Elevators: Balance, Springs and Bungees

[takair]

7 minutes ago, PT20J said:

This is interesting. I ran down the specs on the PA24-250 at https://www.skytamer.com/Piper_PA-24-250.html.

Since the dimensions of the M20J and the PA24-250 are almost identical except for the tail feathers, it is interesting to compare.

The Mooney horizontal tail is 2 ft² larger than the Comanche (Mooney 34.5 ft², Comanche 32.5 ft²)

The Mooney vertical stabilizer + rudder is slightly larger than the Comanche (Mooney 14.15 ft², Comanche 13.4 ft²)

So, the Mooney's empennage design (swept forward surfaces and trimmable stabilizer with trim assist bungees) didn't end up smaller (less drag) than the Comanche's empennage (stabilator and swept-back fin). 

Skip

Speaking of Comanche tails, I was reminded of this flutter video.  As I recall, there were a few flutter related accidents that occurred on boot equipped Comanches.  
 

 

[PT20J]

19 minutes ago, takair said:

Speaking of Comanche tails, I was reminded of this flutter video.  As I recall, there were a few flutter related accidents that occurred on boot equipped Comanches.  

Gee, wouldn't it be fun to be a test pilot? 

It's a PA30. Not sure if a PA24 would do the same thing and the test conditions aren't stated; maybe they were way over Vne or something. Stabilators must be mass balanced just like any control surface to give margin against flutter. There is a big balance weight on a tube out ahead of the stabilator inside the fuselage on a Cherokee. I assume that the Comanche had a similar balance. Maybe @M20Docknows about this.

One thing I always liked about the rectangular stabilator on a Cherokee is that it was just the right height for a nice picnic table if you brought along a couple of folding chairs. Just rotate it level and throw a table cloth over it 

Skip

[Blue on Top]

1 hour ago, takair said:

Speaking of Comanche tails, I was reminded of this flutter video.  As I recall, there were a few flutter related accidents that occurred on boot equipped Comanches.  

Way freakin' cool video!  Yes, Flight Test is fun!  I spent 30+ years of my career in it … so far.  Because they had close-up video of this, they had to know that it was there and that it was not instantly divergent.  We typically shake the wing and tail, symmetrically and anti-symmetrically, from 0-50 Hz. to excite any coupled modes.  Natural frequencies are typically a little under or a little over 10 Hz.  Lower Hz for larger more compliant parts and higher for smaller, stiffer parts.

Again, way cool video @takair!

[Blue on Top]

@PT20J  … and now you're seeing the complexity and tradeoffs of design.  Another huge one is part commonality.  There are parts from the Bonanzas that are used on the 1900s … including most of the certification basis.  It's a very complex world.

There are pros and cons with a stabilator design, most of which is aero-elastic considerations and pilot control force considerations (which drive internal control system loads and weights).

Bottom Line:  It's a big tradeoff with typically little differences.

Thanks, Ron

PS. A high wing is actually faster than a lower wing, but landing gear are very long and don't fit in a high wing well.  Hence the ugly Cessna landing gear.

[PT20J]

12 minutes ago, Blue on Top said:

PS. A high wing is actually faster than a lower wing, but landing gear are very long and don't fit in a high wing well.  Hence the ugly Cessna landing gear.

Interference drag?

[Blue on Top]

1 minute ago, PT20J said:

Interference drag?

Yes, and a lot less dihedral.  You can also change the fuselage shape (make it smaller earlier … before the wing trailing edge).  Which, btw, causes drag on an M20  

[Guest]

7 hours ago, PT20J said:

Gee, wouldn't it be fun to be a test pilot? 

It's a PA30. Not sure if a PA24 would do the same thing and the test conditions aren't stated; maybe they were way over Vne or something. Stabilators must be mass balanced just like any control surface to give margin against flutter. There is a big balance weight on a tube out ahead of the stabilator inside the fuselage on a Cherokee. I assume that the Comanche had a similar balance. Maybe @M20Docknows about this.

One thing I always liked about the rectangular stabilator on a Cherokee is that it was just the right height for a nice picnic table if you brought along a couple of folding chairs. Just rotate it level and throw a table cloth over it 

Skip

That video is of a NASA flight test series on a PA30 Twin Comanche.  The twin has a Vne of 230 mph with a central mass balance in the fuselage, like the Cherokee.  Single Comanches have several different Vne speeds depending on model and modifications.  Rudders required external mass balance to raise the Vne to 202, and external mass balances on the tips of the stabilator to return the the original 227.  
Also required were hinge stiffeners for the trim tab hinges, lack of these was supposed to be the cause of the flutter in the video.

Clarence

[bradp]

So what’s the matter with the raptor aileron design - seems like it flutters or oscillates at 40 kts.

[PT20J]

Clarence @M20Doc, Since the Comanche layout, size and wing are similar to the M20, but the control surfaces are different, I'd be interested in your comparisons of handling and especially control forces between the two airplanes.

Skip

[PT20J]

11 hours ago, Blue on Top said:

There are pros and cons with a stabilator design, most of which is aero-elastic considerations and pilot control force considerations (which drive internal control system loads and weights).

I used to fly Cherokees a lot. The trim always seemed to take a bit of fiddling to get just right. There just wasn't a lot of control force feedback near the trim point. Is that a characteristic of stabilators? 

Skip