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Forces and Moments

9/5/2001 by George Hicks

Copyright:© 2001 Horizon Hobby, Inc.

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In this article...

Page 1: For Starters
Page 3: Torque
Page 5: P-Factor

IMAC is different from other model aerobatic disciplines because the basic configuration of the airplane is more-or-less predefined.We are permitted to deviate from scale by utilizing the 10% rule, which allows us to do some fine-tuning of the airplane for our specific model aerobatic needs.In the end our ultimate goal is to produce an airplane that requires the least amount of pilot input to perform the task at hand while still maintaining a close resemblance its full-scale counterpart.The demands on the airplane's ability certainly increase as you move up in the class hierarchy.An airplane that is setup to perform the Sportsman sequence well might not be competitive in the advanced class without modification.The additional power requirements are only a small part of the problem since the airplane must now be setup to perform as well inverted as it does upright.The requirements on power and proper setup become even more important in the Unlimited class since your airplane must be able to perform every conceivable FAI catalogue maneuver. For many, having your airplane perfectly setup for the sequence isn't enough because the same airplane is expected to be capable of performing all the latest 3D techno-wizardry that has become so popular.When all is said and done you really want to "have your cake and eat it too"...nothing wrong with that, but what do you do when you have a trimming problem that looks too complicated to be solved easily?This is where a solid, fundamental understanding of how the airplane works becomes indispensable.This understanding will allow you to break things down into manageable parts so you can eventually arrive at a logical solution.Some of the problem stems from the way we approach the setup and trimming process in general.The standard method is what I call the "cookbook" approach where we mindlessly follow the orders dictated by some trim chart with little or no thought.Surprisingly, this will get you pretty far but it does little to enhance your problem solving ability when something happens that isn't covered by the method.Furthermore, the material limits you in that you only make adjustments that are provided in the chart itself.This kind of thinking and approach can be detrimental.We should all try and think out of the "box" and learn to conceptualize the airplane in different ways.The trimming and setup process becomes much more useful when you truly understand why you need to make certain modifications and more importantly how these modifications will affect other characteristics of the airplane.

I'm going to do my best to refrain from very specific technical jargon and long equations because I feel that many times they cause more confusion than understanding.On the other hand I feel that a thorough explanation that covers all the major facets of the problem is necessary otherwise you cheapen the quality of the information.My goal is to explain concepts that appear difficult at first, because of the way they are often presented, but in fact are nothing more than simple ideas once you break them down into their basic components.Much of the information you will already know but the idea is to make it all fit together such that you can visualize how the airplane works as a whole.For the newcomer, the learning curve can be very steep and at first glance seem quite overwhelming but remember that nothing is ever as complicated as it looks.I can remember all the questions I had when I started flying aerobatics.Back then information wasn't as easy to find and the answers I did find didn't always prove true.These days the novice should consider himself/herself lucky because all the necessary information is virtually at their fingertips. This availability of information can literally shave years off the time it takes to learn the "art" of precision aerobatic flying.

If the flying is the "art", then the design, setup, and trimming process is the "science" and this is what I intend to cover over the next few articles.I figured that we would start by talking about how propellers affect the airplanes we fly. Last time I checked every IMAC pilot I know had a propeller on his or her airplane so this topic is pretty universal.And now for the good stuff...

Remember when you built your first trainer and you noticed that the plans called for a couple of degrees of right thrust and an equal amount of down thrust?I do... My first question was why do I need to offset my engine?Zero-zero seems like the logical answer to me.After pondering the situation I had totally confused myself by trying to invent some theory to make sense of the situation.What's left to do but ask the local experts?The club gurus immediately started talking in some other language and I heard words like spiraling airstreams and uneven disc-loading and then I heard about the evils of engine torque.After my lesson I was certain that if I bolted the engine on with no right or down thrust my airplane would undoubtedly crash.

I think we have all struggled with trying to figure out whether or not our aerobatic models actually benefit from an offset thrust line.There are many schools of thought and opinions on the matter.You'll find this in every technical aerobatic topic and it's often comical at how heated the discussions can become among the enthusiast.In the end the right answer is the one that makes the airplane perform and handle the way you want it too...period.I've found that every situation is slightly different and it's up to the competitor to make the final judgement.Unfortunately, many pilots don't know how they want the airplane to handle.Don't fret...this is how we all start in the beginning.Most importantly, don't try to force yourself to see things that aren't really there.Eventually your skills will progress to the point where you will start to notice how small modifications can really help your precision flying.Remember that learning is the fun part of all this...it's what life's all about.

The things the aerobatic competitor needs to know about power effects can be counted on one hand so get your fingers ready because when we're finished you'll have memorized the five major effects propellers have on your airplane.