Speed Modifications for Fly Babies

Oh, all right. The picture on the right IS a bit of an exaggeration.

But it's funny, I do occasionally get asked questions about how to make a Fly Baby faster. The question always does give me a bit of a pause...the Fly Baby is *not* designed for speed. It was designed for ease of construction and classic 1920s appearance, but little was done to optimize for a higher cruise speed.

But people ask. So allow me to discuss how to get a Fly Baby going faster.

The Obvious Route Doesn't Work

The first thing that comes to mind for most people is a larger engine. Unfortunately, it just doesn't work out that way. Fly Babies have a plethora of drag, and overcoming the drag takes more engine power than would be reasonable. The largest engine Pete wanted to see in Fly Babies was the Lycoming O-235, at about 115 HP. Big engines like that really don't improve the speed all that much. They help climb, and cruise speed does increase, but remember that the horsepower increase for a given speed increase is the cube of the desired speed increase! So if you want to double the speed, you need eight times the horsepower. Doubling the horsepower would only give you about a 25% increase in speed. So skip the big-engine route.

The First Step

Want a faster airplane? Build it light.

It takes power to accelerate a mass, it takes power to maintain that mass' speed. A lighter airplane will accelerate faster, take off quicker, climb better, and cruise faster.

There is a surprising amount of variation in Fly Baby empty weights. The chart shows the results of a survey Pete sent out. Note the range of empty weights for the 65-HP airplanes... the heaviest is about 150 pounds heavier than the lightest. That's 25 percent! The lighter plane will be overall a better-performing aircraft.

Remember, this isn't just about not making parts thicker or using heavier materials. This is also about your design choices. For instance, adding a full electrical system may add from 50 to 100 pounds to your total aircraft weight. If you don't want to hand-prop, leave off the generator and regulator and just install a lightweight battery and one of the new high-tech lightweight starters. Include a small socket to let you trickle-charge the battery, and you'll be fine.

Drag Reduction is the Key

Face it, the Fly Baby is a "drag bucket." It was designed to be quickly and easily constructed, with little attention paid to drag reduction. Let's take a look at how to clean up some draggy areas

Cockpit Canopies

Personally, I *love* open cockpit flying. But I fully agree open cockpits produce a lot of drag.

Several Fly Baby builders have added closed canopies to their aircraft. Not, usually, because they want faster airplanes, but because they live in the cold country and want to be able to fly year-round.

The removable turtledeck area of the Fly Baby was originally designed to allow swapping with a unit that includes a canopy. In fact, that's why the standard baggage door is less than half the width of the turtledeck...to support making the open-cockpit turtle deck in left and right halves, and allow storage under a closed canopy arrangement.

So it's possible to build an airplane to let you fly with your head in the breeze, and put a canopy on it for longer trips or when the weather turns cold. You get the best of both worlds, that way.

Let's take a look at some of the drawbacks:

Drawbacks of Canopies

Time to design, fabricate, and build. Unfortunately, there never has been a "standard" Fly Baby canopy. Individuals have made them, but detailed design information just isn't out there. In addition, most people want a sliding canopy like a fighter, and these are probably the most difficult to do.
Safety problems: Many are the airplanes...homebuilt and otherwise...that have been lost when the canopy departs in flight and takes out some of the tail surfaces with it. Not to mention conking the pilot on the noggin on the way past. You must ensure the canopy will stay attached to the plane, and include good stout latches to hold it in place while you're flying. Adding a canopy is not a trivial change, from the safety point of view.
Comfort Issues: If you decide to build a canopy, dummy it up first and try it on. An easy mistake for budding canopy designers is to not leave enough room. Remember, your head gets waggled left and right in turbulence, and you want to be able to comfortably look around without touching your head on the plexiglass or frame. Also, keep in mind that you'll want some sort of air vents to give you fresh air on those days when it's too cold to fly with it open but the sun is beating down on you and heating you up.
Weight: Plexiglass is heavy.

Closed Cowlings

The stock Fly Baby cools its engine by hanging the cylinders out in the breeze with a set of scoops atop them to force air down through the rear cylinders. This method is identical to numerous older aircraft, like Piper J-3s. However, there's no question that exposed-cylinder cooling is draggy. Covering up the engine with a clean cowling can eliminate a good bit of drag.

The nice thing is, there are literally thousands of production-type airplanes that you can use as models for the cowling and attachment design. It's probably a lot easier to do than a canopy.

Drawbacks of Closed Cowlings

Complexity: It's not just a matter of wrapping the engine with aluminum. A "pressure plenum" is necessary to force all of the incoming air to flow past the cylinders to cool them. Given its choice, air will NOT travel through those little fins on the cylinders...unless that's the only direction it can go, the air will escape and not do any useful cooling. You'll need baffles and seals, just like a production-type airplane. If a baffle is wrong or a seal doesn't quite make contact, your engine is going to run hot. When you copy the cowling of an Aeronca or Taylorcraft, you have to include all the internal elements as well. As you might expect, building the canopy will take quite a bit of time.
Safety: If you use the attachment systems that production airplanes use, the cowling should be pretty secure. However, keep in mind that it'll probably take some time to troubleshoot the cooling path. Watch your engine temperatures during the test-flying period. Also, a closed cowling will be more subject to problems like vapor lock.
Convenience: Personally, I like the exposed-cylinder look because it makes the engine easy to look at during preflight without having to undo anything. With most exposed-cylinder layouts, you don't even have to open a door to check the oil, and, with a quick-drain valve on the oil tank, you can probably change oil without even removing the cowling.
Weight: More aluminum, both in the form of external cowling and internal baffles. Not too bad, probably.

Wheel Pants

Face it, those big fat tires the Fly Baby uses are draggy. Wheel Pants are a nice-looking way to clean up both the appearance and the airflow.

Putting pants on a Fly Baby gets into a bit of a trade, though. Smaller tires mean less drag and less-expensive pants, but the tires are the only source of shock absorber the Fly Baby gear has. New pants to fit the 800x4 or 800x6 tires will probably run $300 a pair or more.

Drawbacks of Wheel Pants

Convenience: It will be more difficult to service and inspect the tires.
Safety: Might cause some problems in muddy fields.

Landing Gear Vee Fairings

When cleaning up the gear with gear pants, some folks apply fabric to cover up the landing gear "Vee." Most probably do it because they prefer the look of it, but it does clean up one source of drag. It turns the unit into one long triangular piece from two individual elements.

The nice thing about this modification is that is fairly cheap and easy to do. You need to install a bar across the open top of the leg for the fabric to attach to, though. But there's little effort, cost, or weight penalty.

Drawbacks of Gear Vee Fairings

Aerodynamics: Adding vertical surface forward of the CG will tend to reduce the airplane's stability. It's like adding a set of fins to the front of an arrow...it isn't going to want to fly as straight. Still, the gear Vees are located pretty close to the CG.

Other Streamlining

There are a couple of other minor touches folks add that cut down the drag

Axle Fairing: Round tubes are draggy, and there's a long axle tube running between the two wheels. Pete shows some ideas for a fairing on Page 2-10 of the plans.
Wing Wire Fairings: The pairs of cables used for the landing and flying wires are spaced far enough apart that each still contributes to the total drag. Michael Van Wyk filled in the area between these pairs with strips of balsa wood, and wrapped them with metal tape.
Exhaust System: Even if you use the standard open cowling, you can reduce drag if you route your exhaust pipe out the bottom of the cowling rather than let them hang from the cylinders. Compare the photo to the right with the previous photo.

Comments? Contact Ron Wanttaja.