john491

My Feedback: (13)

Hello,
Can anyone tell me why there is a sawtooth on the horizontal stabilizer of a F-15? What purpose does it serve? Is it relevant to models such as ours or is it only for scale purposes?
An enquiring mind wants to know.
John

Halcyon66

To reduce the potential for flutter which can cause catastrophic failure of the stabilizer and the loss of the airframe.

Regards,

john491

My Feedback: (13)

Thank you for the information.
This could be why I lost my left stabilizer to flutter, causing the plane to ultimately crash.
John

Halcyon66

Aerodynamics on models verses real aircraft are very different, so no way to accurately say that was the cause.

Servo/link could have given way or a failure of some other kind could be responsible for the loss.

I always mass balance full moving surfaces, yet there has been some debate on that practice.

Regards,

john491

My Feedback: (13)

In this particular case the crash was due to flutter, and a bit of bad piloting.
I had made four passes around the field on the maiden flight when my spotter said he thought he saw an aileron flutter. So I did a slower circuit and saw none and throttled back up. I was at about 3/4 throttle on a pass right in front of us when the stabilizer went to a high speed flutter and fell off the plane. My natural reaction when a part falls off the plane at higher speeds is to slow down. Unfortunately this was in a tight turn and I slowed too far and I spun in.
But it was definitely flutter that started it all ( I keep telling myself ).
I know how to fix it now though.
John
This was with a stabilizer and elevator combination, as a matter of fact the stabs and rudders were salvaged from a successfully flown plane that I lost to engine failure into some pepper trees.

Zeeb

My Feedback: (41)

There is something to be said about balancing control surfaces on models and the "stabilator" setup causes more arguments. I would argue that it if wasn't important, BVM would not include weights for the forward/inboard side ot the elevons on the F16 models along with a supplemental "manual" update.

That said; as Halcyon66 mentions, full scale aerodynmics do not directly convert to models as explained by Reynolds numbers. Briefly, that means since there is a huge size difference but the air molecules are the same size, they do not behave the same way on the model.

Last question; the model wasn't an FEJ was it? They were/are notorious for flutter issues on the control surfaces......

john491

My Feedback: (13)

Zeeb,
The model in question was a rebuilt and reprocessed Parkinson Regal Eagle. It seems that some parts, like the fully sheeted wings, survive almost any crash, numerous times, so I recycle them. In this case I reused the wings and fins with their built in rudder components. I also reused one stabilizer. It was the new stabilizer which broke and came off the plane. In the past I have used spun carbon cloth instead of fiberglass when I covered the part. I didn't on this occasion. Evidently the wood was too thin, a case of 3/16" as opposed to 1/4" stock.
John

Halcyon66

There are many influences when it comes to all moving surfaces, AOA, center of gravity, center of inertia, center of pressure, surface twist and on and on. They are complex to resolve.

What is also forgotten is the simple point of a surface hanging down and putting pressure on the servo gears when RC turned off and potentially forcing overtravel can create serious issues.

The above can defiantly be addressed with mass balancing, also when RC is on you don't have the constant pressure on the servo.

Strong links and high end servos are always the best starting point.

Regards,

Zeeb

My Feedback: (41)

On that note, most of the higher end servos now come with "slow on" if you will? Power up, they slowly move to their neutral positions. Not usually and issue with Turbines but may be useful on the flying stabilators. They are almost a g*d send for larger IMAC birds who's elevators are almost always hanging all the way down.