is the same but the bungee losses energy very quick.

Bob
Yep, I use two strands of 3/8" surgical tube, about 5m long, for my F104, and stretch it to 5 x the model's weight. This gives a quick "twang" type launch so the model is on the bungee for the minimum time possible. It's because of this method that positioning the hook relative to the 3D CG position is so critical, as it will be accelerating close to 5G off the ramp, and inertial effects will totally dominate over aerodynamic effects at that point.

Stu.
Mar 30, 2010, 06:02 AM
My project: FAIREY DELTA 1
Erik v. Schaik's Avatar
It is the angle that one needs to keep to a minimal on edf jets. One doesn't need height. Speed and straightforward accelleration is more prio. When the bunghee gets released from the hook you can gain heightif desired.

Hook pos is not that critical but I'd recommend the max angle as on the pic. Vid shows a steep climb which is about the max for an edf jet without exessive G-force on the main wing spars.
Just keep in mind the hook lower as CG or one wouldn't be able to produce lift. Mounting a hook all the way in front of a Sniper or any low winged edf may cause launching issues. That is why a dolly is used for increase AOA at launch.
Uriah BU 1 (1 min 6 sec)


less angle:
stevejets Huckebein 1/2 (2 min 35 sec)
Mar 30, 2010, 06:12 AM
Registered User
CaseySP's Avatar
Hi Stu and all,

Speaking of long noses and high CG's, you have me wondering about the bungee hook on my Alfa MiG21. I selected (guessed if I'm honest) a spot between the CG and nose, but closer to 1/3 distance from the nose.

This might not be close enough to the CG as has been recommended, so it might tend to launch "flat". On the other hand, the battery, RX, ESC, and elevator servo are all above the duct, so the CG is quite high. Hopefully I've stumbled on a happy medium - your thoughts?

BTW I have had four planes I have successfully set up for bungee launching using a similar 1/3 of the distance from the nose to CG (all prop), however this is the first EDF that I have fitted a hook to.
Mar 30, 2010, 06:37 AM
Julian T
I agree with Stu and Fuzz! Having bungee launched several hundred times now I have only ever experienced two problems. First and most common, insufficient bungee tension - 5 times model weight is fine. Second is failing to notice a wind direction change - launching even at 20 degrees out can lead to tears. Other than that, I always err on the side of hook too far forward as I prefer speed to height. 50% is a very good rule of thumb I got from Herb. Too far back is a nightmare to be avoided. Jets are not the same as gliders!
Mar 30, 2010, 06:38 AM
The blade numbers go up to 11
stumax's Avatar
Casey, hang the model by the hook and look at it from the side. Take a pic and draw some lines on it to calculate the angles. BTW, I would have though you could hand launch an Alfa foamie?

Stu.
Mar 30, 2010, 06:53 AM
Registered User
CaseySP's Avatar
Quote:
Originally Posted by stumax
Casey, hang the model by the hook and look at it from the side. Take a pic and draw some lines on it to calculate the angles. BTW, I would have though you could hand launch an Alfa foamie?

Stu.
Thanks Stu,

I can't do that at the moment as the airframe is only mocked together and the electronics and fan are in another model. I was reading in the Alfa manual that in high alpha attitudes the long nose acts like a chine, increasing the nose-up pitching tendency. All in all with what I've read here and the proposed set-up of the model I'm feeling reasonably happy with the hook's positioning.

BTW - I don't hand launch. I just don't like the feeling of being out-of-control for those first few critical moments of a flight. And anyone who has seen some of the Alfa MiG21 videos (admittedly on stock setups) would probably be scared off a hand launch too.
Mar 30, 2010, 07:31 AM
My project: FAIREY DELTA 1
Erik v. Schaik's Avatar
As far in front as possible is good for midwing.
been there, done that
Mar 30, 2010, 07:38 AM
Registered User
CaseySP's Avatar
Thanks Erik. Good to have advice from a couple of the EDF guys who I most respect here. I've always liked how you launch without a ramp - nice and easy on the airframe. Unfortunately we don't have such nicely manicured fields here in drought ravaged Oz, so a ramp is a necessary evil.

Very nice Huckebein too.
Mar 30, 2010, 08:31 AM
Registered User
Thread OP
I got this message from Fanjetusa

The Jetapult® operates similar to a glider high start system. Except the jet flies out parallel with the ground and not straight up like a glider. It utilizes the same type of attachment to the fusalage by means of a tow hook on the underside of the fuse. Place a tow hook on the bottom of the fuse half way between the nose of the aircraft and the leading edge of the wing. It is necessary to have this tow hook reinforced with epoxy, a plywood piece to glue the tow hook into and glass cloth to avoid the hook being ripped out of the fuse.
Mar 30, 2010, 08:37 AM
My project: FAIREY DELTA 1
Erik v. Schaik's Avatar
Quote:
Originally Posted by CaseySP
Thanks Erik. Good to have advice from a couple of the EDF guys who I most respect here. I've always liked how you launch without a ramp - nice and easy on the airframe. Unfortunately we don't have such nicely manicured fields here in drought ravaged Oz, so a ramp is a necessary evil.

Very nice Huckebein too.
Thx!

It is the Alfa Huckebein. The Stevejets Huckebein flys even better! They are retooling the moulds at this momentYou can see the hook position is not critical.


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