john491

My Feedback: (13)

I am soliciting opinions here in an effort to make a decision on a build I am designing. As the title suggests the question is whether to go with a flying stab or a fixed stabilizer and an elevator.
For a little background, the plane is sort of a variant on the F-15, which is presented by models with both choices. The model I have built in the past has been a fixed stab with elevator. I have been going around with myself if it is worth it to try a flying stab or not. There are engineering concerns for the flying stab, chief among them the bearings that the stab would be mounted on. At the moment the design is pretty fluid, so I can incorporate almost anything I can think of.
What are the benefits of a flying stab over a fixed system? I just don't want to be spinning my wheels on a design change that only has marginal benefits if incorporated.
Thanks,
John

Boomerang1

I think full flying stabilizers were designed into full size aircraft to meet certain performance requirements.

If you are building a scale model with a full flying stab it's just another problem to overcome to maintain
the scale appearance. Apart from looking cool I can only see disadvantages with full flying stabs for the
average model.

causeitflies

Unless you are going to be approaching the speed of sound, go with the conventional stab and elevator. It's simpler and more effective.

rgburrill

Modern fighter aircraft are designed to fly on the edge of instability - some even past that edge. To do that they require flight control computers (FCCs). There are many model aircraft FCCs but some are better than others when talking about intentional aircraft instability. Those models with full-flying stabs also likely have good FCCs.
Also, flying on the edge of instability puts incredible stresses in the aircraft. The US Air Force lost several early F-16 pilots because the plane could outfly the pilot's G level capability. The RC aircraft with a full-flying stab must be very well built and strong.

Txmustangflyer

The full stabilator was implemented first on the F-86 sabre as far as US military aircraft. The reason was elevator "lock". At high speeds, like on the P-38 lightning, the pilot simply did not have the strength to move the elevator due to air pressure on the surfaces. So a hydraulic assisted flying tail was implemented in production.

Aerodynamic compression can occur at speeds in excess of 500 knots. As rc aircraft don't fly that fast (yet) the chances of it happening are slim. However, it still is possoble that, with enough air pressure you could stall some servos. Just use good servos and a atandard stab/elevator set up should be fine.

The reason the USAF lost early viper pilots was the software used in the fly by wire, not the stabs themselves. The software didn't limit the g-force. In the F-16, when pressure is put on the control stick, it only moves maybe an 8th of an inch, max. It uses pressure sensors at the base of the stick to tell the fly by wire how much to move the surfaces. It takes a very light hand on the stick. But, the computer still has final say so on it. If the computer calculates that what the pilot is asking for will overstress the airframe, it will just, simply max out the air frame.
That was before the G limiter was installed. Yes, some died due to G-LoC before implementation. But 100s more times, the air frames were over stressed as well and rendered unflyable. Pilots were "bending" the aircraft.
The flight controls are so sensitive, that pilots transitioning to the aircraft on attempting a turn for the first tine would inadvertantly snap roll the aircraft 3 to 4 revolutions just trying to do a simple bank turn.
Yes, it was still possible to turn off the AoA limiter and the G limiter in the C and block 52 models with a flip of a switch but only recommended if the aircraft was in a stall. It stalled and fell like a tree leaf, balancing on its cg..it didn't stall and drop the nose, so a pilot had to disable the limiters and try to rock the plane using the stabs muliple times to get it to pitch nose down and regain airspeed.
The F16 manual actually dedicates an entire 28 page section to stall recovery and procedures.

In the popular flight simulation of the late 90s and beyond, these flight characteristics were very detailed. The sim was called Falcon 4.0...the manual that came with it, was a carbon copy of the F-16 flight manual, only omitting classified information.
For you guys that own rc Vipers, that manual would be a huge insight to your aircraft. Even with airfoil changes to make it more forgiving for rc, theres still a lot of good info on the performance of the aircraft.

Halcyon66

You can get that manual here, it is a great read at 600 pages????

https://www.f-16.net/downloads_file19.html

F-16.net is also a great F-16 resource site.

Regards,

patf

My Feedback: (4)

there is a big difference between controls at sub-sonic and super-sonic speeds. for 99% of what we do, we do not 'need' full flying stabs...

Txmustangflyer

Correct. We don't, however, full flying stabs are much less complicated in the end, needibg only a tube, bushings or bearings, a carrier for said busings or bearings and a control horn and linkage. No hinges, etc. The carrier can easily be done with laminated ply between fuselage formers, a bearing under the skin of the fuselage, another on the inside, and a way to mount the control horn at or just aft of the tube on the stab, or a hidden linkage setup.
But from an aerodynamic standpoint, you are currect, rc aircraft don't need flying stabs.

Halcyon66

Yet they look so cool

Unless this happens,




F-22 that i had for years over 200 flights and lost a stab.

Regards,

Txmustangflyer

That wasn't because of it being a flyibg stab, but wear.

I hate to say it, but that sort of failure is preventable with a good maintenence routine each off season including cheching bearings, bushings and retainers.

Sorry you lost your aircraft, but if it moves, it needs to be periodically checked for wear, and the more flight time, the more often it needs checked.

Halcyon66

It was the port stab that failed not servo or links.

At least the turbine was ok, quick clean with a rag and she was good as new



Regards,