Dan Nachbar’s

Motor Balloon

I am building a hot air balloon with several propellers driven by small electric motors. The goal of this work is to create a better way to give hot air balloon rides to more people. The balloon envelope is complete and work on the bottom-end is currently underway.

Overview

One of my great joys is giving rides in small aircraft.

Giving hot air balloon rides Is particularly fun.

Hot air balloons typically fly for about an hour during the morning and evening hours when atmospheric conditions are most calm. One-hour rides are OK but not great. For passengers, the fun and wonder start to diminish after about 15 or 20 minutes. One-hour flights violate the First Law of Showbiz: Leave ’em beggin’ for more!

Rather than a single flight, I’d rather take several load of passengers on shorter rides. Doing that requires remaining above or very near the launch field. To accomplish this, I’m adding a set of small motor/propeller units to an otherwise conventional hot air balloon.

Schematic Drawing

Here is a structural schematic drawing of the bottom-end (a.k.a. the basket) of the aircraft

The “V1.0” configuration uses four 1-meter diameter (40 inch) propellers driven by electric motors to provide enough thrust to push the balloon around at something like 5 miles per hour. That should be enough airspeed to comfortably remain over or upwind of the launch field in very light winds.

The envelope (the big fabric bag) is a custom-built conventionally-shaped hot air balloon.

The bottom-end (for the people, fuel, etc.) is built around on a retired type-certified Aerostar wicker basket, burners, fuel tanks, etc. I am adding a battery box at the bottom and a pair of armatures to the left and right sides. Each armature will support two prop/motor units.

FAQ

I’ve done something rather like it before – personalblimp.com

The motors and props are the same ones used on the quad-copter drones that are being increasingly used for agricultural applications. This approach to generating static thrust works great but typpically generates a lot of prop noise. Having several loud buzzing nasty props near the basket will likely ruin the experience for everyone involved.

The most effective way to reduce the noise generated by a prop is to run it more slowly. Drone props are usually buzzing along at 6,000-7,000 RPM. My plan is to run the props nearer 1,000 RPM. Turning the props more slowly also reduces the amount of thrust they generate. In order to get enough thrust to be useful, I’ll need several large slow-turning props.

The other thing that one can do to reduce noise in the basket is to move the props further away. This design mounts them on arms that are at least a few feet away from the basket. The “best” place for the props from the noise and an aerodynamic perspective is at the equator of the envelope. That’s structurally very tricky. So I’m leaving that as an improvement for down the road.

I presented an overview of this project at EAA’s Airventure in Oshkosh in July of 2024.

The slides for the presentation can be found here.

Construction Status

With a lot of help from friends, I have a fully-built 140,000 cubic foot, conventionally shaped hot air balloon envelope.

The envelope had its initial inflation in September of 2023. That inflation test is shown in the photo to the right. Since then, I’ve been focused on designing and fabricating the components of each of the four novel aspects of the aircraft: the suspension system to support the added weight of the batteries, the battery box located below the basket, the arms on which the motors are mounted, and the shrouds around the props.

I hope to start flight testing in the Spring of 2025.

Photos from the Shop

Here are some photos of those components in progress.

Battery box in the closed position.

Battery box in the open position.

Motor support arm. This also shows the basket in the inflation position. A bevel on the bottom side of the battery box help keep the heavy basket tipped up a bit to make inflation easier for the pilot.

One of shrouds (rings) that will go around the props. The shrouds will provide separation between the scary sharp spinning things and the people - both in the basket and on the ground. I suspect that they will also help reduce prop noise in the basket.

A view of the suspension system geometry. The suspension lines from the envelope will attach to a aluminum blocks added near the top of the pre-existing support posts and lines will run from those blocks to the battery box attached below the pre-existing basket.