Aileron Hinge Design

[lr27]

The Sky Pup, which is a well regarded (as far as I can tell) part 103 ultralight, uses fabric hinges on the tail surfaces. Very simple, and presumably very light. I forget whether they are the alternating under and over strips or the type with two strips sewn together so that, in cross section, you have an X. Then there's the old sewn hinges that the CL fliers used to use. I've used all three successfully on my RC models. For the first two methods, it might be a good idea to use a little reinforcement at the ends. There's a drawing showing the cross section in the Sky Pup newsletter, issue 8. machnone.com/news2.html
I can check the plans for a little more detail if you like.

There are no ailerons on tbe Sky Pup, but I suspect this hinge method could be easily adapted.

BTW, it's claimed that the Sky Pup was designed by engineers from Cessna. The main ingredients are wood, foam, and fabric, with a few small bits fabricated from 4130.
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It seems to me that it must be possible to make ailerons that are stiff in torsion but not in bending. I haven't figured out an elegant and simple solution for fore and aft bending, though. 3 pieces joined by pins seems like a total kluge.

The Sky Pup plans are thorough, and I'm sure you could learn a lot from them, if you have the budget. Not a metal airplane, though.

 

[blane.c]

The wing design on the Sky Pup wouldn't allow the addition of ailerons easily. It doesn't fly fast enough for ailerons to be of much use. Remember ailerons are the first control to lose effectiveness as you get slower and the Sky Pup never gets fast. Dihedral and rudder, dihedral and rudder.

 

[lr27]

I should have been clearer that I wasn't suggesting ailerons for the Pup, since the wings as designed aren't meant for it.

OTOH, the Pup is supposed to cruise at about twice stall speed. This is also true of the Volksplane, which uses ailerons. Do people complain that the ailerons on the VP aren't effective? I know they're said to be heavy. My own experience with models is that ailerons seem to work ok at slow speeds, but not without rudder. I've definitely experienced innecective ailerons when used without rudder at slow speeds. I added a rudder to a model I had just for that reason. Also, I put a flying wing UAV prototype into the bushes that way. Fortunately, it was slow and bouncy. Before that I had mostly been flying conventionally configured models, so I didn't have the right habits.

 

[Victor Bravo]

The story I remember reading is that Steve Wood, the Sky Pup designer, was in fact a Cessna angineer.

If the Sky Pup wing cannot handle aileron loads, you could get halfway there without structural changes, by using (differential) roll spoilers.

On a perfectly calm day being flown gently, a super light 2 axis airplane like the Sky Pup or the Quicksilver (rudder and a lot of dihedral) will fly just fine, the dihedral induces a roll command very quickly after the yaw command.

But a super light airplane is far more sensitive to gusts, winds, and thermal lift/sink. So with a super light airplane, when the weather goes from perfectly calm to just a little bit of air movement, very quickly you have to start using larger control deflections and maneuvering much more harshly than if you were in a GA airplane.

That's when it becomes no fun, and nauseating, and you can't lift a wingtip out of the dirt with just the rudder quickly enough on a gusty landing. So the weather parameters you can fly safely in are just far less than if you had 3 axis control on the same airplane.

 

[lr27]

I suspect that diagonal grain sheeting and maybe a thin ply shear web would make the wing torsionally stiff enough, and maybe strong enough too. Then it's just a matter of structure to support a sub spar for the aileron. Or maybe just two extra strong ribs. All significantly more complicated, of course, than the original wing.

When I lived near an airfield, I would only see ultralights in the evening when the air was calm.

 

[cluttonfred]

I doubt very much that the presence or absence of ailerons has any practical impact on the usability of something like a Sky Pup, it's more about speed, wing loading, and inertia magnifying the impact of wind gusts. It's easy to see how that would play out from this quick comparison....

Sky Pup
Wing area: 130 sq ft (12 m2)
Gross weight: 400 lb (181 kg)
Wing loading: 3.1 lb/sq ft (15.1 kg/m2)
Stall speed: 26 mph (42 kph)

J-3 Cub
Wing area: 178.5 sq ft (16.58 m2)
Max takeoff weight: 1,220 lb (550 kg)
Wing loading: 6.84 lb/sq ft (33.4 kg/m2)
Stall speed: 38 mph (61 kph)

Ercoupe 415-C
Wing area: 142.6 ft2 (13.25 m2)
Max. takeoff weight: 1,260 lb (572 kg)
Wing loading: 8.83 lb/ft2 (43.17 kg/m2)
Stall speed: 48 mph (77 kph)

C-152
Wing area: 160 sq ft (15 m2)
Gross weight: 1,670 lb (757 kg)
Wing loading: 10.4 lb/sq ft (50.5 kg/m2)
Stall speed: 49 mph (79 kph) flaps down

Also, I don't have the references handy but I seem to remember that the maximum crosswind for a two-control Ercoupe using the crabbed landing technique is quite high, something like 20-25 knots. That number actually goes down in Ercoupes with rudder pedals installed, which suggests that two-axis controls are not as limiting as one might think, it's just a question of pilot training.

 

[lr27]

In addition to wing loading, aspect ratio matters too. High aspect ratio wings are more sensitive to changes in angle of attack. And vice versa, which is probably how Barnaby Wainfan survived his trip to Oshkosh in the Facetmobile.

When flying an RC glider, ailerons make it somewhat easier to land precisely, especially when trying to land gently. My guess is that this also applies to ultralights.

 

[Ollie Krause]

Hi All,

Sorry I haven't been able to post an update recently but I just finished up a rough assembly of my wing design (see attached photos). As described in a previous post, I opted to use a thick(ish) torque tube within some greased nylon bushings. The two bushings directly adjacent to the aileron have a slightly smaller ID than those at the root of the wing so that slight wing flex doesn't cause the torque tube to lock up but the aileron doesn't flop around and flutter during flight. At this point, my primary concern is that the aileron is only attached to any of the main ribs at either end making it potentially susceptible to flexing and flutter (because of it's unsupported design). This is going to be used on an ultralight aircraft flying at slow speeds where I imagine that the aerodynamic forces on the aileron won't be extreme though and I'd like to keep the design as simple as possible. Are there any other aircraft that use a similar torque tube assembly and have the aileron completely suspended between two of the main ribs or is this inherently unstable and should be avoided? When this design is used, I typically see at least 3 attachment points such as in the Belite wings or the SS-1 video linked above. Anyway any help is greatly appreciated.

My latest wing design can be viewed in 3D on my Public Onshape Document (to make up for the low resolution screen shots).

 

[proppastie]

Faa Glider Criteria has a specification for control surfaces based on speed. In mycase it is 12 lb/sq ft limit load. After you have the load calculated then you calculate if your attach points and ribs are strong enough. Normally you calculate for a safety factor of 1.5 or 2 for composites. (Load 18 or 24 lb/sq ft for example) You would load test to 12...and that is the only number (if it passes) you are sure is correct.

 

[proppastie]

Load distribution is shown in the same publication.

 

[cluttonfred]

Very nicely done, Ollie, though I wold be concerned about the torque tube binding in the bearings as the wing flexes under load. You'll want to minimize the number of bearings, maybe just the two on each end of the control surface and then the torque tube unsupported (clear space around it as it passes through the wing ribs) until you get to the wing root.

 

[Aerowerx]

.... some greased nylon bushings....

Are you sure that is a good idea?

I don't recall seeing anyone mention putting grease on their plastic bearings.

Some greases and and some plastics don't get along well together.

And what is to keep the grease from dripping down on to the fabric causing more problems.

It will also cause maintenance problems, since it would have to be replaced regularly.

If you are concerned about friction, use PTFE. Delrin (acetal) is another popular choice and is less expensive than PTFE.

 

[Hot Wings]

Some greases and and some plastics don't get along well together.

But grease and grit/dirt really like each other.

Delrin - good stuff...maybe with just a light spray of PTFE containing bicycle chain lube.

 

[Ollie Krause]

Very nicely done, Ollie, though I wold be concerned about the torque tube binding in the bearings as the wing flexes under load. You'll want to minimize the number of bearings, maybe just the two on each end of the control surface and then the torque tube unsupported (clear space around it as it passes through the wing ribs) until you get to the wing root.

I could take the bushings out entirely but I would worry that natural vibrations of the torque tube would cause it to bump up against my ribs during flight and damage them (even if the hole was super loose). I'll just make the bushings near the root have 1/2" of wiggle room for the torque tube so they are more like "rib protectors" and less like bushings.

 

[Ollie Krause]

Are you sure that is a good idea?

I don't recall seeing anyone mention putting grease on their plastic bearings.

Some greases and and some plastics don't get along well together.

And what is to keep the grease from dripping down on to the fabric causing more problems.

It will also cause maintenance problems, since it would have to be replaced regularly.

If you are concerned about friction, use PTFE. Delrin (acetal) is another popular choice and is less expensive than PTFE.

Good point! Unfortunately I can't change the material of these bushings as their unique size and shape mandates that I 3D print them out of nylon (which is probably the strongest and best suited printable material for bushings). Do you think Hot Wing's suggestion to use PTFE chain lube would also attract too much dirt?

 

[Ollie Krause]

Faa Glider Criteria has a specification for control surfaces based on speed. In mycase it is 12 lb/sq ft limit load. After you have the load calculated then you calculate if your attach points and ribs are strong enough. Normally you calculate for a safety factor of 1.5 or 2 for composites. (Load 18 or 24 lb/sq ft for example) You would load test to 12...and that is the only number (if it passes) you are sure is correct.

Thanks for the info and super helpful links! Before construct the wing though I'm going to try to figure out how to use some sort of finite element analysis software so I can refine the design a bit before I go ahead and start breaking stuff irl. Our outreach team is also still in the process of finding sponsors and such so we probably won't begin construction until at least this summer.

 

[Hot Wings]

Does anyone else find threads like this to quite frustrating? We have a seemingly intelligent young person wanting to create a flying machine, but is lacking experience. There is just so much assistance and knowledge that many of us could provide in a relatively short time with an in person, hands on, mentor session.

Trying to do this by remote control via an internet group is just so......inefficient.

There must be someone in the Bay Area that could meet up with the OP in a shop somewhere for a couple of hours of question/answer/demonstration?

Maybe one of Boku's Akaflieg sessions?

 

[Victor Bravo]

Ollie there are engineers here who know a lot more than me, but your aileron design seems to be reasonable for a light and slow airplane. HOWEVER, since you only have two hinge points, and you are driving the aileron from a torque tube, you will need to consider a few specific things :

1) Please make sure that your aileron is as short (spanwise) as possible for the task it needs to do.

2) I would suggest that the aileron leading edge and torque tube should be made out of as stiff of an alloy as possible. You will probably find that using 2024 or 7075 aluminum instead of 6061 makes the aileron stiffer (torsionally), and that this improves the resistance to torsional resonance or vibration.

3) More importantly, if the aileron is attached at the ends only, the tube also has to be far stiffer in the bending direction. This also points you toward 2024 and 7075 I believe

4) You may also find that a slight increase in wall thickness of the tube gives you a large benefit, or a larger safety margin, and is worth the few ounces of weight.

5) Also, you may well have to put an external mass balance on the aileron (a weight forward of the hinge line). This may not be needed if your speeds are slow.

 

[proppastie]

Does anyone else find threads like this to quite frustrating?

The old saying "A camel is a horse designed by committee." You will learn much more about aircraft design by building a proven design than by listening to 20 different people each who has a different idea as to what an aircraft should be. Be sure to static load test because finite element is a difficult specialty to understand and use properly.....Your life is at risk unless you get almost 100% right..... (71% a "C" normally will be failing)

 

[Aerowerx]

...Do you think Hot Wing's suggestion to use PTFE chain lube would also attract too much dirt?

Have you ever seen a clean bicycle chain?

I'm by no means an expert on this, but I would still be concerned about the compatibility of PTFE chain lube and the nylon. Not so much the PTFE itself, but the other components of the spray.

Remember bicycle chain is made of steel, not nylon.