WWI Replica Safety

[Joe Perkel]

Originally Posted by Machinbird

The intent of the above tour of the V-n diagram is to give a better understanding of the use of the diagram from a piloting viewpoint. I hope it is helpful.
Sid

Sid and Hank,

More than a bit helpful! Very informative, worthy of further study and future reference.

What I've done is to copy and paste both responses along with the diagram into a dedicated text file in case this thread eventually disappears.

Excellent stuff, Bravo and thanks!

[Brad]

I'm glad to see the discussion on manuvering speed, flutter, etc. as it is VERY important.

I think it is equally important to discuss the other end of the spectrum, particularly when we are talking about test flying a new replica.

A couple of things that got drilled into my head by several different instructors come to mind... and they both relate to energy management.

To begin with, in cruise flight... pitch controls altitude, throttle controls airspeed. Ok, that makes sense.

But when we are talking about approach to landing... those functions are reversed. Pitch controls airspeed, throttle controls altitude.

My first instructor taught me the "conventional" approach to landing... pull power to specified RPM, bleed off speed, lower flaps, turn crosswind, next flap setting, turn final, etc... all the while trying to judge how much power you need to put the airplane where you want it. That's all well and good as long as you have power.

What happens if you have an engine "issue" while that is going on?

My last instructor taught me a different approach... power to idle abeam the numbers, bleed off speed, make your turns to place the airplane where you know you can make the runway. Too high? Slip it. Too low? Ok, dummy, you screwed up. Add power, let's try that again... get it right this time.

Now it REALLY irks me to see spam can pilots fly a 2 mile final with power. It is their life they are risking, not mine... but they're also making my pattern more difficult, because once I pull power I have every intention of being able to land the aircraft without applying any throttle... so I have to adjust accordingly.

So that brings us to WWI replicas. Most of them have a VERY steep glide due to light weight and lots of drag. I suspect that many of them, if you pull the power to idle abeam the numbers you would have just enough energy for a constant curving "short approach" pattern... remember you have to maintain enough airspeed for a flare.

It may even be of benefit to have some "ultralight" instruction since the energy curve (so to speak) is probably pretty similar. Just a thought.

As for how this relates to flight testing... well... a new aircraft is more likely to have a power failure than an existing one that has been flying for a while, simply because of the "dummy factor." I'd rather plan ahead for my own stupidity than take risks.

[hank jarrett]

OK, I am working on some more from what Sid said but thought I should comment on part of Brads message right away.
Brad, if your instructor told you that the elevator controls altitude and the throttle controls speed (at ANY point in the flight) he may need a quick dope slap to the back of the head. That is ABSOLUTELY wrong! It all relates to trim speed and although you may get an initial increase in altitude from a new elevator position, the plane will seek the new trim speed and stop climbing. It should return to the original altitude (with some oscillations that can be mathematically defined and should be a part of the test program) and stabilize at that new speed.
If you increase throttle, the plane may initially accelerate, but its speed will oscillate around the original speed (usually VERY few times) and the plane will CLIMB. Some of these effects may be slightly changed at test points like maximum altitude (NOT service ceiling!) or where the aircraft is ready to stall, but the plane will react to changes in elevator trim and throttle the same all the time in normal unaccelerated flight.
Hank

[Joe Perkel]

Originally Posted by hank jarrett

"...if your instructor told you that the elevator controls altitude and the throttle controls speed (at ANY point in the flight) he may need a quick dope slap to the back of the head. That is ABSOLUTELY wrong! Hank..."

Many pilots may not be able to put it into words (explain precisely what's happening to the airframe) but, few maneuvers test a pilot's successful inputs to the airplane quite like maintaining the glide-slope and localizer to decision height. I highly recommend instrument training to anyone not already so trained.

[Machinbird]

Hi. For those still curious about flutter, I've put together a bit more information.
I found a youtube posting by an amateur rocket oriented person discussing fin flutter on rockets. He ran some computer simulations using NASTRAN.
Look at the various ways the simplistic fin he modeled can vibrate. This has application to wings and control surface flutter, although the aircraft situation is more complex. Each vibration mode is unique and has its own frequency. As you go to the higher order modes, the frequency of vibration is increasing. Just energizing one of the oscillation modes at its characteristic frequency is sufficient to cause a flutter problem.
Here is the flick:

YouTube - Fin flutter mode shapes--2nd version

YouTube - Fin flutter mode shapes--2nd version

The Wikipedia article on Aeroelasticity has some good info, look at the section on flutter.
http://en.wikipedia.org
/wiki/Aeroelasticity
When something flutters, the energy of the airstream around the surface is extracted by the flutter process to drive the oscillation.
So how does this energy couple to the surface? I'm not a specialist in aerodynamics, but I believe a process akin to 'Karman Vortex Street' generation is at the root of this coupling. See this Wiki article for yet more info:
Kármán vortex street - Wikipedia, the free encyclopedia

As usual, I'd love to hear from someone who has better understanding of the subject than myself. My formal background is in physics and not aerodynamics.
Sid

[hank jarrett]

Had to happen. NASTRAN has even invaded WW-1 aeroplanes! I just can't escape it!
The nice thing about NASTRAN is the parts don't break. Imagine while you watch that video how much force it takes to bend that fin that way. The loads built up are HUGE! That is why flutter failures are often mistaken for overload. It only cycled once and the part failed.
A good way to see the forcing functions that cause flutter is to watch a flag wave. What you are seeing is 2nd, 3ed and sometimes even higher degree flutter in a VERY flexible structure (the cloth in the flag). Imagine the air hitting the flag pole and a vortex rolling off one side. As the vortex sheds on one side of the flag it deflects the flag so that the air on the other side is influence to shed a vortex there. The cycle repeats back and forth causing the familiar "flutter" of the flag. If you load all the parameters for a flag and pole in NASTRAN it gives back all the movements you have seen so many times at ball games. Using the variables in the computer program will even let you see the little "snaps" at the end of the flag that overload the weave of and cause it to tear along the stripes and the trailing edge.
You can use NASTRAN to design a flag that will last longer in different winds (but some of them are pretty expensive fixes and one makes the flag flutter strange and just doesn't look pretty.
Hank

[Joe Perkel]

A fascinating exercise in complex aerodynamics, I think the lesson here for us is care in control rigging and assembly.

For the purposes of WWI replica building and operation, directly from the AC 90-89A….

i. What can be done about it?

Having described how flutter happens, the following suggestions
should help reduce the possibility of it happening
to the amateur-builder’s aircraft:

(1) Perform a mass balance of all flight
controls in accordance with the designer/kit manufacturer’s
instructions.

(2) Eliminate all control ‘‘free play’’ by
reducing slop in rod end bearings, hinges, and every
nut and bolt used in attaching flight controls.

(3) Ensure that all rigging and cable tension
is set accurately to the design specifications
using a calibrated cable tensiometer.

(4) Re-balance any flight control if it has
been repaired, repainted, or modified in any way.

NOTE: If the pilot experiences flutter, or
believes he did, reduce power immediately
and land as soon as possible. Do not attempt
further flight until the aircraft has been
thoroughly inspected for flutter induced
damage. This inspection should include all
wing/tail attach points, flight controls, their
attach points/hinges, hardware, control
rods, and control rod bearings for elongated
bolt/rivet holes, cracks, (especially rod end
bearings) and sheared rivets.

[Machinbird]

Originally Posted by Joe Perkel

A fascinating exercise in complex aerodynamics, I think the lesson here for us is care in control rigging and assembly.

For the purposes of WWI replica building and operation, directly from the AC 90-89A….

Joe, What you have here applies to control surfaces. Suppose you needed to add equipment into a wing, say a strobe power supply near the wing tip. Suppose it was real convenient to mount the power supply near the rear spar.
That might actually be a bad idea. Just imagine what happens when the wing experiences the effect of g load. The weight of the power supply twists the wing very slightly toward a higher angle of attack. Now imagine the same wing tip vibrating in an up and down manner. The power supply's weight causes the wing tip to twist in a direction that increases the motion. Now suppose further that the new characteristic twisting oscillation frequency aligns with one of the wing's spanwise vibration modes. This is how you get into flutter below the Vne speed.
In general, mass should be added to surfaces at or slightly ahead of the local center of gravity of that section. There are a lot of knowledgeable and capable certified aircraft mechanics that do not understand this concept.
Mass distribution can be critical.
Sid

[Joe Perkel]

Originally Posted by Machinbird

In general, mass should be added to surfaces at or slightly ahead of the local center of gravity of that section. There are a lot of knowledgeable and capable certified aircraft mechanics that do not understand this concept.
Mass distribution can be critical.
Sid

Sid,

Funny you should mention this, the flutter discussion has me looking at the documentation in various references regarding this very issue. I have the advantage of adding density of materials data to my CAD software to calculate this for me, but I need to relearn the basics the old fashioned way. The control surfaces, (all four ailerons, the vertical stabilizer, as well as the rudder), are all early targets for fabrication on my project.

Your point regarding the addition of apparently subtle but potentially significant changes, is a reminder that these designs are likely best left as they were.

[N4085B]

Originally Posted by Machinbird

There is a condition you do not want to encounter during the test flying build up on your new aircraft. Flutter. Few living people have any experience with the subject. The subject aircraft in the following sequence was flown by Fred Haise of Apollo 13 fame. The sequence ends just as the flutter is beginning to dampen as the chase pilot wisely backs away from an incipient catastrophic failure.

From the relatively low frequency of the oscillation, I think the mode being excited was the spanwise bending mode of the stabilator, but I certainly wouldn't mind someone with more knowledge of the subject providing better information.
There is quite a bit more basic information available on the subject of flutter which is a part of the general subject of Aeroelastic phenomena.
The purpose of this topic is not to put fear into prospective test pilots, but to arm them with the information necessary to recognize that they might be approaching an aeroelastic limit on their flying machine.
Let me know if you would like to see more along this topic and of course, feel free to contribute.
Sid

Thanks for posting that nasa video. I learned how to fly in a Comanche 180 (single engine version of the aircraft in the video) ...and that was truly sobering to see!