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I stumbled upon a nice photo manipulation on aviationhumor.net.

Bike Rack on Cessna 172

Source

My question is short and concise:

  • Is it practically possible to do that? Is it okay in terms of aerodynamics, weight, CG?

  • Bonus Question: If it was possible, would there be any chance it gets certified for use?

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    $\begingroup$ You might have more luck hanging them under the wing $\endgroup$ – Pondlife Dec 18 '17 at 23:17
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    $\begingroup$ That would be a pretty bad day if one of them lets go & takes out your rudder... and even if it didn't damage your aircraft, it could ruin somebody's day on the ground. I don't really see the FAA agreeing to something like that, since every single time you fly with bikes, it's a new installation (of the bike on the rack) and the need for checks that nothing will let go at an inopportune moment. $\endgroup$ – Ralph J Dec 18 '17 at 23:33
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    $\begingroup$ @Pondlife Hmm.... I don't think that's what "bicycle landing gear" is supposed to mean. $\endgroup$ – reirab Dec 19 '17 at 6:06
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    $\begingroup$ The wing is pretty much designed to be at the center of mass of the vehicle, so putting something above the wing is actually the best place for it. $\endgroup$ – dotancohen Dec 19 '17 at 6:46
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    $\begingroup$ Perhaps you should make it clear whether you are asking whether such a bike rack is practical out of purely academic interest, or whether you actually want to carry bikes in your 172? $\endgroup$ – jamesqf Dec 20 '17 at 3:39
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Is it practically possible to do that? Is it okay in terms of weight, CG?

Bikes (depending on make and model) are not all that heavy and in this picture they are more or less center mass and would most likely fall in the CG range.

aerodynamics

Again this depends on the bikes and the placement but with proper care you could be ok. Admittedly a bit different these bikes would create no more drag than say a wing walker would and people have been doing that for some time,

enter image description here

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keep in mind bi-planes fly a bit slower than a 172. But on the small vehicle transit front the PBY-Catalina carried a full sized rowboat under its right wing for use when you arrived at your destination. Interestingly it seems to be mounted backwards which I would think is the non aerodynamic way to mount it...

enter image description here

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It looks like at least one guy has actually managed to pull this off in New Zealand

enter image description here

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In case you were wondering if it’s legal, the Civil Aviation Authority of New Zealand has approved the design.

And he used a Cessna 185. Important to note that these are mounted under the wing which keeps them out of the way of the rudder as Dan mentions but more importantly should anything go wrong the bike would shear away and fall down away from the aircraft instead of back into the rudder.

It looks like this kit plane also offers a similar rack.

....If it was possible, would there be any chance it gets certified for use?

In New Zealand yes, in the US under the FAA, depends how good you are at paperwork.

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    $\begingroup$ "these are mounted under the wing which keeps them out of the way of the rudder" ...but your horizontal stabilizer is probably toast if the bicycles somehow separate from the wing in flight. (It doesn't look like they'd have time to drop sufficiently in cruise to clear below the tail, and they definitely aren't going to clear above the tail.) I'm not sure I'd prefer either to the other. Something like a snapped elevator cable is bad enough; I feel that a bicycle (or two!) hitting the front end of the stabilizer at 150+ km/h likely wouldn't be better in terms of resulting aerodynamics. $\endgroup$ – a CVn Dec 19 '17 at 8:34
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    $\begingroup$ @MichaelKjörling I'm a physicist not a pilot, but if those bikes came off they wouldn't hit the stabiliser at anything like you airspeed. They'd start at the airspeed and decelerate due to their drag. If the bike decelerated instantly on detachment it would fall about 4cm before the stabilizer hit it (@95kt and scaling off a photo). At the same speed the drag force on a bike with rider would be ~420N, which would decelerate a 10kg bike by 42m/s². The bike would hit the stabilizer about 0.4s after detachment doing 30kt (actually much less as no rider). It would fall 30cm in that time $\endgroup$ – Chris H Dec 19 '17 at 9:19
  • $\begingroup$ Still too much for comfort. BTW I couldn't find figures for the drag on a bike without rider. The forces aren't very much higher than on top of a car (you should see how fast some people drive with roof loads round here) $\endgroup$ – Chris H Dec 19 '17 at 9:20
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    $\begingroup$ @MichaelKjörling The points where struts meet the wing on Cessna 185 look like about same distance from the centerline as tips of horizontal stabilizer. Racks seem to be hanging further outside, which gives you slight chance that a falling bike will miss the stabilizer horizontally as well. Or clip only the outer half of the stabilizer. $\endgroup$ – Agent_L Dec 19 '17 at 11:56
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    $\begingroup$ Related question at bicycles.se. The drag on the bike (assuming it kept the same orientation, which it wouldn't) would be <1/10 of my estimate with a rider, meaning that after detachment it would take over a second for the stabilizer to catch up and have a closing speed over under 10kt. But in this time it would have fallen clear of the plane by a few metres. This is the other extreme, as once the bike started tumbling it would present much more drag surface $\endgroup$ – Chris H Dec 19 '17 at 16:53
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Just eyeballing your image, the CG looks OK, and it's possible to put that much weight into the interior of the plane just fine, so the weight is OK.

The aerodynamics on the other hand - yeah, no. Imagine the bikes are not strapped down and you have to hold them in place - in a 120 MPH wind. That's a LOT of drag and the turbulence from it could easily mask the rudder.

As for certification - in theory you could get an STC for all Cessna 172s or file a form 337 for a modification for a single named aircraft. You would have to prove that this was safe and the aircraft was airworthy.

Good luck with that.

It would be far cheaper/easier/safer to just buy a folding bicycle and carry it inside the cockpit.

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    $\begingroup$ Don't know about a 172 specifically, but a mountain bike (plus camping gear) is easily carried in a Piper Cherokee. You just remove the upright part of the rear seat. (I eventually also removed the base, and replaced it with a plywood sheet cut to fit.) Then pop the wheels off the bike, and it fits in fairly easily. Use a cargo net to keep things from bouncing around. I'm pretty sure two bikes would be doable. $\endgroup$ – jamesqf Dec 19 '17 at 4:10
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    $\begingroup$ If you take the question literally, then it's probably possible to install the rack, as long as you never, ever put bikes in it. Except when the plane is parked. $\endgroup$ – dreamcatcher Dec 19 '17 at 4:48
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    $\begingroup$ @dreamcatcher Very good point. I think it's possible to go even further and get approval for transporting bikes while taxiing at very slow speeds. $\endgroup$ – Emilio M Bumachar Dec 19 '17 at 13:11
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    $\begingroup$ A folding bicycle is not going to substitute for a mountain bike. Still, overall I like you answer. $\endgroup$ – paparazzo Dec 19 '17 at 14:16
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    $\begingroup$ @dreamcatcher That'd be a way to get a lot of attention at Oshkosh. $\endgroup$ – Harper - Reinstate Monica Dec 20 '17 at 16:53

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