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EAA Experimenter

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A Tool for Understanding Power, Drag, and Prop Design

New perspectives, techniques, and a working model

By Howard Handelman, EAA 111399, for Experimenter

Drag

Howard Handelman learned to fly in the late 1970s. He’s owned a Cessna 150-M and a Moni for which he built new wings. His current aviation pleasure comes from the RV-7A N17HH (www.N17HH.net) he built and uses as a test platform for the ideas found in the “tools.” He claims no professional qualifications—just a retired “IT guy” and compulsively curious.

Most experimental aircraft builders, when faced with a repetitive task in the shop, will build a tool, jig, or fixture designed to ease the workload. This article gives you a tool as well. It doesn’t do anything you couldn’t do with a pencil and paper or a pocket calculator, but it does it the same way every time. And it does it instantly. No calculations are hidden, and all are proven standard factors or methods. It’s a template and a measuring tool.

At EAA AirVenture Oshkosh 2010, I presented an explanation of how, with common instrumentation, the drag curve of a general aviation aircraft can be determined with reasonable accuracy. Surprisingly, the accuracy may be an improvement on some CAFE aircraft performance tests. That presentation was an upgrade of an earlier article we published in Experimenter. The upgrade clarified the concepts, simplified the methods, and corrected some weak areas.

The benefits of knowing your drag curve include:

  • understanding and predicting flight profiles quickly and easily
  • improving propeller design by matching it to the airframe
  • providing a framework for evaluating test data and isolating results that don’t fit
  • closely estimating horsepower and propulsive efficiency.

This article presents an improved tool - using a spreadsheet - for recording, testing, and manipulating the results of testing. Its prerequisite is that the “key point” on the drag curve has been established as described in the 2010 presentation. It can be found and downloaded here. The sample is set up for the CAFE RV-6A so that known data can be verified. If you save it to your computer you’ll be able to use your data. Here’s a picture:

Table

The tool uses these values:

Minimum pounds of drag

Target density altitude

Speed at minimum drag (Vld)

Estimated specific fuel consumption

Weight of airplane when drag established

Estimated glide ratio (iteratively)

Weight of airplane for prediction

Test value for propulsive efficiency

Target true airspeed

 

For extra convenience, there's a built-in calculator for converting knots to mph, calibrated airspeed (CAS) to true airspeed, plus pressure altitude and temperature to density altitude, if you don’t have an E6B flight computer handy.

Albert Einstein is credited as saying: “As far as the laws of mathematics refer to reality, they are not certain; and as far as they are certain, they do not refer to reality.” That’s true for this tool. It’s useful within the levels of accuracy we can obtain, but it’s not perfect and ignores many small factors. It’s also not accurate for laminar flow wings.

The value of this tool is that it gives us an integrated picture, and the value of that integrated picture is that no one factor can change without “causing” a corresponding change elsewhere in the tool. Everything is connected to everything else in a known way. It’s a working math model of your airplane—some of it, at least.

To Use It, Begin With What You Know
Use CAS for minimum drag, the pounds of drag at that speed, and the weight of the aircraft then and now. There is no place to enter the minimum drag! Instead, you must enter the glide ratio and iterate that until the calculated drag matches the known. Now you have the CAS glide triangle and the deduction part is complete. The triangle is simple physics used in a novel way. Please note that this glide triangle is not the same as an engine-out glide; this ratio will always be higher (better) than with an engine out (single engine, of course). Idle power glides can be better or worse.

From there, you can make predictions. Tell the sheet how fast you want to go and at what altitude. If you supply estimated SFC (specific fuel consumption) and propulsive efficiency, it will tell you the BHP (brake horsepower) you need to do it—whether or not you have that much power. Those predictions can be used in reverse to check your estimates against reality.

You’re right: A separate tool that would be useful would be an engine power calculator. There are many, including manufacturer charts. Sadly they disagree to an extent suggesting that some of them can’t be correct. Soon I’ll add to the confusion (in a future article), but while this tool uses only accepted factors and formulas, my engine power calculator will go into disputed territory. Meanwhile, I included one from Kevin Horton with his kind permission. 

What speed improvement would a more powerful engine get you? (Not much, usually.) If you’re matching a new prop to your plane, what numbers to use? What’s an optimum flight profile for cross-country? If someone says his engine or prop or wing mod produced a given change, does it look reasonable when viewed with this tool?

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