Aerodynamic Balance

[ePilot22]

I'm doing my CFI checkride and I'm asked about mass balance and aerodynamic balance. I know that the ailerons, rudder and elevator of a Cessna 172RG (most cessna and aircraft in general) are mass or horn balanced and I understand that it makes control inputs by the pilot easy and cuts down on "flutter". But how and why?

 

[Lead Sled]

OK, remember when a control is deflected a low pressure area forms on the cambered side. This tends to pull the control back into alignment with the wing, stabilizer or fin as the case may be. However, the control surface has mass and therefore momentum. If the center of gravity of the control surface is behind the hinge, the control tends to overshoot the point of alignment. The result is a tendency for the control to flutter. To solve the above problem the control must be balanced, so that its center of gravity is in line with the hinge.

'Sled

 

[Stifler's Mom]

I don't know if it is me, but I don't ever remember having to know this much **CENSORED****CENSORED****CENSORED****CENSORED** about flying. You guys make my head hurt sometimes.

 

[Tinstaafl]

Really? This was basic Private stuff where& when I learnt.

To expand on Lead's comments and address another part of the question:

Mass balance: Moves the hinged trailing edge's moment closer to the hinge line to reduce the tendency to flutter. As Lead said, the aerodynamic forces are trying to move the deflected surface back to the 'in trail' position however the inertia of the surface means that it will 'overshoot' the neutral position. Now the mirror image of the forces occurs. Flutter is more likely as speed increases because the aerodymic force increases. Adding a weight forward of the hinge line offsets the inertia of the mass of surface behind the hinge line.

Aerodynamic balance: The 'horn' surfaces forward of the hinge line aren't just to house weights. The area of the surface ahead of the hingle line helps to reduce the force needed to deflect the surface into the airstream.

 

[Lead Sled]

Aerodynamic balance: The 'horn' surfaces forward of the hinge line aren't just to house weights. The area of the surface ahead of the hingle line helps to reduce the force needed to deflect the surface into the airstream.

Oops, I knew that I was forgetting something.

'Sled

 

[sqwkvfr]

Really? This was basic Private stuff where& when I learnt.

I'm with Stifler's Mom on this...I'm working on my CFI and I haven't seen this anywhere.

To expand on Lead's comments and address another part of the question:

Mass balance: Moves the hinged trailing edge's moment closer to the hinge line to reduce the tendency to flutter. As [/b]Lead[/b] said, the aerodynamic forces are trying to move the deflected surface back to the 'in trail' position however the inertia of the surface means that it will 'overshoot' the neutral position. Now the mirror image of the forces occurs. Flutter is more likely as speed increases because the aerodymic force increases. Adding a weight forward of the hinge line offsets the inertia of the mass of surface behind the hinge line.

Aerodynamic balance: The 'horn' surfaces forward of the hinge line aren't just to house weights. The area of the surface ahead of the hingle line helps to reduce the force needed to deflect the surface into the airstream.

Thanks for the info...this is good stuff.

 

[JetSpeed219]

I'm just about to take my CFI checkride and I have never heard of this either.......

 

[Lead Sled]

I'm just about to take my CFI checkride and I have never heard of this either...

There is stuff that you need to know; there is stuff that you should know; there is stuff that is nice to know; and finally, there is stuff that you don't need to know. Personally, I think that this falls somewhere between nice and don't need to know.

'Sled

 

[ePilot22]

Passed the Ride!

Thanks for you responses! I know a lot of you do not deal with single engine Cessnas any more but again I thank you for your time and effort. Evertime we pre-flight and count the three counter weights under the aileron surface and check the balances on the elevator and rudder we should know what they are for and how to explain why they are important to a student. I have always checked and counted them but never gave a second thought to what they were or are for until my oral yesterday. I now understand the importance of mass balance and how it affects us as pilots and the control surfaces including the wings in flight.

Thanks again!

Tom

 

[Lead Sled]

Congratulations on passing the CFI ride. The FAA holds CFIs to a higher standard and all of your rides should only get easier from now on.

Now for a couple of CFI rules...

Rule #1: Do what I mean, not what I say.
Rule #2: Always stay at least one chapter ahead of your student.

'Sled

 

[ePilot22]

Thanks! It took more than one attempt. I had my first oral on Wendesday and didn't last 45 mins. I knew everything really well, I just couldn't give all the information I know in a way that someone who didn't could understand. What gets me are my friends (fellow pilots) that say they don't want to be instructors bacause they don't what to fly traffic patterns and do steep turns for 800 hundred hours. I just tell them be glad someone was willing to do it for you or you'd be a Burger King salting the fries right now! I hoping to get on with a 141 school in Oklahoma but it'll be a few weeks before I know anything for sure.


Thanks again.

Tom

 

[Lead Sled]

What gets me are my friends (fellow pilots) that say they don't want to be instructors bacause they don't what to fly traffic patterns and do steep turns for 800 hundred hours.

Tom...
It's too bad your friends feel the way they do. There are very few shortcuts to experience in this business. Personally, when I'm looking to hire someone, the fact that the candidate has either held or holds a CFI certificate is a big plus for me. I don't really care how much they used it - the fact that they were able to successfully jump through all of the hoops is enough. (In fact, based of the theory that there's a big difference between 1000 hours of experience and one hour of experience repeated 1000 times, I tend to start discounting dual given after 1000 hours or so.)

It wasn't always so. I had an ATP and over 3,000 hours before I got my CFI. I thought that somehow I had "dodged a bullet" by being able to go directly into Part 135 (charter) flying after I got my commercial and instrument rating. I was wrong. In any teaching situation, it's always the teacher who learns the most. The same thing applies doubly in aviation. Getting your CFI and actively instructing for a while will teach you things about flying that you will only learn through instructing. I learned a lot and to this day the lessons are very valuable and useful. Becoming a CFI, in my opinion, is one of the most important steps you can take in your aviation career.

It may not alway be easy, but the overall experience will be worth it. Enjoy.

'Sled

 

[ePilot22]

One of my friends got a 1900 type rating and he only has 300TT and barely has his CommAMEL. He chose to do that instead of CFI. I hope he makes it with a carrier but I think the time is way to low. Anyway, I wish the examiner understood your first rule, that would have helped me of so much! I had a FAA guy sit in on the oral and after I had failed he said that I needed to pretend I was trying to teach a waitress at IHOP how to fly or someone how to eat a bag of cookies. You can't just tell them to open the bag! Do what I mean...not what I say!!! Now I'm just afraid I going to ruin the first few students I get. But your right, I've heard I'll learn more in the first six months of teaching than I learned in the last four years of flying! I'm looking forward to it at the same time - dreading it! Thanks again!

 

[Lil Jon]

I've heard I'll learn more in the first six months of teaching than I learned in the last four years of flying! I'm looking forward to it at the same time - dreading it!

that's the truth. i don't know how many times i said to myself: huh! hmm, look at that! now i get it! when teaching/watching my students fly. don't dread instructing too much, its a great exprience (i never thought i'd say that). the grass is always greener......but carry a sick sack for yourself, you might need it after the 30th lazy 8 of the day! best of luck.

 

[Catbert]

I wish the examiner understood your first rule, that would have helped me of so much! I had a FAA guy sit in on the oral and after I had failed he said that I needed to pretend I was trying to teach a waitress at IHOP how to fly or someone how to eat a bag of cookies. You can't just tell them to open the bag!

Yep. My CFI instructor told me pretty much the same thing when I was working on my ticket. When I started instructing I spent the first 100 hours or so making my instruction progressively simpler. I find myself occasionally explaining to the student what I mean by "horizon". It's not that they're dumb, it's just that for most people, this is a completely new world. We've had PhD's who flat out don't understand a word you say and high school kids who are aviating geniuses. You really have to start at ground zero, assuming nothing.

 

[nosehair]

there's a big difference between 1000 hours of experience and one hour of experience repeated 1000 times, I tend to start discounting dual given after 1000 hours or so

I used to think that same thing...back in the few thousand hour level...but now I see another level of knowledge and experience begins to appear when you have many, many thousands of hours of repeating.

For one, repeating the same exact thing will get to you...break it up, mix it up, spin it up, teach to the student - not to the lesson. Make the lesson work for you - not you work for the lesson. Burn out occurs when you keep doing the same thing over and over and over and ov...well, you get the point...

After a while you begin to get it that you can continue the experience long after the thrill is gone....

 

[ePilot22]

I love the advice! It's much more than I expected when I posted the question, but now I have another situtation. I earned my CFI/I in Oklahoma and all my other ratings in Denver (where I live, PPL, INST, COMM-ASEL) but I now have a job offer in NJ. 2000 miles from home. 0 dual given. I have never been on the northeast coast so the lifestyle and things are a big change. The nice thing so far is the varity of experience. From high altitude - no actual in Denver to low altitude and a lot of actual in Oklahoma to NJ and what the east coast has to offer. The pay is not great and with around 700hrs in a year (part 61 with recommend Jepp syllabus), but it's more than what I've got in Denver, in fact my home FBO is not happy I went to Oklahoma to do the CFI/I. What do you guys/girls think? Thanks!

Tom

 

[Pilot_Ryan]

You really have to move for a flight instructor job? Well, if you wanted to move anyway, I could understand. But generally CFI jobs should not be THAT hard to come by in your local area, unless you're in a really weird spot (you're not.) I'd be careful about moving just for a temporary job like CFI.

 

[Flechas]

Congrats on passing your ride man!!!

I agree with the above posters, I had a great time instructing and learned a lot.

Some advice:

- Don't milk your students, not even when you only have a couple. Instead try to build a good reputation, that will get you a lot more students, it's a snowball effect.

- Take at least a day off every week, you will need it (I wish a had done this).

- Always keep studying and learning

- When a student has a question that you don't know, tell the truth and LOOK IT UP, don't just ask around, a lot of the information flying around, someone made it up or assumed it was the answer.

- Have lots of fun!!

Best of luck finding a job!

Flechas

 

[ePilot22]

Love the advice! Keep it coming! The real problem with Denver is a high CFI market and low student base. One student flying twice a week and I'll starve to death before they solo! Or I'll overwhelm them with technical knowledge, their eyes will glaze over and I'll never see them again. My flight club isn't happy I left to do my CFI/I at another location and they're not really interested in hiring me because of it. It sucks because I did my PPL,INST, and COMM ASEL with them. I love the Denver area but moving to another location is great experience. Thanks again, you all have given me more than I could have asked for!

Tom

 

[Lead Sled]

I used to think that same thing...back in the few thousand hour level...but now I see another level of knowledge and experience begins to appear when you have many, many thousands of hours of repeating...

That may indeed be true for some instructors, but I've yet to see it and I've been a CFI for 30 years. As a working corporate chief pilot and flight department manager what I have seen, time and time again, is high-time (up to 7,000 hours of dual given) CFIs who can't make it in the "real world". It doesn't matter if they've jumped through the hoops to get a type-rating or two (or three or four), if they don't have the practical experience of dealing with weather, ATC, etc they're going to be behind the "power curve" if you know what I mean. When you climb into the cockpit with a 5,000 hour pilot you expect to have a certain amount of experience and background. High time CFIs can fly and explain the various training manuevers; they can quote you day in and day out what the various FAA books say about weather, ice, etc. but they often don't have the "hands on", "real world" experience that you must have to suppliment all of that book learning. Put those guys in a cockpit when you've got some serious IFR flying to do and they're propbably not going be able to contribute at the level of your expectations.

'Sled