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