Aerial refuelling challenge

What do you think?

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Karthick Ashwath

Junior Member
Hey guys, I believe its time for another exciting and difficult flitetest challenge!(been a while since you did a viewer request challenge)
The target is to build an aerial refuelling aircraft which can refuel/recharge a fleet of 1-3 small fighters.(fairly small, maybe 20" wingspan maximum?)(doesn't have to be fast)
1.The aerial refueller can be anything(plane/helicopter/blimp etc.)
2.The aerial refueller can use any power source(lipo,li-ion,solar PV,solar thermal,gas,etc.)
3.The refueller does not need to have the capacity to allow fighter planes to land and recharge on it.(airdeck is optional)
4.The fighters must be electric.They don't have to be fast at all.In fact, they can even be a highly efficient motorglider.
5.All the planes must be in the air throughout the operation.They are not allowed to land on the ground.
6.A safe fast charging to 80% of capacity is enough.(balance charging not needed)
7.You can choose how many aircraft you want to refuel/recharge at a time.
8. You can go for a robotic fully controlled boom or a free flying hose/wire/small quadcopter(like a tiny whoop) with magnetic contacts to attach on metal plates on the plane or anything else. Mode of refuelling is upto you.
9.(edit)You may use supercapacitors to greatly reduce the charging time.
By using a supercapacitor, you can greatly reduce the charging time, but this will cause the flying time to come down and will also require proper circuitry to be charged safely.Use a microcontroller(like the arduino) to check that the proper polarity is received and start charging only after that(there is a chance that the wrong terminals on the charging side get attached)
10.(edit)You may use hooks/rails to keep the fighters in the air.

If the rules are too difficult please do tell me!
Aerial refuelling.jpg KC-45 USAF Aerial Refuelling Tanker Aircraft.jpg
Warm regards,
Karthick Ashwath.
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Cardboard Boy
Charging while in air will be nearly impossible because it will take too long, even just docking two aircrafts together is extremely hard so you should start there and be happy if it works.
You are not the first to attempt that and I havent heard of the other ones.


G'day Mate
When I first saw thread topic this is the first thing that came to mind, Josh Bixler air to air refuelling in a F-16 :p


Old and Bold RC PILOT
As previously stated this idea has been around for ages but almost always fails because of the charge current required, the difficulty of connecting aircraft whilst airborne and the energy density of the chosen energy source.

Even a small aircraft can use around 10A continuous in flight which means that before a single microamp of charge can be given the charging aircraft must provide 10 Amps continuously. If the receiving aircraft has a tiny battery, (say 1000mA/Hr), and you wish a 5 minute charge window, (a VERY long time to have 2 aircraft connected together),you would require a charge current, (ignoring losses and inefficiencies), then you would need to provide 12A of charge current on top of the 10A of holding current.

With the idea being for 3 or so aircraft to be charged at the same time your charging setup would require to supply, (in this example), 66A of charge current capacity.

So now comes the weight issue. Assuming that the batteries are not to be damaged you would need some form of charger/current limiting device for each plane being "refueled". Then the power source or reservoir for the charging would need to be larger than the required total energy discharge into the receiving planes.

Charging Time for the current range of batteries would require that the refueling be done whilst the planes are turning in formation whereas full size liquid refueling is normally done in straight line flight. The model requirement for refueling whilst turning is because by the time the model aircraft have hooked up and completed their recharge could be anything from 5 minutes to around 20 minutes assuming that the receiving aircraft have a maximum flight time, without refueling of 15 minutes. So unless all of the aircraft involved are able to be flown in a straight line for the 20 minutes for a recharge cycle, (as suggested by the figures herein), the group of aircraft will be required to turn whilst charging. Whilst not impossible it is extremely to get model aircraft to fly in close formation in a straight line for any serious length to time and drops considerably when adding co-ordinated maneuvers as well.

Whilst it may seem like a nice idea, (it must be because it keeps coming up), it is a difficult to execute idea in the extreme. It is not quite impossible but close enough to wear that classification at this time in history!

Have fun!
Probably have better luck developing some kind of battery retrieval system that can pick up a new battery connect/disconnect and expel the used battery...


Maneuvering With Purpose
Simulating it would look good. Being able to pickup an inflight towline reliably would impress me.


New member
I think it could be done but not in the traditional sense of flying behind a plane and attaching a hose. And I also see this as more of a thought experiment than a practical project.

Anyway, picture this:

You have a large "tanker" plane flying slow holding patterns. This plane has a flat bottom with a small electromagnet under it. The smaller planes that will receive the "fuel" have a metal plate on the tops (light enough to still allow the plane to fly but big enough to allow for the electromagnet to snap onto it). When ready you switch the on magnet in the tanker and slam the smaller plane onto the belly of the tanker which will then get held on by the magnet. I'm sure you could workout a way to make the charging contacts connect. You have large pads (positive and negative) on the tanker to allow some room for error and a set of contacts on the smaller receiving plane that make the connection. Once charging is done you turn off the electromagnet and allow the smaller plane to drop free.

This would avoid the issue of the plane drawing power to keep itself flying during the processes of refueling since it will be attached to the bottom of the tanker.

Boom, air to air refueling for RC planes. Totally impractical and there are some other random issues. Like making sure the prop of the receiving plane doesn't destroy the tanker (EDFs maybe?) or how the flight characteristics would change when the two planes are connected ect.

There's my idea.


Old and Bold RC PILOT
Interesting idea but flawed. You will encounter the same issues the saw the USS Akron abandon the refueling of aircraft experiments and eventually be withdrawn from service early.

Venturi effect will tend to cause the charger plane and the plane to be charged to tend to accelerate towards each other as they get closer to each other. Due to the air flow between the 2 aircraft accelerating and creating a partial vacuum, which will increase as the planes get closer to each other the risk of a serious impact is extremely high. This should also be seen as a hard to judge issue as you may be trying to judge distances or as little as 6 inches, between aircraft, from a distance of many hundreds of metres.

Electromagnet! Heaps of power required. Power means weight! Metal plates mean weight. The early experiments used a hook up rail on the tanker and a hook on the receiving aircraft. Release was effected by dropping the rail or releasing the hook.

Making the contacts large means the receiving aircraft contacts would need to be widely spaced. It is easy to declare that something should be easy but in fact the mechanics involved are more difficult than flying into a trailing boom.

It might be possible to have a retracting rail system on the tanker which could retract a "Hooked" plane up to the bottom of the tanker whilst at the same time centering it to allow accurate contact with charging contacts. The servo would need to have sufficient power/torque to lift the receiving aircraft and to ensure firm contact into the charger connector.

If the rail was deployed far enough below the tanker aircraft the venturi effects and collision hazard could be reduced to close to zero. The bar would require to be retracted as close to vertical as is possible on or around the tanker CG point. The hook on the aircraft should be Mounted higher than any propeller tip, be above the CG point, and have a grab/release mechanism to facilitate not only connection to the tanker but also release from.

The receiving aircraft would be required to do a climb and glide approach to attempt hook up. The bar could have a couple of trailing streamers to assist with visibility at a distance.

If the pilots are extremely experienced the above may work enough to allow for a reasonable chance of charging success.

Have fun!
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Building Fool-Flying Noob
What about using magnetic leads. I heard a guy trying to use magnets to mount cameras and power it.

it is on my list to try it out.

Captain Video

Well-known member
Logistical and electrical challenges aside, why not simulate the recharge/refuel process. Determine an approximate charge length and the planes must be veritably linked for the predetermined period to qualify success.


Master member
Aside from what else has been mentioned, I wanna chime in one little development that could benefit doing something like this for whoever wishes to tackle it. In fact, it might be a neat venture for Peter Sripol since he's like the mad scientist type.

In recent years, smart phones have been utilizing a technology to do wireless charging. It would be really cool if such a thing could be applied in this refueling challenge. Either have the planes close enough together to where they could send the wireless charging signal or possibly a wired tether to a charging pad that the other plane could touch or be close to. I find it funny that it's called wireless charging yet you still need a wire lol. So if you could find a way to utilize that technology somehow and obviously make it charge batteries at a safe current then that would be a big step in making this a reality. That would cut out trying to find a way to "connect" the 2 planes which I think would be extremely difficult to do.


New member
I thought about wireless charging when coming up with my "solution". The reason I didn't run with it is because I use wireless charging every day when I go to bed. I have an iPhone 10s Max that I wireless charge. The problem is, even though the surface area on the back of a phone that size is pretty big, you have very little usable wiggle room in terms of when it will be able to begin charging. You have to get that sucker right on the center or else the coils wont line up right and the system will start to get super hot. Most wireless chargers (including the one I use) has a built in fan to try and mitigate this. Even then the accuracy required for a solution like that to work is pretty high. Plus its not a very efficient means of energy transfer.

Apple is working on a way to layer the charge coils in slightly different positions so a user can drop their phone anywhere on the charging plate and a micro processor will decide what coil to energize for charging. But that's apple, and their R&D budget is probably a little bigger than anyone's here lol.


Well-known member
Here's my take on this one. Yes, extremely hard, but possible. My plan would be a modified version of Hai-Lee's retracting rail.

The only way I see this working practically is that OP mentions that he would allow the tanker to be a blimp. A tethered blimp would be able to both carry a large battery pack (or even solar panels to charge its own batteries) and support the weight of a charging aircraft. If the blimp had a pair of tapered rails hanging beneath it, with a servo to clamp them open and shut, the aircraft to be refueled could fly its wings into the rails and be caught. Set the rails at the right attitude, and you might be able to stall your way into them at low speed and high alpha. Excess collision speed will just push the blimp around.

Power could then be transferred through the rails into strip contacts on the wings, and on completion of charge the clamps can be opened, allowing the aircraft to drop out.

FPV would be an absolute must, otherwise the blimp will not last very long. i.e. the first attempt would result in a prop strike that pops the blimp.


Old and Bold RC PILOT
Just a few thoughts on the idea of a tethered balloon!
In WWII they used tethered balloons to actually deter dive bombing and to slice the wings of those dive bombers foolish enough to try.

A tethered balloon is not much different really to hanging a charging point off of a pole or similar ground connected structure.
No matter what there is almost a guaranteed speed differential between a tethered balloon flying at zero airspeed and a flying model even at stall, unless the aircraft is 3D capable! For safety it is always best to minimize speed differential, especially where there is increased risk of some form of impact.

With the open charger contacts you definitely do not want to use hydrogen as the lifting gas, without recording the entire setup and refueling attempt on a video recording device! It could have a very interesting result:eek::LOL::rolleyes:! You would also need to specify the weight of the receiving plane and look into the charging station weight as Hydrogen and Helium actually require quite large volumes to produce much serious lift.

If a plane flying high alpha has its wing LEs hit an obstacle and stop suddenly the inertia of the fuselage motor etc will tend to pitch the nose of the aircraft upwards, (unless it has a low wing layout or a low wing forward staggered biplane layout). The pitch upwards could result increased risk of prop strike especially if the rails upon which the plane lands and into which there is the possibility of a forceful stop could cause a deformation of the balloon. As the rails are pushed forwards, and upwards the resulting increase in gas pressure could cause the surface of the balloon to bulge or deform, in the direction of the aircraft propeller itself.

I hate to admit it but trying to hook up with a fixed wing aircraft, or a Zeppelin, (as the charger station), would be somewhat simpler and safer if such a project was ever proceeded with!

Just my opinions of course!

have fun!


Well-known member
Interesting thoughts! I'll be honest, I don't have much experience with lighter than air vehicles. The reason I thought of a blimp is because a fixed wing aircraft capable of a long enough flight for this job, as well as carrying the required battery capacity, is starting to get so big that it's going to require launching from an airport! And the reason I suggested tethered was to avoid having to deal with ballast and wind. A tethered balloon can have a lot of excess lifting capacity while a free floating one will be forced to push itself either up or down while idling.

I wonder if while docking, the tether could be slacked so that the blimp could match speeds with the airplane.

I was imagining a zeppelin type blimp with the docking area mounted like the zeppelin cabin. Docking clamps would hang fairly low, underneath a hard surface that could withstand a prop strike, because as you mentioned, the aircraft will likely be pitching all over the place as it stops, as will the blimp.

And yes, I was thinking of a 3d capable aircraft for this as I can't think of a better way to get the speed down. Climb and glide would result in a lot of missed approaches and would require flying at greater than stall speed. Perhaps a better concept, if we are flying 3d, is to just fly straight up vertically into the clamps. Zero speed differential. There are definitely going to be some ugly airflow effects as the prop disc closes in on the blimp, though - any idea what they would be?


Cardboard Boy
Using a probe and hopper thiny with maybe small magnets would be nice to simulate a mockup refueling.

Using a fpv camera would help, also you can drop the idea of charging any significant amount that way unless you only want to charge some supercaps.


Active member
Using a probe and hopper thiny with maybe small magnets would be nice to simulate a mockup refueling.

Using a fpv camera would help, also you can drop the idea of charging any significant amount that way unless you only want to charge some supercaps.
Or you could put supercomacitors on the drone then have it connect to the plane and "refuel it."

Karthick Ashwath

Junior Member
Thanks for the feedback everybody,especially your detailed and valuable feedback, Mr.Hai Lee.I have incorporated some of the suggestions to make the project feasible.(added points 9 and 10, modified other points)

Karthick Ashwath.
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