Speed, props and power

Model Airplane News, Mar 1998 by Hahan, Greg

WHILE ON A MUCH-NEEDED break from the building shop, I happened to be sifting through a past issue of Model Aviation and came across the rules meeting report and a list of proposed changes for '97. Of course, I skipped right to the scale section and noticed a couple of proposals from Ken Walters pertaining to his favorite topic, "scale-like speed." Ken has been hashing and rehashing scale speed and scale realism for years, mostly focusing on ways to mathematically quantify speed and therefore give it a place of its own on the score sheet. Ken has a very good point, and I agree 100 percent with his ideas but have found that his articles are, at best, technically difficult to understand. The point gets lost and, unfortunately, so does most of the interest, although I do agree that there needs to be more emphasis placed on the issue of speed.

"Scale-like speed" is thrown into that catch-all category called "scale realism," which is the lOth maneuver in the AMA rule book and at Top Gun is figured into each maneuver at the end. Either way, speed is low priority on the score sheet and also to most of the judges. I have to admit, though, when the speed is right, everyone notices-especially the judges. In the effort to keep scoring as simple as possible, we'll probably never see speed in a category of its own. Even so, most judges seem to put quite a bit of emphasis on it; this means you should, too. If you give judges what they want (a good, smooth presentation at the right speed), then they'll give you what you want (a high flight score). If you happen to question a judge and he says that you're flying too fast and you realize you can't do anything about it, you're hurtin'! This brings me to the main topic of how I achieve scale-like speed.

Scale speed and flight presentation are issues that I like to address in the planning stages of any competition project. Along with 3-views, color photos and chips, etc., you should also research flight characteristics so your model can be powered and configured to fly accurately. Of course, the easiest and safest way to go on power is to stick with the plan or kit manufacturer's or designer's recommendations, which are usually more than adequate for the size and weight of the model. I like to look at the full-scale performance figures, particularly the horsepower-to-gross-weight ratio (not overload), to decide on the power requirements of my models. I've used this ratio to configure power for several models, and I've found it to be very accurate in providing the proper flight attitude and speed to achieve good flight scores.

To use this system correctly, you must first estimate the finished weight of your model. Generally, you'll want to take the expected weight from the plan or kit manufacturer and then figure in your normal building style: light or heavy. You don't have to get it down to the ounce, but try to get it within a couple of pounds. You must also know how much horsepower your prospective engine puts out at cruise rpm, or 3/4 power. To keep this simple, a good rule of thumb for any normally aspirated, gas or methanol 2-stroke is 1hp per cubic inch. Anyone who tells you that their 2ci engine puts out 4 or 5hp is dreaming or doesn't understand the math. Normally aspirated (carburetor, non-turbo-charged) 4-stroke engines come in around 0.7hp per cubic inch.

From there we look at the performance figures of the full-scale version. Try to get the specifics as to gross weight and horsepower of the particular model (B, D, J, etc.), as they usually differ. Divide the horsepower by the weight down to where you know how many pounds each horsepower is carrying. During WW II, most fighter aircraft had a power-to-weight ratio of 1:8, or 1hp to every 8 pounds; bombers were usually around 1:10. If you look at different types of aircraft, you'll find that these numbers are pretty much universal to most piston-engine, propeller-driven aircraft, with the exception of aerobatic types and a few other special-use planes. Now take these numbers and apply them directly to the model you want to build. This will give you the engine size required to fly that model with as close to scale speed and performance as possible. To clarify things, here are a few examples.

Say you want to do a big P-47D model with a 92-inch wingspan. The plans call for a finished weight of about 30 pounds and for minimum power of 5ci or higher. You've read a review of the plans in your favorite magazine, and the builder said that the finished model came out right on at 30 pounds and, with a 5.8ci engine, would go straight up out of sight and knife-edge well. (Oh boy, that's just what I want to see: a Jug that can fly like an Extra!) You're an average builder, but you're going to detail this model for competition so want to add 5 pounds to make up for the details. This will bring the finished weight to 35 pounds. While scouring your documentation sources, you notice that the P-47D was powered by a Pratt & Whitney twin-row radial rated at 2,300hp and had a loaded gross weight of 18,000 pounds. Dividing down the fullscale numbers gives us a power-to-weight ratio of 1:7.8, or 7.8 pounds per 1hp. With this information, take the full-scale ratio and apply it directly to your proposed model weight (35 pounds). You'll come up with a required horsepower of around 4.4. This translates into a 4.2 to 4.5ci engine or, for those of you who like to think in metric, 65 to 70cc. I understand that the 4.2ci engine, which the numbers recommend for this size and weight model, is a long way from a 5.8ci, but we're talking "scale" here. It's very difficult-if not impossible-to create the "illusion" with an over-powered model. The key: if your model is powered right, you won't have to create the illusion, it will do it for you by flying correctly!