Fix for sloppy control horns?

[Flying Dutchman]

I just finished my Prelude and plan to maiden it tomorrow. However, I have a bit more slop in the control horns on the flaps and ailerons than I like (potential flutter problem).

I used the supplied MP Jet clevises (with brass pins) and FG horns, but probably drilled out the holes in the horns a bit too much. The servo end is a Z bend, and seems OK. Are there any tricks for fixing this problem, other than cutting out the horns and replacing them?

@Wayne in his Knief Racer build report mentioned putting some thin CA on a Z bend link to tighten things up, but I wonder whether that would also work on the MPJ clevis and FG horn? I don't want to risk turning a hinge point into a fixed solid joint.

Thanks in advance,
Steven

 

[Konrad]

I too would be concerned with the MPJet plastic housing bonding the the control horn. I don't think the prelude will get going fast enough to flutter. But you might see more centring issues than you would normally want.

Do you have a set of wire drills? If it was me I'd fly her as is. If need be I'd drill the horn for a hole 0.3mm oversize, pack the hole with 15minute epoxy. The after it cures redrill the horn to size. In the end I'd be looking to have a 0.15mm to 0.2mm walled epoxy bushing in the horn.

 

[Flying Dutchman]

I too would be concerned with the MPJet plastic housing bonding the the control horn. I don't think the prelude will get going fast enough to flutter. But you might see more centring issues than you would normally want.

Do you have a set of wire drills? If it was me I'd fly her as is. If need be I'd drill the horn for a hole 0.3mm oversize, pack the hole with 15minute epoxy. The after it cures redrill the horn to size. In the end I'd be looking to have a 0.15mm to 0.2mm walled epoxy bushing in the horn.

Thanks Konrad - very clever idea to oversize and refill the hole. I do have a small hand drill with lots of bits, just not the diameter of the MPJet clevis pin. I found out that 0.1mm difference is huge!

But indeed, I'll find out once it's in the air. Your Samsara write up sensitized me to flutter issues...

 

[Andrias]

A little drop of glue. drive the servo back and forth while the glue dries ?

 

[Konrad]

Yep, 0.1mm is 0.004 in the king's units! Are you aware of what is called a pin vise.

The flutter in the Samsara is as a result of structure and trim, not linkage. But basically folks try to fly the Samsara just too fast. I hope you have been able to see some of the videos I posted showing the performance of the Samsara even with a broken rear spar. She is no slouch!

Andrias, would you still use CA glue with a fiber horn and plastic clevis body of the MPJet clevis?

P.S.
I guess one could wax the MPJet black housing and then do the CA trick.

 

[Flying Dutchman]

Yep, 0.1mm is 0.004 in the king's units! Are you aware of what is called a pin vise.

I'm not familiar with the "pin vise" name, but that is basically what my "hand drill" looks like. I grew up metric, and all my bits are in mm.
Unfortunately my caliper has only 2 significant digits, so that leaves more room for error than I like. 2 seemed enough when I bought it...

 

[Flying Dutchman]

Unrelated question about flying the Prelude: how do you keep the flaps from damage or worse during landing?
Unless I have enough head wind I typically use full crow all the way, so either I need to have the presence of mind to retract them at the last second before touch down, or come up with some protection scheme. My previous flapped gliders had a bit more ground clearance for the flaps, wing skids was enough.
I already put wing skids at the polyhedral point, but they won't keep the flaps from touching the ground. A disadvantage of polyhedral designs.
Our desert strip is rather unforgiving (hard dirt & gravel).

 

[Konrad]

2 significant digits in the metric system is more than adequate. My quick metric mikes are spec'd to 0.02mm

 

[Konrad]

Unrelated question about flying the Prelude: how do you keep the flaps from damage or worse during landing?
Unless I have enough head wind I typically use full crow all the way, so either I need to have the presence of mind to retract them at the last second before touch down, or come up with some protection scheme. My previous flapped gliders had a bit more ground clearance for the flaps, wing skids was enough.
I already put wing skids at the polyhedral point, but they won't keep the flaps from touching the ground. A disadvantage of polyhedral designs.
Our desert strip is rather unforgiving (hard dirt & gravel).

Generally it is a timing issue. There is a sharp wind gradient when the model is about at hip level. To maintain flying speed we retract the flaps as the ship passes through that altitude. I actually try to dive a bit to maintain air speed to touch down.

Now I have to be honest and admit that I often am a bit late retracting the flaps. In that Samsara thread you will find details on how to make an override push rod.

I don't see how the wing dihedral has any effect on this.

 

[Flying Dutchman]

Generally it is a timing issue. There is a sharp wind gradient when the model is about at hip level. To maintain flying speed we retract the flaps as the ship passes through that altitude. I actually try to dive a bit to maintain air speed to touch down.

Now I have to be honest and admit that I often am a bit late retracting the flaps. In that Samsara thread you will find details on how to make an override push rod.

I don't see how the wing dihedral has any effect on this.

Good advice, I'll practice retracting a bit sooner.

Not with dihedral, but polyhedral. With my straight wing gliders (e.g. ASW28, Discus), a skid at the tip also raises the inboard portion high enough to keep the flaps off the ground. In my experience the tips generally lend themselves better to mounting skids or other spacing devices than the polyhedral inflection point.

 

[Konrad]

Di, Poly or multi-hedral isn't the issue. The problem is when the flaps extend past the bottom of the fuselage when fully deployed . I still fail to see how the placement of the skid makes much difference in protecting fully deployed glider flaps.

 

[Flying Dutchman]

Di, Poly or multi-hedral isn't the issue. The problem is when the flaps extend past the bottom of the fuselage when fully deployed . I still fail to see how the placement of the skid makes much difference in protecting fully deployed glider flaps.

They don't extend below the bottom of the fuse - there is still about 1 cm ground clearance. But they will touch the ground with the least amount of tilt.

 

[Konrad]

When talking about wing skids, I hope we are talking skid pads and not protruding fins. Protruding fins from the bottom of the wing have an adverse effect of the stability one is trying to gain with dihedral. They lower the EDA.

I made these from a plastic spoon. They offer no protection for the flaps. They protect the bottom of the wing from small pebbles.

 

[Wayne]

That spoon would probably be more aerodynamic if you turned it around. (Or better yet put some pudding into and place it into your mouth.. Yummy..)

I lift my flaps just before touchdown. This method is not perfect, sooner or later you will screw up the timing, but it does do the trick. Knock on wood, I have yet to damage a plane with this method.

 

[Konrad]

I think you are correct.

 

[Flying Dutchman]

Maidened my Prelude today, it flew beautifully. First thing I checked after trimming for level flight was pitch-up response to flaps and crow, which was mild. Since I had not programmed any down elevator in the mix, I was a bit concerned about using them on final, but the headwind was strong enough that I didn't need them to slow down. Per Konrad's advice I made sure they were retracted early, and it worked out well.

And yes Konrad, I'm using the dreaded fin-type skids, they stick up about 3/4" - see below. As you can see, the total area is less than that of the servo cover, so I doubt that this will have any effect. BTW, those wingtips are about 2" off the ground when the polyhedral 'knee' touches down, so that would require a pretty serious size spoon. For now I'll just make sure I'm clean when touching down.

 

[Konrad]

Now I like to use a tablespoon at that poly kink. Again to protect the bottom of the wing not the flaps.

Are you using crow or just flaps?

Did you do any work to find the CG?

 

[Flying Dutchman]

Now I like to use a tablespoon at that poly kink. Again to protect the bottom of the wing not the flaps.

Are you useing crow or just flaps?

I use crow.

 

[Konrad]

...

And yes Konrad, I'm using the dreaded fin-type skids, they stick up about 3/4" - see below. As you can see, the total area is less than that of the servo cover, so I doubt that this will have any effect. ...

View attachment 6775

Another reason I like top drive control surfaces

 

[Glenn Beer]

2 significant digits in the metric system is more than adequate. My quick metric mikes are spec'd to 0.02mm

View attachment 6772

Isn’t 0.02mm = 2 microns ? Pretty small.