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Flying Wing launch dolly

Old 07-25-2016, 12:59 PM
  #1  
severnside
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Default Flying Wing launch dolly

Just finished building an Orion 7 (Horten) electric pusher flying wing 3.2M span.
I had not realized that it is not suitable for hand launching & needs a bungy or dolly launch. Not keen on bungy so am looking at making a dolly. Anyone have experience of this please. Model is rather heavy at 2.68K.
Also posted in flying wings
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Old 07-25-2016, 02:07 PM
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fhhuber
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That will need a WIDE dolly

Typical is a trike dolly with steerable nosewheel. We have found that you can plug the dolly nosewheel servo to the aircraft RX with the servo connector filed down a little to make it slide out of the extension plug reliably with just the weight of the typical "standard" servo.

Main wheels appx 1/3 to 1/2 wingspan track width. The mains will need to be on appx 50% MAC line.

Nose wheel forming an equilateral triangle with the other wheels. You really don't want the oversensitive steering of having it further back. Further forward is fine if desired.
That will most likely put your nose wheel forward of the model's nose.

The frame cradling the wing high enough for decent prop clearance and preventing the wingtips or prop from hitting if you have the 3 wheels on the ground and the wing takes a 30 deg up angle.

simplest release is best. We've used trapping a small dorsal fin in a slot. Appx 1/2 root wing chord X 1/16 inch wide plywood fin with well rounded leading edge and straight vertical trailing edge appx 1.5 inches tall in a 1/8 inch slot 1/4 inch longer than the fin. (excess slot length to the rear)
Fin height chosen for our paved runway... if the model tends to bounce out of the dolly early make the fin taller.

The fin also acts as a landing skid. Make sure it is VERY well attached to the model.

I strongly recommend skid-fins appx 1/2 span and at the tips on that model to reduce damage from landings.

Mount the nosewheel steering plug forward of the dorsal fin

Cradles for the wing sections at the width of the main wheels and centerline should hold the wing at appx +1 to + 3 deg AOA relative to the ground. More than +5 can make the model stall when it lifts itself off.

Don't force it to lift early... your elevons should have authority to pull the nose up long before the plane is ready to fly. When you see the model start to leave the dolly apply gentle up for appx 10 deg climb and if nothing is binding, the dolly should never leave the ground.
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Old 07-25-2016, 03:47 PM
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severnside
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Thanks for reply. Will try to absorb your info & do a drawing.
Never used a bungy or dolly before. Included a pic showing dorsal fin. Velcro can now come off! Could not get anywhere near enough speed with hand launch.
The drawing for this model shows 2 bungy positions on the fin, the second one is under the velcro (close to CG). Why 2?
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Old 07-25-2016, 08:58 PM
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That fin might work to keep the model centered in the dolly... but may be marginal for dragging the dolly

Maybe add verticals to the front of the center wing saddle to catch the leading edge.

Don't put posts out at the side saddles.... they always release one before the other and cause the plane to yaw severely as it comes off the dolly.
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Old 07-25-2016, 09:24 PM
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That's a really nice looking model you have there. The Horten designs are sooo graceful, she will look great in the air I'm sure.

I've no experience of dolly launching anything that big but on the smaller models i've used a dolly on the challenge was having a secure enough attachment to the model so that the dolley stayed secure and aligned while on the ground, yet also 'loose' enough so that it didn't snag and remain attached to the plane when it lifted. A servo operated release is helpful but would be a hard retrofit.

A bungee should also work but i'd imagine that finding the correct attachment point would be very critical on a tailless design. It might be good policy to make a smaller scale 'chuck glider' and experiment with bungee launches on that?

Good luck with however you get it in the air!
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Old 07-26-2016, 07:17 AM
  #6  
severnside
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Much to ponder on. Do not hold your breath.
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Old 07-26-2016, 09:26 AM
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You can go to a servo released dolly... but that adds complexity and potential failure points.

If you go to a servo release, you should work to have the CG the same with the dolly attached as without. The model is very likely to be able to fly with the weight if it doesn't alter CG. (been there)
With that option... you could get it flying, then do a low relatively low speed pass over grass and drop the dolly. Landing in high grass at low speed shouldn't break it.

The primary potential failures are:

Failed release... the dolly stays with the plane the whole flight. No big deal.
Partial release... the dolly hangs unevenly on the aircraft. This can cause a crash due to the CG shift.
Uncommanded release... which has the model come off the dolly before its ready to fly.

******************

Looking back a bit you asked about the multiple hook points.

the forward point is typically for high power or high wind launch.
The more rearward would be for a gentle almost kite-like launch.

The basic idea is the towline acts as a weight and the further forward, the more it tends to pull the nose down. The nature of a flying wing is to nose up with increased airspeed. High power launch pulling at the aft location would potentially bend and break the wings. Pulling from the front tends to not launch as high, but with higher airspeed.

Towline/bungee/catapult launch of a wing is a little tricky due to them being unstable until they have some airspeed.
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Old 07-26-2016, 11:03 AM
  #8  
severnside
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Had this response from the person I bought the plan from (Raimund).
`You must start the orion with a little rubber (20 Meters). You can not start the orion with hand launch the speed is to slow.
You begin the test Start with rubber shot and fly 200 Meters with no curves and looking the model.
Make Elevators with high 2mm then the CG is ok and you can testing with elektric Motor and high fly.'
Any thoughts?
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Old 07-26-2016, 11:10 AM
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JetPlaneFlyer
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Sounds like he has tested with a bungee. See if you can get more details of the attachment point he used and the 'pull' of the elastic.

Generally with a bungee the further back the hook is located the more that the nose pitches up and the faster it climbs. Too far back and it becomes unstable and will spin into the ground. Too far forward and it doesn't climb.. Start forward and work back is the safest approach.
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Old 07-27-2016, 11:17 AM
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severnside
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Got this back from Raimund,
`The bungee rubberpower middel -> strong, lenght rubber 20 Meters then strain about 15 Meters. Front hook position.'
Does this make sense?
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Old 07-27-2016, 11:54 AM
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Yep... more of a catapult launch than a high start. Not a lot of height from it.

Actually, it helps to know the tension at the hook when the rubber is stretched.
Different rubber will have different "spring constant" kg tension per meter of stretch.
Knowing the tension he used will allow you to use a fish scale to prevent applying more power than the aircraft can survive.
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Old 07-27-2016, 12:11 PM
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severnside
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OK will try again.
He has asked for my build pics so will ask when I send them.
Might take a while. Is this tread the best way to stay in touch?
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