Down thrust?

[Russ P]

I've built a Junior 60 with 2 channels, after a few of flights resulting in crashes I am looking for some advice.

Take off is fine, but after the climb out the model continually loops until it hits the ground! On the one occasion where I was able to close the throttle at the right moment the model glided to a perfect landing, all be it a long way a way.

I assumed that the c of g is correct due to the glide being ok, and adjusted the thrust line to give a couple of washers down thrust. But it still looped under power

How much down thrust is safe, if indeed that is the issue.

Thanks

[Dwain Dibley.]

Russ, more info will be needed.

Power train, weight, where you have the C of G etc etc

Have you checked trims on elevator, not much movement needed on these old birds.

Indoor models can have immense amounts of Downthrust, wing incidence is more important probably.

D.D.

[Dwain Dibley.]

P.S.

Can the wing lift while in flight ??

D.D.

[Frank Skilbeck]

On the vintage models there is a lot of drag on the high wing which combines with the engine thrust to give a large pitching up moment, how much down thrust is safe is however much it needs to stop the plane from pitching up under power. You can either achieve this with down thrust (and vintage models often have a lot of down thrust) or mix in some down elevator with throttle, that is you have throttle control (you say it's 2 channel, is that rudder/elevator only?)

Or go with a lower power setting.

[will -0]

I think the point Dwain is alluding to is that if your wing is held on with rubber bands, but not all that tightly, the wing can lift at the front giving a very large pitch up.

I assume when you say "2 channels" and "closed the throttle" this means that you have throttle and rudder only as controls, is this correct?

[will -0]

If so, you have a fixed elevator/wing incidence* which means that as you go faster the thing will always try to pitch up.

You could get round this with

• lots of down thrust,
• or by using a finer pitch prop/less power (fly slower with the power on),
• or by moving the cg back a little (and reducing the "elevator" setting to match), though I doubt this is the route to go unless it is excessively far forward.

or of course a combination of all three in moderation.

 

*also known as "decalage", not to be confused with decolletage which is something else.

Edited By will -0 on 09/05/2019 13:09:07

[Nigel R]

Plan is here

First, I would check you have the amount of downthrust shown.

Second, check CG.

Third, a Junior 60 doesn't need a lot of power, it's really a slow flying free flight model - even for RC we're only talking in order of 200W or a 0.15 glow. What engine or electric setup do you have?

[Peter Miller]

The old dodge that we used to use in free flight days (And the Junior Sixty is basically a free flight model) was to put the prop on back to front. This reduced the power significantly

Edited By Peter Miller on 09/05/2019 13:55:44

[Don Fry]

If you loop a Junior 60 you are risking the wings breaking under the loads imposed.

Throttle it so from take off the climb out is a gentle climb. Nose up, 5 degrees. When you get to hight, close the throttle so it ceases to climb, and turn it in the direction you wish it to go in.

If you have built it according to the plan, the down thrust is right.

The Junior 60, by reason of its undercambered wing flys at one speed. Feed power in it goes up, same speed. Lose power, it comes down, same speed. It is a good design, but that's all it does

[Nigel R]

I seem to remember looping mine quite a bit. I don't think it was going fast enough to have very much force applied to anything. I only had a wheezy VT 25 four stroke though, so overpowering wasn't an issue.

It will also fly inverted, although it requires "a bit" of down elevator and "a bit" of power and isn't very happy doing it.

And barrel roll - you have to get it inverted somehow.

That thick undercambered section is a bit of a drawback for RC flight. I'd be tempted to make a Super 60 wing for it.

[Don Fry]

A loop is executed from a spiral dive, exiting into wind. No power.

A roll in executed, exit cross wind, rudder mementry into wind to put some weight on the outside wing of the roll, then opposite rudder. No power.

Single channel manoeuvres.

But as you say, it's fine in a fall off loop, but the original poster is describing whooshy multiple things at constant power

[Russ P]

Thanks I'll chech out the points raised. Plenty to think about.

[cymaz]

I had a Junior 60 ( until the wings clapped). It went straight up to the moon on the first flight. I built up the rear of the cabin to raise the wing trailing edge over 1/2”.

I left the engine thrust as it was. 

 

Edited By cymaz on 09/05/2019 17:31:46

[Bob Cotsford]

If it's capable of continuous loops maybe the OP should consider putting a smaller motor in or limiting the throttle movement! More power with models designed for free-flight/rudder only won't help matters, they are pretty much single speed machines. When I built an Eros 84" cabin job weighing 7lbs powered by a wheezy old OS 48 4 stroke I had to do like Cymaz and pack up the t/e of the wing in addition to adding 3 or 4 degrees additional downthrust. It still needed down elevator mixed with the throttleto limit the climb rate.

[PatMc]

Posted by cymaz on 09/05/2019 17:31:09:

I built up the rear of the cabin to raise the wing trailing edge over 1/2”.

I left the engine thrust as it was.

That's effectively adding upthrust.