Chris Walby Posted May 7, 2018 Share Posted May 7, 2018 I have a ducted fan ARTF that is proving to be troublesome on take off! The main legs are narrow as they are located and retract into the fuselage and the nose U/C is well forward. The issue is that as it accelerates on the grass it says drifts right and I steer to correct it rolls right (it can touch its right wing tip while heading left direction in severe cases). The front nose leg has an Oleo and is vertically straight (side view) and has an dog leg so that it is in line with the retract from the front view. The question I have is could the steering be improved by raking the wheel back slightly so it has in effect a little trail. I sort of remember bicycles with straight forks have very fast steering and moving the wheel spindle away from the vertical axis helps (bikes normally move it forwards for convenience), Its only a semi scale so function over form is okay...just be nice to get it off the ground! All constructive assistance welcomed and yes it has been suggested to go PSS ....I'll give it a few more goes yet! Quote Link to comment Share on other sites More sharing options...
Former Member Posted May 7, 2018 Share Posted May 7, 2018 [This posting has been removed] Quote Link to comment Share on other sites More sharing options...
J D 8 Posted May 7, 2018 Share Posted May 7, 2018 The problem with ducted fan or jet is there is little airflow over the rudder at low speed. Sounds like you have an F16 or similar type with narrow main UC track not well suited to grass operation. A little rake may help as would making sure there is no slop in the steering and the steering servo is man enough for the job Quote Link to comment Share on other sites More sharing options...
David Ovenden Posted May 7, 2018 Share Posted May 7, 2018 I'm no EDF expert. However, maybe a 3-axis gyro might help. They are so fast in responding to changes in direction that maybe "instantaneous" rudder input from the gyro would stop the veering off course in the first place. And if the model did veer, help with aileron correction too. There are some cheap, small, light ones available now. Some added rake may help but I can't see how removing the steerable nose leg would assist as I imagine rudder alone would be insufficient at low speeds on takeoff. Quote Link to comment Share on other sites More sharing options...
Denis Watkins Posted May 7, 2018 Share Posted May 7, 2018 As you know with Jet nose wheels Chris, they look good straight down and are usually small This results in the steering being very light on the ground I would not cant it back due to not looking right Single heading hold gyros use by helis can be introduced to the steering/rudder servo very easily And as you say, more rudder authority Quote Link to comment Share on other sites More sharing options...
Bob Cotsford Posted May 7, 2018 Share Posted May 7, 2018 I get the same with prop driven models if the u/c is a little tall, my Marabu is particularly bad. I'd suggest that you keep the steering but reduce it's rate, or even have dual rates, a higher one for low speed taxying anf low for take-off and landing. Bicycles and motorcycles have raked forks in order to assist with the way they lean when turning, I don't think it will help here. Quote Link to comment Share on other sites More sharing options...
Piers Bowlan Posted May 7, 2018 Share Posted May 7, 2018 That is a problem with a narrow track landing gear on the ground; rudder left = rolls right and visa versa. I agree, a small giro might help with directional stability - a small correction early is much better than a large one late. I don't know how helpful raking the nose leg back might be, could be worth a try but don't hold your breath! Only other thought I have is to add a couple of wire skids to the wingtips to limit the amount of roll on the ground. Not very scale, although I don't know what the model is, and it might not improve things either Quote Link to comment Share on other sites More sharing options...
Chris Walby Posted May 7, 2018 Author Share Posted May 7, 2018 Percy, I don't taxi anywhere, with 3 min 20 seconds of flight time I plonk it on the end of the runway and want to fly but to do that it has to get off the ground first. It has underneath cheat holes but from the advice I think I'll do the following. Lower the U/C legs Fit a single axis gyro I have lying about Rake the wheel back a bit Change the sloppy steering mech to some very small ball joints Reduce the steering rate I flies quite well once its in the air, its just getting it there under it own power is the problem Thanks for all the advice Quote Link to comment Share on other sites More sharing options...
Allan Bennett Posted May 7, 2018 Share Posted May 7, 2018 If you adopt the other measures on your list, I don't think that raking the wheel back will help any further. It might be worth trying the last two measures first, as they require the least modifications. Then try the gyro if that hasn't made it any better. My DF Learjet tracks perfectly well on grass despite the slightly forward facing front retract (it retracts forwards) and little or no rudder authority at low speed. Edited By Allan Bennett on 07/05/2018 11:17:45 Quote Link to comment Share on other sites More sharing options...
Chris Walby Posted May 9, 2018 Author Share Posted May 9, 2018 Had a bit of bench time with some success And started working on the list in a slightly different order Change the sloppy steering mech to some very small ball joints - Done Reduce the steering rate - Will do! Lower the U/C legs these are 15mm longer than the standard - see question Rake the wheel back a bit Fit a single axis gyro I have lying about - this will be a lot more tricky if it needs to go at the C of G point So the success was changing out the original sloppy fitting link (Oleo retract pin and clevis) as both had lateral play that meant the wheel could (only my theory here) allow the aircraft to turn and then tuck in. the new arrangement has very little play. The new one is a ball joint with some oversize washers to limit the lateral movement. Not easy to see but I used two of the pin to clevis fittings (facing each other) and bolted the ball joint which seems to have removed a lot of the play, . New one installed and old link at bottom of picture The question is do I lower the retracts by 15mm to match the originals (although mine has larger wheels) or try it as it is? I am also toying with the idea of scrapping the Oleos and bending some springs up (once the tool turns up), doe anyone know if a ali Oleo is lighter than a 4mm double coil wire leg? Quote Link to comment Share on other sites More sharing options...
Martin Harris - Moderator Posted May 9, 2018 Share Posted May 9, 2018 Simple gyros work by sensing angular movement so, despite common opinion, they can be mounted anywhere on the airframe as the rate of change will be the same - unless you have an exceptionally floppy model! Edited By Martin Harris on 09/05/2018 22:54:28 Quote Link to comment Share on other sites More sharing options...
Geoff S Posted May 10, 2018 Share Posted May 10, 2018 Posted by Chris Walby on 07/05/2018 07:36:09: The question I have is could the steering be improved by raking the wheel back slightly so it has in effect a little trail. I sort of remember bicycles with straight forks have very fast steering and moving the wheel spindle away from the vertical axis helps (bikes normally move it forwards for convenience), The straightness or otherwise of a bicycle fork leg has no effect on the trail. Trail is determined by the head angle and the fork rake, which, even if the legs are straight, can be set at an angle to the fork crown. Curved fork legs are only used to give a degree of flex and hence suspension. Trail is the distance between a point on the ground which is an extension of the steering head and a line vertically down from the axle. I once had the fork legs of a tandem straightened a little to increase the trail because the steering was too light. BMW motor cycles with Earle's leading link forks had 2 positions for the axle so the trail could be reduced when a sidecar was fitted. Geoff Quote Link to comment Share on other sites More sharing options...
Trevor Posted May 10, 2018 Share Posted May 10, 2018 Well put Geoff! All of which means that, with the oleo/steering setup illustrated above, to achieve a 'trail' or castor effect, the whole retract unit would have to be angled so that the leg was raked forwards. I'm not advocating this, just trying to emphasise the need to think very carefully about these things as your post describes. Trevor Quote Link to comment Share on other sites More sharing options...
Nigel R Posted May 10, 2018 Share Posted May 10, 2018 Percy, trikes can work on grass without any problems (I have two in the hangar right now) - I don't think suggesting trikes are inherently unworkable from an unpaved runway is that helpful, no? Chris, I would be inclined to try what you now have. If you've cut out play within the steering links then the wheel will now at least be pointing the direction you tell it to, which sounds like 90% of the battle. "bikes normally move it forwards for convenience" Geoff covers it quite well, but bikes most defintely do have "rake backwards" (trail). Usually an inch or two. And more trail = slower/heavier steering. "I am also toying with the idea of scrapping the Oleos and bending some springs up (once the tool turns up), doe anyone know if a ali Oleo is lighter than a 4mm double coil wire leg?" You have a wire bender, I assume? I'd imagine the simple sprung wire would be as light or lighter than then Oleo. But you can work out the weight of a coiled leg quite easily, if you know how long the leg is. FWIW the last coiled nose leg I bent (single strut and spring with three coils, from 10swg / 3.5mm wire) needed about a foot of wire at 0.8 oz per ft; next size up 4mm wire is 1.25 oz/ft. However, unless there is directional play within the Oleo itself, would a new leg help anything? Quote Link to comment Share on other sites More sharing options...
Neil R Posted May 10, 2018 Share Posted May 10, 2018 Sorry not helpful - but recently a colleague of mine described the unwelcome manoeuvre as the 'Robin Reliant Roll' - I'd not heard that in an aviation context before, and was quite tickled at how perfectly it fits! (I'd guess any lowering of the cg would help, but I wouldn't expect that to be practicable). Quote Link to comment Share on other sites More sharing options...
Chris Walby Posted May 10, 2018 Author Share Posted May 10, 2018 Neil R, thanks for the advice and comments + the others as for theres have been most helpful. Are yes the RRR manoeuvre describes it well. The current U/C are 15mm longer than the originals that self destructed on the first take off and if I cut and re-drill/tap the front Oleo I can get it back to the original height. The rear oleos are just a case of shorting the pins and spacers only thing is once removed I can't put it back easily. I think shortening will help so I'll do it. and then give it a go. Apologies as it was off at a tangent, but I keep ripping the retracts out of the foam and was thinking the Oleo has difficulty compressing with a lot of lateral force hence a coiled wire version would spring back and up which might be better for me. Quote Link to comment Share on other sites More sharing options...
Nigel R Posted May 10, 2018 Share Posted May 10, 2018 "I keep ripping the retracts out of the foam" The plot thickens... Is the retract on a plate? The old school trick for retracts in foam was to poke some dowel through the corners of the plate and into the foam, it increased the glue surface area and reduced the leverage from the legs that your hard structure had to resist. Shorter legs would also help! What sort of size/weight is the model? As a last resort, what about a dolly? Quote Link to comment Share on other sites More sharing options...
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