Trailers and wheel placement

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rtfm

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I'm sure a bunch of you guys know the answer to this: how do you work out where to place the wheels/axel on a trailer?
 

TiPi

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It depends! What is the trailer for? Where will the CG be empty/loaded? The axle needs to be just behind the CG so that you have a "small" downforce on the tow ball. "Small" is relative, on a trailer with a tinnie it would be 10-20kg, on a 2.5t campervan it can easily be 200kg.
 

rtfm

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So, about 20kg on the tow ball. That's all I need really. I'm thinking of building a trailer/hangar for the Fleabike.
Giving my imagination free rein, it might be possible to design a trailer for both the Fleabike and my Smart car to be towed behind my motorhome. We will definitely need to tow the Smart car for use in the cities/towns//supermarkets and so on. But I don't want to do a lap of Australia without the plane, so both have to go...
 

rtfm

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I initially thought of this:
1661504185580.png
but that makes for a very long trailer. Tilting the Fleabike up and forward would allow me to decrease the length of the trailer by over a metre and a half. Something roughly like this.1661504413797.png
 

rtfm

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Initial rough sketches to visualize possible configurations only (as you can see). But do-able, I think.
I thought I could winch the plane up onto two wheel cradles, to ensure it is secure, and placed correctly, would pitch the tail up, allowing the Smart car to ride up and under the tail.
The motorhome is capable of towing 3500kg. My trailer, car, plane etc comes to a mere fraction of that.
 

dog

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back the plane in ,its weight will not be a handling
issue even without the car,use standard wheel straps to hold the mains,and then lift the nose all the way(small block and tackle) and drive the car under,maybe pull the nose gear if it is supper easy.
 

Matt G.

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Ideally you want ~10% of the weight of the trailer on the tongue. Since you probably won't be able to get the loaded and unloaded CG in the same place, there is going to have to be a bit of a compromise.
 

PMD

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Let's start by pointing out that a trailer to run across a continent is a VERY different thing from the one to drag an airplane a few klicks from home to airport.

Yes, 10% is the MINIMUM for good yaw stability. The more you can put on the tongue, the more stable the trailer will be. Do your W&B calcs carefully and conservatively. Also: the longer the trailer is, the more stable it will be and easier to back. A short trailer behind a long motorhome is THE most difficult thing on this planet to back (as a single trailer, not the place to discuss multiple trailer trains here).

Since your airplane will be riding back there for a considerable amount of distance, worth thinking long and hard (intentional pun) about the suspension you intend to use. Over-the-counter trailer stuff is just plain garbage and so stiff it can literally shake the plane apart. The Smart can help a bit by adding some mass, but still a challenge. Then it comes to the mounting of the airplane fuselage and wings. Remember that this thing is going to be bouncing a LOT and you have to consider the movement that WILL happen and minimize stress and chafing of the component parts. Thus why you will NEED a much softer and better dampened suspension system that any trailer parts house can provide.
 
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Derswede

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PMD hit the nail on the head. Might also want to consider sides to allow tarping of the airplane and car. When we built a boat trailer that my father had us build for a boat he snagged, we added several features to allow us to use it to haul airplanes. A rectangular structure for the wheels of the airplane would support it well, but getting the plane onto it without lots of friends or a hoist may be tough. Using an articulating structure with a hydraulic jack to lift it would make it simpler to get the airplane up and stable. It would have to lift the tail assembly high enough to clear the smartcar's hood. Please use PMD's suggestion on suspension.....airbag suspensions are popular with lots of the kids today. Less "bounce to the ounce"! As for wheel position, look at a couple of caravans (campers for us NA types), the dimensions will give you a good ratio to use (from tow hitch to axle).

Good luck with the project!

Derswede
 

AdrianS

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To add to what others have said re suspension, you really need shock absorbers as well as compliant springing, especially if you plan to tow it empty / plane only.

Airbags give a soft ride, but bounce and bounce without shock absorbers. I wouldn't use them for a round Aus trip - we had an airbag trailer for work, and airbag troubles let it down more than once.


A Smart car weighs approx 850 kg, plus the plane, so that's say 1000 kg plus the trailer weight itself. That means you can get away with a single axle trailer.

ps I hope you post a rough itinerary - if you pass through Melb. I'll buy you a beer.
 

Vigilant1

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So, about 20kg on the tow ball. That's all I need really.

The rule of thumb is 10-15% of the total trailer weight should be on the hitch.
If total trailer length is an especially important criteria, consider making good use of the meter or so of length from the main trailer bed to the hitch itself. Sure, it tapers, but the nose of your plane could stick into that slot.
Putting a car onto the trailer, even a small car, changes the scope of the project considerably. It'll need to be a much beefier and heavier frame than if this was just a trailer for a light plane. Trailer brakes and associated controllers may be highly advisable or required by law, maybe dual axles etc.
FWIW, In the US a trailer weighing over 3000 lbs (gross) must have brakes on all wheels in most states. A Smart Car has a curb weight from about 1500 lbs to about 3000 lbs (wow, I had no idea they could be so heavy!). Add in the trailer itself, plane , ramps, etc.
A couple of small 200cc, 250 lb scooters back there would be a whole different thing, some folks might consider that a good option. Fast enough to be safe on the roads, able to do okay on a dirt road, can take a passenger in a pinch, a bag of groceries is no problem, and able to park nearly anywhere in town or at a campsite.
 
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Vigilant1

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Note: You'll want to also consider the weight on the trailer hitch/tow vehicle suspension after you've taken that car off the trailer (at the campsite, etc). If designed for the weight of a car, the trailer's wheels may be pretty far aft. You don't want to be in a situation where, after the car is downloaded at campsite, the weight on the hitch exceeds the weight permitted by the tow vehicle's receiver/ structure/suspension.
 

Riggerrob

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Note: You'll want to also consider the weight on the trailer hitch/tow vehicle suspension after you've taken that car off the trailer (at the campsite, etc). If designed for the weight of a car, the trailer's wheels may be pretty far aft. You don't want to be in a situation where, after the car is downloaded at campsite, the weight on the hitch exceeds the weight permitted by the tow vehicle's receiver/ structure/suspension.
Then you need a second set of internal tie-downs to allow you to move the airplane closer to the trailer's C. of G.
 

Vigilant1

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Then you need a second set of internal tie-downs to allow you to move the airplane closer to the trailer's C. of G.
Right. Having to move the plane within the trailer every time the car is offloaded doesn't sound very convenient (unless you'd be taking the plane out anyway).
 
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Right. Having to move the plane within the trailer every time the car is offloaded doesn't sound very convenient (unless you'd be taking the plane out anyway).


Is keeping the Smart car inline with trailer centerline absolutely mandatory? The car is pretty short. He might be able to get away with parking the car at 90 degrees on trailer and building sideramps, like many ATV trailers. If that's possible, the car could be on/off-loaded without moving the plane, no matter what position the plane is in.
 

Vigilant1

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Is keeping the Smart car inline with trailer centerline absolutely mandatory? The car is pretty short. He might be able to get away with parking the car at 90 degrees on trailer and building sideramps, like many ATV trailers. If that's possible, the car could be on/off-loaded without moving the plane, no matter what position the plane is in.
A Smart Car is 8.5 feet long. The trailer width would require special permits in most states, maybe an advance flag car etc.
 

rtfm

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Wow. Excellent responses. Thanks guys. Exactly the sort of info I was hoping for. I even checked out the scooter idea - I love it - but I think my wife would divorce me...

Cheers,
Duncan
 
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