This is probably a well discussed subject, but I can't find it. What determines VNE in RVs? With the same basic elevator design on all RVs, why does one have a higher VNE than another and then a P51 Mustang can streak along at over twice our VNE. Even though the ailerons and elevators are made differently on the two airplanes, why would flutter not be a factor on the P51? I'm curious to know if it's control design or the way the surfaces are built.....beefier, thicker skins, more attach points...?
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Flutter?
- Thread starter DEWATSON
- Start date
Mark Albery
Well Known Member
The current issue of KITPLANES has another great article by Barnaby Wainfan on this very subject "Critical Flutter Airspeed".
Kitplanes continues to publish a monthly submission by Wainfan, and over the years he has articulated the various aspects of the contributors to the onset of flutter, as well as many other aerodynamic design details. His is the first article I read each month in Kitplanes.
Kitplanes continues to publish a monthly submission by Wainfan, and over the years he has articulated the various aspects of the contributors to the onset of flutter, as well as many other aerodynamic design details. His is the first article I read each month in Kitplanes.
Excellent!!!!
Interesting question...
I feel the answers will be even more interesting!!!
Interesting question...
I feel the answers will be even more interesting!!!
The NTSB report on the Reno accident a few years ago blamed a loose nut for allowing trim tab flutter which caused....resulting in the loss of the aircraft.
Flutter is so complicated in part because there are so many 'modes', eg, what's bending and in what direction. And it is possible that small variations from aircraft to aircraft can be important.
Do a search here on VAF on Flutter, and when you've read all the links, report back with a summary! You'll know more than all of us put together...or you'll be totally confused! 
LAMPSguy
Well Known Member
Chemistry
You know how in chemistry, they tell you "this is how things happen...always". Then the next semester, they explain how they lied and here are the exceptions...then the next semester....
Well, here is a simple explanation that is sorta right and sorta wrong, but may be a quick and dirty to get you far enough.
You know how some props have no rpm restrictions, some do. Some engines with some props don't, some other prop on that same engine does. Harmonics. Certain combinations of things just work, some don't. Same with women too. I am sure there is someone who would be plenty happy with MY practice wife!
You know how in chemistry, they tell you "this is how things happen...always". Then the next semester, they explain how they lied and here are the exceptions...then the next semester....
Well, here is a simple explanation that is sorta right and sorta wrong, but may be a quick and dirty to get you far enough.
You know how some props have no rpm restrictions, some do. Some engines with some props don't, some other prop on that same engine does. Harmonics. Certain combinations of things just work, some don't. Same with women too. I am sure there is someone who would be plenty happy with MY practice wife!
I'll take a shot at this.
With a aero engineering degree and 30 years experience in aircraft design here are my answers.
Question 1: Engineering Analysis verified by flight test.
Q2: Magic
Q3: answer to Q1 plus some of answer to Q2
Q4: yes, yes, yes, yes, yes .......
lostpilot28
Well Known Member
I think the problem with just doing a search for "flutter" will result in 98% returns on topics associated with the VNE of a specific model of RV.
I could be mistaken, but I think the OP is asking for what mechanical attribute makes the P-51 tolerate the higher speeds...ie. thicker skins, etc.
This is a great question...and I'm looking forward to a great answer.
I could be mistaken, but I think the OP is asking for what mechanical attribute makes the P-51 tolerate the higher speeds...ie. thicker skins, etc.
This is a great question...and I'm looking forward to a great answer.
SmilingJack
Well Known Member
flutter
Lostpilot28 understands my original question. If the shape of the ailerons and elevators are very close to the same shapes of those on a P51, why can the P51 tolerate such greater airspeed over these surfaces without experiencing flutter? I think they are flying modified P51s around 500 MPH at Reno.
Lostpilot28 understands my original question. If the shape of the ailerons and elevators are very close to the same shapes of those on a P51, why can the P51 tolerate such greater airspeed over these surfaces without experiencing flutter? I think they are flying modified P51s around 500 MPH at Reno.
ChiefPilot
Well Known Member
It's not just about shape, it's also about mass, balance, rigidity, damping, and numerous other factors. The Wikipedia page is a good introduction to the basic theories.
Weight, construction, material, balance point, airfoil ahead of it, wing loading, TAS, air density.... And they're totally different airfoils. Just for starters.
P51
In addition to the fatal accident at Reno, there have been several incidents that likely involved flutter. Voodoo many years ago had a tape across the elevator trim hinge. The tape peeled up in flight, the tab fluttered and the airplane did a violent pull up which put the pilot to sleep for a bit. He woke up at quite a high altitude and landed safely.
I believe there have been two P51's that had the rudder trim tab depart during Reno races. Both landed safely.
The P51 derived racer with the Lear wing crashed fatally from flutter in the rudder that tore the airplane apart.
The P51 fixed empennage components are much heavier than RV's.
The elevators and rudder are fairly light, but stronger hinges and mass balance located well inboard from tips. Elevators are all metal, rudder is fabric covered.
In addition to the fatal accident at Reno, there have been several incidents that likely involved flutter. Voodoo many years ago had a tape across the elevator trim hinge. The tape peeled up in flight, the tab fluttered and the airplane did a violent pull up which put the pilot to sleep for a bit. He woke up at quite a high altitude and landed safely.
I believe there have been two P51's that had the rudder trim tab depart during Reno races. Both landed safely.
The P51 derived racer with the Lear wing crashed fatally from flutter in the rudder that tore the airplane apart.
The P51 fixed empennage components are much heavier than RV's.
The elevators and rudder are fairly light, but stronger hinges and mass balance located well inboard from tips. Elevators are all metal, rudder is fabric covered.
AX-O
Well Known Member
Some funny post here but it does not answer your question.
All control surfaces are not created equal and are not subjected to the same local airflow or stresses.
For example lets use an elevator.
The rigidity of the inside structure contributes to how flexible the overall elevator is and what frequencies could excite a harmonic mode on the elevator.
The thickness of the skin also affects the rigidity of the elevator and the associated harmonics.
The way the elevator is attached to other structures also introduces issues because now you will have potential harmonics on both structures playing off each other. i.e. horizontal stab
The way the elevator is attached to the actual "control stick" also plays into how flexible "the collective" structure is.
Local imperfections i.e. dents, fat/narrow trailing edges, affect aerodynamics.
The airflow around the elevator is also affected by other structures i.e. fuse, horizontal stab, rudder, main wing, aileron/flap configuration, etc. That interaction will be different on different aircraft model based on where surfaces are located with respect to each other even if you are using the same elevator on two different aircraft.
The problem is further complicated by the atmosphere conditions.
You can have flutter below Vne if you get into the right (more like the wrong) conditions. There is no way to flight test every combination out there.
An elevator traveling at 200 mph straight and level is not under the same conditions as the same elevator traveling at 200 mph and 5 Gs. So speed is not the only factor affecting flutter. It takes many factors combined to get flutter.
That is a very short and simplistic answer but it should provide some elementary level knowledge. Hope that helps.
All control surfaces are not created equal and are not subjected to the same local airflow or stresses.
For example lets use an elevator.
The rigidity of the inside structure contributes to how flexible the overall elevator is and what frequencies could excite a harmonic mode on the elevator.
The thickness of the skin also affects the rigidity of the elevator and the associated harmonics.
The way the elevator is attached to other structures also introduces issues because now you will have potential harmonics on both structures playing off each other. i.e. horizontal stab
The way the elevator is attached to the actual "control stick" also plays into how flexible "the collective" structure is.
Local imperfections i.e. dents, fat/narrow trailing edges, affect aerodynamics.
The airflow around the elevator is also affected by other structures i.e. fuse, horizontal stab, rudder, main wing, aileron/flap configuration, etc. That interaction will be different on different aircraft model based on where surfaces are located with respect to each other even if you are using the same elevator on two different aircraft.
The problem is further complicated by the atmosphere conditions.
You can have flutter below Vne if you get into the right (more like the wrong) conditions. There is no way to flight test every combination out there.
An elevator traveling at 200 mph straight and level is not under the same conditions as the same elevator traveling at 200 mph and 5 Gs. So speed is not the only factor affecting flutter. It takes many factors combined to get flutter.
That is a very short and simplistic answer but it should provide some elementary level knowledge. Hope that helps.
Ok. I'll bite
Really simplified answer... The stiffer a part the higher the natural frequency at a given weight which normally translates to a higher VNE.
The magic trick is figuring out which parts have to be made stiffer, and not inducing other flutter modes by fixing the first one.
By mode I mean the parts of the airplane that interact to "spring" back and forth.
Unfortunately it is complex, so we can't model all interactions.. In the real world this is how VNE is set....
Engineering uses their best techniques, but often miss a mode or two....
https://m.youtube.com/watch?v=OhwLojNerMU
Then test pilots try their luck... The lucky ones don't find flutter...
http://www.youtube.com/watch?v=pEOmCkZyXzk
The test pilot dives to max speed... And hits the controls to induce flutter... If he doesn't get any, he add speed and tries again....
If the pilot is lucky and induces flutter at the right speed it looks like the above.. he pulls g's to increase stiffness, slow the plane and stop the flutter he lands, changes his shorts and discusses the experience with the engineer.
If the test pilot tries again 5 or 1 knots faster it looks like an explosion....and the test pilot dies. Many who encounter flutter die which is why the FAA mandates the analysis before flight testing.
Then the engineers figure out what broke and make it stiffer...or reduces the VNE.
The next pilot finds the next flutter mode....(springy part that explodes) Which could be anywhere. Classic examples... Fuse twisted by the tail, wing twisted by aileron, horizontal stab twisted be elevator, fuse bent by elevator....
Repeat until you can hit your VNE at all edges of the design envelope, or the plane become to heavy to fly... In which case you install a bigger engine (Merlin comes to mind)
Then they release the plane to the public and joe pilot gets in a spiral in the clouds, blows through VNE, and they find the next mode and make it stiffer. (Bonanza) or do more models on the mode that broke the plane and reduce the VNE.
Then someone puts bigger fuel tanks in the wing, which reduces its natural frequency.....and....the cycle continues
I hope this helps....
Derek
Really simplified answer... The stiffer a part the higher the natural frequency at a given weight which normally translates to a higher VNE.
The magic trick is figuring out which parts have to be made stiffer, and not inducing other flutter modes by fixing the first one.
By mode I mean the parts of the airplane that interact to "spring" back and forth.
Unfortunately it is complex, so we can't model all interactions.. In the real world this is how VNE is set....
Engineering uses their best techniques, but often miss a mode or two....
https://m.youtube.com/watch?v=OhwLojNerMU
Then test pilots try their luck... The lucky ones don't find flutter...
http://www.youtube.com/watch?v=pEOmCkZyXzk
The test pilot dives to max speed... And hits the controls to induce flutter... If he doesn't get any, he add speed and tries again....
If the pilot is lucky and induces flutter at the right speed it looks like the above.. he pulls g's to increase stiffness, slow the plane and stop the flutter he lands, changes his shorts and discusses the experience with the engineer.
If the test pilot tries again 5 or 1 knots faster it looks like an explosion....and the test pilot dies. Many who encounter flutter die which is why the FAA mandates the analysis before flight testing.
Then the engineers figure out what broke and make it stiffer...or reduces the VNE.
The next pilot finds the next flutter mode....(springy part that explodes) Which could be anywhere. Classic examples... Fuse twisted by the tail, wing twisted by aileron, horizontal stab twisted be elevator, fuse bent by elevator....
Repeat until you can hit your VNE at all edges of the design envelope, or the plane become to heavy to fly... In which case you install a bigger engine (Merlin comes to mind)
Then they release the plane to the public and joe pilot gets in a spiral in the clouds, blows through VNE, and they find the next mode and make it stiffer. (Bonanza) or do more models on the mode that broke the plane and reduce the VNE.
Then someone puts bigger fuel tanks in the wing, which reduces its natural frequency.....and....the cycle continues
I hope this helps....
Derek
Toobuilder
Well Known Member
And let's also not forget that the Corsair ailerons are made of wood - the metal ones fluttered in flight test.
flutter
OK....let's leave the P51 and talk only about RVs. First, I'm only familiar with the RV8. I know VNE for the RV8 is 230 MPH. I think VNE for the RV6 is 210 MPH, yet it is said that the RV6 has the strongest wing. The RV7 has the same tail and wing as the RV8 (I think....not sure about that). I've always heard that air speed should be kept at or below VNE to prevent flutter. Maybe I'm beating a dead horse, if so, I apologize. It just seems to me that RVs would all have the same VNE because of the very close similarities to design (skin thickness, control input design, airfoil, aileron attachments, elevator attachments, etc.) Obviously there's more to this flutter thing than I will ever understand.
OK....let's leave the P51 and talk only about RVs. First, I'm only familiar with the RV8. I know VNE for the RV8 is 230 MPH. I think VNE for the RV6 is 210 MPH, yet it is said that the RV6 has the strongest wing. The RV7 has the same tail and wing as the RV8 (I think....not sure about that). I've always heard that air speed should be kept at or below VNE to prevent flutter. Maybe I'm beating a dead horse, if so, I apologize. It just seems to me that RVs would all have the same VNE because of the very close similarities to design (skin thickness, control input design, airfoil, aileron attachments, elevator attachments, etc.) Obviously there's more to this flutter thing than I will ever understand.
David Paule
Well Known Member
Not all RV-s have the same surface designs. Differences abound and subtle ones can play a huge part in flutter. Even with scaling the design to different sizes, the differences that result in mass and in stiffness can be very important. And frequently, the stiffness of the fuselage or wing can play a significant part in the resulting flutter speed or characteristics.
As you indicated, it's very involved.
Dave
Maybe this will help.
Aerodynamic flutter is like ticking off a woman. To truly understand the condition leading to this requires knowledge of many interrelated variables and several intangible conditions. The variables are similar with certain groups of women, but no two are truly the same.
Best to simply stay well clear as once you trigger the condition, your odds of surviving are low.
Another perspective ...
First, a few words to look into on this matter:
Harmonics
Natural Frequency
Excitation
AX-O and others have given good info on this but here is a totally different look at it (from a LOT safer perspective )
On my RV6, there is some natural frequency associated with multiples of 9 KTS rolling (ground speed) that EXCITES my right gear leg to shake ("flutter"? ).
If I do nothing, I feel like the wheel pant will tear apart. If I speed up or slow down, it goes away until I reach a harmonic.
Now, even though there are many RV6's built the way mine is the suble differences in mass make a big difference as to whether the shake occurs as well as the magnitude. For example, I recently changed tires from recaps (heavier) to first run new (lighter and different treads) and the magnitude "feels" like it is twice as much though still at the same speeds.
Now, take all the stuff said about structures and mass in other posts and you can see where not only might flutter occur at different points for RV8 v RV7 v RV6 but even within the RV6 family it may be different. The analysis that many do helps to see where flutter is not likely to occur. All other regions are off limits. A roll of the dice.
James
First, a few words to look into on this matter:
Harmonics
Natural Frequency
Excitation
AX-O and others have given good info on this but here is a totally different look at it (from a LOT safer perspective
)On my RV6, there is some natural frequency associated with multiples of 9 KTS rolling (ground speed) that EXCITES my right gear leg to shake ("flutter"?
).If I do nothing, I feel like the wheel pant will tear apart. If I speed up or slow down, it goes away until I reach a harmonic.
Now, even though there are many RV6's built the way mine is the suble differences in mass make a big difference as to whether the shake occurs as well as the magnitude. For example, I recently changed tires from recaps (heavier) to first run new (lighter and different treads) and the magnitude "feels" like it is twice as much though still at the same speeds.
Now, take all the stuff said about structures and mass in other posts and you can see where not only might flutter occur at different points for RV8 v RV7 v RV6 but even within the RV6 family it may be different. The analysis that many do helps to see where flutter is not likely to occur. All other regions are off limits. A roll of the dice.
James
LAMPSguy
Well Known Member
Like I said about the ex!
BillL
Well Known Member
All the jokes and confusion won't make sense, but a new perspective to the complexity of the phenomenon will be revealed with this link.
http://www.ltas-cm3.ulg.ac.be/AERO0023-1/ConceptionAeroAeroelasticite.pdf
The jokes may become funny.
http://www.ltas-cm3.ulg.ac.be/AERO0023-1/ConceptionAeroAeroelasticite.pdf
The jokes may become funny.
F1Boss
Well Known Member
Well, MAYBE! Be Careful.
Recall that several fellas have spoken to frequency - increase frequency and increase Vne. Increase frequency by adding stiffness. Increase stiffness by adding structure. Increase structure, and increase weight. The process follows that path.
So, let's have a look at the P47 - a ship with almost no Vne. It is heavy. Google an exploded view of this ship - especially the aft section - and count the parts. Looking at that view, you MIGHT get a clue as to how to get your RV up to 300KT. In a dive.
Later, a follow-on version of the P47 was developed, and it's empennage fell off in dive testing: the aft fuselage failed as one engineer had predicted. The pilot survived by following that engineers' instructions (the heavy G forces would oscillate - "jump out when the Gs are negative") - I talked to him about this flight. It was not a fun ride, or so he told.
So, it is not gonna be easy to simply 'increase your Vne'. You WILL be making some parts. Keep in mind that the RV empennage is based on the Midget Mustang design, which is light, and doesn't really go that fast (0-200 power). Parts count remains the same with the RV (3,4,6,7,8), so Vne remains pretty much the same for the empennage.
I would stop there - but you can see where it all leads. You cannot simply fix one area - the aircraft structure aft of the spar is a system. Be ready to work back from the main spar to, and including, the emp structure, and then you can do your testing for a higher Vne.
The RV wing structure is ready for just about anything you think it can take. I had heard a very fast number associated with the aileron flutter margin, but I cannot speak to the wing itself, tho it is associated with the aileron system and it's flutter number. Seems the RV4/RV6 wing is VERY solid.
I have seen, and supplied replacement parts for, several F1s that have been flown to ridiculous speeds. I will volunteer that beefing up those parts will certainly allow those parts, and maybe the ship, to survive those speeds a second time, but the spar-aft system is not 'fixed'; other internal parts are required to make those speeds 'safe'. I have also been personally involved with one particular flutter event; the part that failed has been beefed up, but which part might fail next?
This flutter discussion is not to be taken lightly: gravity always works.
FYI the P-39 had an aft fuselage problem that let the empennage fall off. More than a 'few' aviators died proving this fault. Big, heavy, external skin doublers allowed that ship to remain in service. This is an example of how to handle flutter, or more to the point - aero elasticity. In this case, the empennage was up to the task, but the structure fwd was not - same as the more powerful version of the P-47. Point B cannot be stronger than Point A, lest Point A fail 1st.
"Gravity always works, and the ground is always hard."
Good luck, and Carry on!
Mark
Recall that several fellas have spoken to frequency - increase frequency and increase Vne. Increase frequency by adding stiffness. Increase stiffness by adding structure. Increase structure, and increase weight. The process follows that path.
So, let's have a look at the P47 - a ship with almost no Vne. It is heavy. Google an exploded view of this ship - especially the aft section - and count the parts. Looking at that view, you MIGHT get a clue as to how to get your RV up to 300KT. In a dive.
Later, a follow-on version of the P47 was developed, and it's empennage fell off in dive testing: the aft fuselage failed as one engineer had predicted. The pilot survived by following that engineers' instructions (the heavy G forces would oscillate - "jump out when the Gs are negative") - I talked to him about this flight. It was not a fun ride, or so he told.
So, it is not gonna be easy to simply 'increase your Vne'. You WILL be making some parts. Keep in mind that the RV empennage is based on the Midget Mustang design, which is light, and doesn't really go that fast (0-200 power). Parts count remains the same with the RV (3,4,6,7,8), so Vne remains pretty much the same for the empennage.
I would stop there - but you can see where it all leads. You cannot simply fix one area - the aircraft structure aft of the spar is a system. Be ready to work back from the main spar to, and including, the emp structure, and then you can do your testing for a higher Vne.
The RV wing structure is ready for just about anything you think it can take. I had heard a very fast number associated with the aileron flutter margin, but I cannot speak to the wing itself, tho it is associated with the aileron system and it's flutter number. Seems the RV4/RV6 wing is VERY solid.
I have seen, and supplied replacement parts for, several F1s that have been flown to ridiculous speeds. I will volunteer that beefing up those parts will certainly allow those parts, and maybe the ship, to survive those speeds a second time, but the spar-aft system is not 'fixed'; other internal parts are required to make those speeds 'safe'. I have also been personally involved with one particular flutter event; the part that failed has been beefed up, but which part might fail next?
This flutter discussion is not to be taken lightly: gravity always works.
FYI the P-39 had an aft fuselage problem that let the empennage fall off. More than a 'few' aviators died proving this fault. Big, heavy, external skin doublers allowed that ship to remain in service. This is an example of how to handle flutter, or more to the point - aero elasticity. In this case, the empennage was up to the task, but the structure fwd was not - same as the more powerful version of the P-47. Point B cannot be stronger than Point A, lest Point A fail 1st.
"Gravity always works, and the ground is always hard."
Good luck, and Carry on!
Mark
David Paule
Well Known Member
The RV-3 is considerably different. The rudder only has two hinges, not three, for example. Both stabilizers lack the middle ribs. Neither the elevator nor the rudder are balanced (but at least on the RV-3B, the ailerons are balanced).
I suspect there are thickness differences and probably other changes as well.
Ain't no way I'll take mine past its 210 mph Vne.
Dave
RV-3B - Still building wings.
As one who has survived wing flutter severe enough to break the rear spar and de-skin a portion of the wing, I can assure you that flutter is not something to take lightly.
This happened on a Moni Motorglider at a speed well below VNE. As a matter of fact, it happened at maneuver entry speed.
This happened on a Moni Motorglider at a speed well below VNE. As a matter of fact, it happened at maneuver entry speed.
rbibb
Well Known Member
Science vs. Engineering
The Science of Flutter is well understood. Without getting into details that would only prove how shallow my knowledge of the subject go back to high school science class where you had a weight (mass) and a spring and excited the combo at different rates. As some rate of excitation all **** broke loose - the resonant frequency.
Now a structure like an airplane is just a complex combination of mass and springs. What springs you say? Well the structure isn't rigid in that it can flex and bend under load. If it didn't it would be too heavy to fly. The material (in this case) aluminum acts as a spring albeit a very stiff one. Hence at some excitation frequency all **** is going to break loose. So science knows why and how. Engineering seeks to model the structure in such a manner that all the various mass and spring elements that make up the structure can be reduced to a simplified model that mere mortals engineers) and predict what the resonant frequency is and hence what excitation by the air molecules at a certain speed can take - and then add a margin just to show they don't trust the calculations. Then do some tests (in a wind tunnel perhaps) to see if they are right. Then with some brave soul - in flight tests to further validate their estimates to be close enough. That it doesn't always work out is evident the results cited of tails departing aircraft, etc.
So bottom line is -everyone knows why it happens but no one knows for sure exactly when it will happen in a particular aircraft. The excitation point can be pushed higher and higher with a stiffer spring (stiffer structure) at the expense of that bug-a-boo of performance - weight. Some changes are obvious from empirical observation - stiffer skins, tighter control linkage tolerances, better balance, etc. But exactly what that will do for you is impossible to calculate with any degree of exactness. Modern numerical methods like finite element analysis help get better approximations - but if it was perfect they would't need flight tests.
Long answer to simple question but the reason an RV can't have the same VNE of a P51 is simple actually. Because it doesn't have the same structure as a P-51. Nor an RV6 versus -8, etc. When it comes to structural dynamics - YMMV with construction details and even things like how many coats of paint you put on it. Remember its a mass and spring thing. Change any one of them and you've changed the natural frequencies when all **** breaks loose.
The Science of Flutter is well understood. Without getting into details that would only prove how shallow my knowledge of the subject go back to high school science class where you had a weight (mass) and a spring and excited the combo at different rates. As some rate of excitation all **** broke loose - the resonant frequency.
Now a structure like an airplane is just a complex combination of mass and springs. What springs you say? Well the structure isn't rigid in that it can flex and bend under load. If it didn't it would be too heavy to fly. The material (in this case) aluminum acts as a spring albeit a very stiff one. Hence at some excitation frequency all **** is going to break loose. So science knows why and how. Engineering seeks to model the structure in such a manner that all the various mass and spring elements that make up the structure can be reduced to a simplified model that mere mortals engineers) and predict what the resonant frequency is and hence what excitation by the air molecules at a certain speed can take - and then add a margin just to show they don't trust the calculations. Then do some tests (in a wind tunnel perhaps) to see if they are right. Then with some brave soul - in flight tests to further validate their estimates to be close enough. That it doesn't always work out is evident the results cited of tails departing aircraft, etc.
So bottom line is -everyone knows why it happens but no one knows for sure exactly when it will happen in a particular aircraft. The excitation point can be pushed higher and higher with a stiffer spring (stiffer structure) at the expense of that bug-a-boo of performance - weight. Some changes are obvious from empirical observation - stiffer skins, tighter control linkage tolerances, better balance, etc. But exactly what that will do for you is impossible to calculate with any degree of exactness. Modern numerical methods like finite element analysis help get better approximations - but if it was perfect they would't need flight tests.
Long answer to simple question but the reason an RV can't have the same VNE of a P51 is simple actually. Because it doesn't have the same structure as a P-51. Nor an RV6 versus -8, etc. When it comes to structural dynamics - YMMV with construction details and even things like how many coats of paint you put on it. Remember its a mass and spring thing. Change any one of them and you've changed the natural frequencies when all **** breaks loose.
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Marc Bourget
Well Known Member
While I didn't read EVERY post in this thread, I have some advice that will increase the safety of flutter test flights.
The procedure outlined to me by John Thorp, who experienced two flutter incidents in the same flight testing the T-18, is to dive the aircraft to 5 mph above the particular excitation speed and put the aircraft into a climb and do the excitation input as the aircraft is slowing through the test speed.
As to the two incidents experienced by Mr. Thorp, the second was identified only after examining the electronic data. The structure fluttered a second time due to the damage incurred in the first and it was not felt through the controls. I have heard of a different incident where the pilot reported feeling something, but couldn't identify what it was, only to realize he had developed a blister in his palm from holding the stick.
Finally, flutter testing protocols typically test one excitation at a time, i.e., "pulse left aileron" or "up elevator" like explained above, and shown in the BD-10 in-flight failure, the tail aft of the spar is a "system". Flutter tests typically mimick a gust load at straight and level cruise, for example. Nobody I've heard of has ever combined two or more excitation pulses as part of their flutter test protocols.
Be careful !
mjb
The procedure outlined to me by John Thorp, who experienced two flutter incidents in the same flight testing the T-18, is to dive the aircraft to 5 mph above the particular excitation speed and put the aircraft into a climb and do the excitation input as the aircraft is slowing through the test speed.
As to the two incidents experienced by Mr. Thorp, the second was identified only after examining the electronic data. The structure fluttered a second time due to the damage incurred in the first and it was not felt through the controls. I have heard of a different incident where the pilot reported feeling something, but couldn't identify what it was, only to realize he had developed a blister in his palm from holding the stick.
Finally, flutter testing protocols typically test one excitation at a time, i.e., "pulse left aileron" or "up elevator" like explained above, and shown in the BD-10 in-flight failure, the tail aft of the spar is a "system". Flutter tests typically mimick a gust load at straight and level cruise, for example. Nobody I've heard of has ever combined two or more excitation pulses as part of their flutter test protocols.
Be careful !
mjb
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