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Wheel fairings

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frans(at)privatepilots.nl
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Posted: Sun Aug 19, 2012 5:46 am Post subject: Wheel fairings

On 08/12/2012 07:24 PM, Bud Yerly wrote:

 	  Quote: 	
		   Frans,

 I'll look for some pictures. I don't have them at the home office today.


	


The reason I was asking is that I'm in the process of making some 

improved wheel fairings. It would be nice to share some ideas to the 

benefit of all of us, which is the purpose of this forum.



Well, I have some ideas to share. I started with the nose wheel fairing 

because there is only one of it so I only have to make one test sample, 

and I disliked the original nose wheel fairing more than the main wheel 

fairings.



To improve on something, one first has to identify the shortcomings. For 

me, the shortcomings of the original fairing are:



1) Poor fit. We all experienced that the thing was impossible to get 

symmetric. (I have been told that the company now corrected this error 

but that didn't solve my problem with the already bought fairing).

2) It is wider than necessary. Apart from the obvious aerodynamic 

disadvantage, the extra width also implies that the fairing has to bend 

in sharply at the underside to meet up with the smaller wheel. This bend 

further creates interference drag, and it increases vulnerability to 

objects on the ground. Ideally, the fairing's bottom should not be wider 

than the track of the wheel.

3) Too weak. Partly as a result of the width, but also due to the 

material itself.

4) The vertical division into two halves. These halves do not match well 

together, they meet at an angle and often bulge out, disrupting the 

airflow up front and negating the main purpose of the rest of the 

fairing behind it.

5) The clutter caused by all the fasteners, all on a separate height. 

Add a hole at the axle for a tow bar, and there are 5 levels (on each 

side!) where the airflow gets disrupted.

6) Too small in height. I would like to hide more of the wheel, and I 

also would like to fair in my 3cm shaft extension. (This mod was 

necessary to allow a larger diameter prop and still maintain enough 

ground clearance).

7) Too short. The length might be theoretically right, but if the 

fairing is not aligned perfectly with the airflow it should be longer to 

allow a shallow decline of the "downwind" side of the fairing to keep 

the airflow attached. And the fairing is likely not aligned perfectly, 

due to the proximity of the prop, and it is also quite possible that 

during take off the nose wheel turns a bit to a side while leaving the 

ground, especially while taking of in a cross wind.



How did I improve on all this?

1) A key decision was to make the fairing from aramide (kevlar) instead 

of glass. Aramide has less tensile strength than glass or carbon, but it 

won't crack and it is highly resistant to impact forces. If it is 

literally(!) bullet proof, it should also cope well with rocks and 

bumps. The downside is that the material is hard to handle, you can 

forget about using normal scissors and knifes. We made part of a test 

fairing with two layups of aramide and had great fun trying to destroy 

it once it was cured. We jumped on it so it pancaked, but it just popped 

back into shape after we left it, without any damage. We actually folded 

it, the epoxy itself cracked a bit but not a single strand broke and it 

folded right back into its original shape (but of course this time with 

a visible fold line). If I were living in the US I would probably also 

have fired some bullets at it just for laughs. This material allowed me 

to make a much deeper fairing and still don't have to worry about soft 

fields and hitting some rocks, while keeping the weight low at the same 

time. On the final fairing we tested it by taking the wheel off the 

ground, putting obstacles below the fairing and putting the aircraft on 

its own weight again, so that the entire weight of the aircraft was 

resting on the fairing. No problem. It will probably survive a flat tire 

with only some paint damage. We could probably land on it without any 

wheel inside at all.

2) A decision was made to keep the axle nuts outside of the fairing. We 

needed an attach point for the tow bar anyway, and it would either be a 

hole or a small protrusion. By keeping the axle ends outside of the 

fairing I could make the fairing more than 5 cm (2 inches) smaller! 

Also, I could eliminate the arms (saving additional width and weight) 

and fasteners (leaving the rest of the fairing smooth), by just 

embedding two metal plates in the layups and using the axle bolts itself 

to fasten the fairing. Now the fairing got significantly smaller, also 

the theoretical length would be smaller. Because I wanted a longer 

fairing, I just kept the length of the original fairing to get the ratio 

I wanted. And because the fairing is so small, it is easy to make a 

natural transition into the wheel at the bottom of the fairing without 

any curve or flat part on the sides of the underside.

3) Instead of the vertical division, I wanted a lengthwise division so 

the cut would be in line with the airflow. Instead of dividing it into 

two halves, Ilona came up with the idea to make a cut so that a part of 

the bottom can be taken off. This allows the wheel to enter, and it 

makes the vulnerable bottom part to be easily replaceable. Also, it 

gives the interesting option to fly with the bottom removed in winter 

time, to give more clearance for the wheel and to allow mud and snow to 

fall out by itself.



A few notes:

The upper part of the fairing is wider than the lower part. This is 

because the upper part must also encapsulate the yoke. If then a 

straight angle to the rear is followed, the rear of the fairing 

automatically gets this "swept back" appearance and its curved 

underside. This is also the reason that the "nose" is above the center 

of the wheel.

Of course the bottom of the fairing is a bit of a compromise to get 

enough clearance on ramps and to blend in the profile of the wheel as 

good as possible.

The "skirts" on the side are purposedly not rigid. They will give way if 

something catches between the tire and the skirt.

I made the shape of blue foam, and decided to leave it inside except of 

course for the cavity of the wheel. This adds further strength for a 

very low weight penalty.

There is a small "bulge" in the division cut on the port side, this is 

to facilitate access to the valve of the tire. I only have to drop the 

underside of the fairing to get access to it.

To remove the entire fairing, the wheel assembly must be taken off. 

There is however not much need to remove the fairing, as even the wheel 

can be taken out of the yoke without removing the upper part of the fairing.

The fairing is reinforced near the shaft. This forms the third 

attachment point, so that the fairing can not rotate over the axle.

There is additional clearance at the rear of the tire, so the wheel can 

never "catch" the fairing.

The yellow appearance of the fairing is due to the color of the aramide 

cloth. The ragged appearance of edges is also due to the aramide. Of 

course the fairing will receive a proper finish.



This is a one of design, however if someone wants to use it as a plug 

that is possible. Keep in mind though that it is designed for nose wheel 

assemblies with a 3cm longer shaft than original.



Frans

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Fred Klein

Joined: 26 Mar 2012
Posts: 503

Posted: Sun Aug 19, 2012 6:12 am Post subject: Wheel fairings

On Aug 19, 2012, at 6:45 AM, Frans Veldman wrote:

 	  Quote: 	
		  The reason I was asking is that I'm in the process of making some improved wheel fairings. It would be nice to share some ideas to the benefit of all of us, which is the purpose of this forum.



Well, I have some ideas to share. 	




Frans...

I tip my hat to you and IIona...I salute your analysis, design, and execution...very nice work!

Fred

 [quote][b]

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craigb(at)onthenet.com.au
Guest

Posted: Sun Aug 19, 2012 12:42 pm Post subject: Wheel fairings

Frans, could you elaborate on the "correct length would be shorter" wouldn't

the longer shallower transition

Create less drag and thus make your longer fairing more efficient.



Regards

craig

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frans(at)privatepilots.nl
Guest

Posted: Sun Aug 19, 2012 2:38 pm Post subject: Wheel fairings

On 08/19/2012 10:41 PM, craig wrote:

 	  Quote: 	
		   



 Frans, could you elaborate on the "correct length would be shorter" wouldn't

 the longer shallower transition

 Create less drag and thus make your longer fairing more efficient.


	


There is also something like skin drag. More surface creates more drag. 

So there is an optimum somewhere: for a 2D fairing (like for a strut or 

a wheel leg) the optimal width to length ratio would be 1:3.7, with the 

widest point at about 30% of the length. For a 3D fairing things get a 

bit more complicated. Shorter carries a higher penalty than longer, so 

if in doubt just make it longer.



Frans

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