Topic: Busa 1/3 Fokker D VII

So the servo is pulling to it's full travel, but the elevator is dropping (with gravity) to it's full extent so making the wire slack?

the servo is pulling the cable to the set degrees of surface travel but after traviling 15 dergress or more the cable gose slack but dose not drop if you know what i meen

What is this measurement?



And what is the same measurement across the servo horn
Is it a straight run from horn to horn?

Sorry for all the questions, the only thing I can think of so far is your ele horn is set back a little from the hinge line 

the control horn is 60mm long 30mm at top 30mm at bottom the hole fore the clevis is at 27mm and the control horn is straight

the control horn is 60mm long 30mm at top 30mm at bottom the hole fore the clevis is at 27mm and the control horn is straight

OK, how does that measurement compare to your servo horn?

the servo horn is about 5mms sorter but i have made an identical horn as the on to the surface and tried that it was still the same

the hole for the clevis is 15mm back from were the surface hinges could this be the problem

Yes.... 

Yes.... 

especially as you have differential on the top and bottom horns.

brill ill alter that then why dose this happen? I'm getting a bit comfused

brill ill alter that then why dose this happen? I'm getting a bit confused

I am confused now too! Someone check me out here.... It's a bit like the differential on ailerons when you drive both from one servo, but the holes are not in line with the centre of the servo horn..

the elevators are driven by 2 servos on this one for each side

I always try and get the hole in the horn above the hinge line to avoid any geometry issues, a bit like this pic



Steve

I think if you alter the geometry so your clevis pulls from OVER (or as close as you can get) the hinge line you should be ok. By moving the point at which the clevis pulls further back (15mm in this case) your inducing a pull 'arc' which is different to your hinge arc. Confused??? Good, cos so am I now 

this is how far back the control horn is from the hinge line                                                                                                                       

Hmm, I would make up a temporary horn, tack/clamp in place as far over the hinge line as possible (90deg up from the leading edge of your existing horn) and give it a try. If it works THEN we can look on how best to put it right.

thanks ill try it know ill let you know later

hi all just tried what mudflap suggested it worked a lot better but the bottom cable still goes slack a bit not as much as before its a improvement

I glad it helped, don't forget you will not need very much 'down' movement anyway.

Ive got some robart horns Ive found i might us those

didnt have robart horns in 1918, whats the problem with cutting your own making them a scale shape, that way you can decide the "quote" clevis hole distance and duplicate this with the hole dimention on the servo arms and link the cable direct insuring the controle horne holes line up with the hinge line. A bit like they have done since early flight, simples..The only A/C I can think of where the controle cables went slack when operated was the tiger moth, and that was for a very different reason. Just a thought but it seemed to work ok way back then so it should work now. regards Ken

Are the cables coming off the horns at 90* to the centre. In yore picture it looked like your horns were vertical but the cables coming off were running down hill, if you see what I mean. That will cause some problems.

Simon

quite obvious now looking at that pic that the horns are way too far back.  The hole for the clevis should be right over the hinge line. That HAS to be it, I could not undersand why it was not working correctly before! Well spotted mudflap!

yes you are right in what you see the horns maybe a bit too big , ill probably make my own horns more scale like

thanks chaps for your help ill post some photos of the nearly completed model when ive sorted the horns

Hi Buzz2, just back from a family New Year thing. From the pics, it does look like the horn is too far back, sorting this will help resolve the issue. I would still have the holes set back VERY slightly though, it really does help with positive centring and less strain on the whole system. Even with all that slack on the bottom cable on your elevator, if the motor was running that cable would be the one doing the work in that situation, so the slack one would be the one not doing any work. The pic Rcfanuk posted shows the horn holes slightly behind the hinge line and is ideal. Also, on the routing cables issue, I too would only guide them when they are changing direction or coming through covering. When coming through covering though, if there is no change of direction, you are supporting the covering rather than the cables. Your DVII looks like a cracker, I'd love to see more pics when you get time

Ian.

Hi Buzz2, just back from a family New Year thing. From the pics, it does look like the horn is too far back, sorting this will help resolve the issue. I would still have the holes set back VERY slightly though, it really does help with positive centring and less strain on the whole system. Even with all that slack on the bottom cable on your elevator, if the motor was running that cable would be the one doing the work in that situation, so the slack one would be the one not doing any work. The pic Rcfanuk posted shows the horn holes slightly behind the hinge line and is ideal. Also, on the routing cables issue, I too would only guide them when they are changing direction or coming through covering. When coming through covering though, if there is no change of direction, you are supporting the covering rather than the cables. Your DVII looks like a cracker, I'd love to see more pics when you get time

Ian.

I never knew that, Ian! I always thought the holes for the clevis had to be exactly on the hinge line. How does it make it more positive?

Exactly on the centre line is perfect if the cables are coming off the surface horn and servo horn at 90 degrees and the horns are the same length etc, but when it comes to scale models, this rarely happens, since the full size rarely do the same. Fokker elevators are an excellent case in question, the horns (surface horn and joystick) are vertical but the cables are far from at the ideal angles geometrically, with the cables both running downhill into the fuselage from the tailplane. If the horns are slightly rearward, it allows a decent tension around centre and no loop tension at full travel, in that instance the cable doing the work will be under tension from airflow. This allows the centred surface to be tensioned just right with no centring issues or binding.

Buzz2, I'd always be tempted to fit a slave belcrank at the front end for a couple of reasons. Practicality, if for some reason you need to remove a servo there is no need to disturb cables (with them disappearing down the fuselage and wrapping themselves round something) or if some trim needs adjusting then again theres no need to disturb cables, just do the pushrod. The second reason is servo life. If the cables go to the horn, if there is too much tension on them, this load is then sent into the servo gear train and can seriously alter their performance or life span. That said however, with the correct tension and cable runs, I have the cables direct to the servos on my Camel with no issues. I used Hitec 645 on each half so they are barely under load.

Ian.

Exactly on the centre line is perfect if the cables are coming off the surface horn and servo horn at 90 degrees and the horns are the same length etc, but when it comes to scale models, this rarely happens, since the full size rarely do the same. Fokker elevators are an excellent case in question, the horns (surface horn and joystick) are vertical but the cables are far from at the ideal angles geometrically, with the cables both running downhill into the fuselage from the tailplane. If the horns are slightly rearward, it allows a decent tension around centre and no loop tension at full travel, in that instance the cable doing the work will be under tension from airflow. This allows the centred surface to be tensioned just right with no centring issues or binding.

Buzz2, I'd always be tempted to fit a slave belcrank at the front end for a couple of reasons. Practicality, if for some reason you need to remove a servo there is no need to disturb cables (with them disappearing down the fuselage and wrapping themselves round something) or if some trim needs adjusting then again theres no need to disturb cables, just do the pushrod. The second reason is servo life. If the cables go to the horn, if there is too much tension on them, this load is then sent into the servo gear train and can seriously alter their performance or life span. That said however, with the correct tension and cable runs, I have the cables direct to the servos on my Camel with no issues. I used Hitec 645 on each half so they are barely under load.

Ian.

Cheers Ian- interesting!