IO-360 -> Maximum safe HP?

Post by **Spike** » Mon May 15, 2006 8:16 am

John, keep in mind that your true airspeed is your speed through the air. The reason that Vne, and more specifically flutter speeds, have to do with how fast the air is moving past the airframe, not how much of it. I believe what causes flutter, as Van wrote in one of his articles, is a harmonic between the aircraft surfaces and the speed at which the air is moving around it. The air is moving around the airplane at true airspeed, while IAS is measuring pressure as you mentioned previously.

I should go digging for that RVator article.

-- John

N537TM · Post by **N537TM** » Mon May 15, 2006 11:19 am

CJ

I remember that RV article... Flutter is one of the problems, but some one tried to explain to me the "over stressing the airframe problem".. Here goes i'll try to explain what I learned.

The fast air moving over the wing is flutter ...

The second is the mass of the plane moving in a straight line, and when you change from the straight line, the mass does not want to change direction with you.. It resists this change, we call it load factor.

Lets look at what happens when I yank back on the stick at ...say 135 Kts true .... 120 indicated. The plane pitches up and since I'm truley traveling 135 kts ... I see X G's.

Now if I yank back at 120 indicated, when I'm 205 true, wow, I see many more G's ... pop goes that clubbed rivet in the spar ( darn i new I should have fixed that sucker) and bang goes the wing. Not a good day.

Same indicated .. but different true.

Does this help???

Mike

cjensen · Post by **cjensen** » Mon May 15, 2006 11:59 am

Whoa, whoa, whoa...

Vne is NOT based on true airspeed. It is an INDICATED speed right on the airspeed indicator-RED LINE. NEVER EXCEED THIS INDICATED speed. True airspeed is a computed speed corrected for temp and pressure.

Here's the best example I can think of (works with ANY airplane)-

The TBM 700 has a Vne of 266kts indicated. The airplane will true at 285kts without any problems whatsoever at altitude. Same with a Learjet, but it's called Vmo, and is moveable with altitude. True airspeed will ALWAYS be higher than indicated as you go up.

True airspeed is how fast you are actually traveling over the ground in a no wind situation (groundspeed is true airspeed corrected for wind). It has nothing to do with how fast the air is moving past the airpalne...

Post by **Spike** » Mon May 15, 2006 2:46 pm

Hate to tell you Chad, Vne for the RV series is based upon true airspeed. Check out the first edition of the RVator 2005, and I believe the one before that. An excerpt,

... As I suspected the idea that a major reason for establishing Vne was based on a true Airspeed number was news to some pilots ...

Another caller quested using True airspeed when FAR Part 23 ... uses indicated airspeed. Again, the answer is about margins. if, for instance, an airplane with a normally aspirated engine is flying above 10,000 feet the diminishing power will offset the increasing true airspeed by an amount that will make it impossible to reach Vne. Whether the pilot is reading true or indicated doesnt matter - hes still within the margin of safety. Its possible that knowing this the writers of Part 23 decided to keep things simple.

But what if that same airplane has gobs more power available through a turbo charger or bigger engine? The designer had to choose a range of powerplants and make some assumptions when he designed the structure and established the Vne. ... But of course we are in the experimental world and can put in any engine we choose. The designer cant possibly provide margins againast every possible powerplant.

Basically, their reasoning for this is because they can't control the engine that people are putting on their aircraft. Part 23 specifies indicated airspeed for certified birds which is ok because the designers know all of powerplants that will be used on their aircraft. Therefore they can predict how much power will be lost at any given altitude. Van's does not, therefore they can not predict the performance profile of every aircraft to determine where to put that line on the airpseed indicator to be safe across all regimes of flight.

Their solution is to state it as an absolute number in true airspeed.

True airspeed is how fast you are actually traveling over the ground in a no wind situation (groundspeed is true airspeed corrected for wind). It has nothing to do with how fast the air is moving past the airpalne...

I do believe that you just contradicted yourself.

cjensen · Post by **cjensen** » Mon May 15, 2006 3:24 pm

Ok, that makes some sense, however it's put in to rather vague terms. Such is the experimental airplane world. I didn't get that issue of RVator since my sub started last summer. I can see their point, but it basically passes the problem on to the builder, which is fine. If the engine you choose is of higher power than recommended, you should be respobsible for figuring out the problem yourself. I won't argue this any further. If that's what Vans says, that's what is correct...

Quote:

True airspeed is how fast you are actually traveling over the ground in a no wind situation (groundspeed is true airspeed corrected for wind). It has nothing to do with how fast the air is moving past the airpalne...

I do believe that you just contradicted yourself.

Where is the contradiction in this statement?

Once the equation of true airspeed is figured (indicated corrected for calibration, then corrected for non standard temp and pressure), you use that to figure groundspeed, which takes in to account for whatever wind component is acting on the airplane (groundspeed equals true airspeed corrected for wind). The air moving past the airplane is simply indicated airspeed. The amount of ram air pressure is (close to) the same amount of air passing over and around the airplane. Less dense air means less ram air pressure, and less molecules circling the airplane at altitude. If your true airspeed is figured to be 200mph at 9000ft, the airplane and systems may be only feeling 170mph of force. Airspeed is just a measure of pressure...that has to be corrected for to figure true and ground speeds (and that's AFTER you correct for calibration errors). I know that last sentence was repetitive...

Am I missing something here, or has all this studying for a CFI gone to my head???

Post by **Spike** » Mon May 15, 2006 3:56 pm

cjensen wrote: Where is the contradiction in this statement?

As you pointed out, in a no wind situation, the speed at which you are moving over the ground is your true airspeed, or, how fast the air is passing by the airframe. If so then how can it have "nothing to do with how fast the air is moving past the airpalne?"

Once the equation of true airspeed is figured (indicated corrected for calibration, then corrected for non standard temp and pressure), you use that to figure groundspeed, which takes in to account for whatever wind component is acting on the airplane (groundspeed equals true airspeed corrected for wind).

Agreed

The air moving past the airplane is simply indicated airspeed. The amount of ram air pressure is (close to) the same amount of air passing over and around the airplane.

Not sure what you mean by this.

Less dense air means less ram air pressure, and less molecules circling the airplane at altitude. If your true airspeed is figured to be 200mph at 9000ft, the airplane and systems may be only feeling 170mph of force.

Agreed

Airspeed is just a measure of pressure...that has to be corrected for to figure true and ground speeds (and that's AFTER you correct for calibration errors). I know that last sentence was repetitive...

The statement that I made bold is incorrect technically. Indicated airspeed is just a measure of pressure, not the airspeed itself. Because of this, in order to find out how fast you are flying through the air (true airspeed) when up high, or during non standard conditions, you have to use an equation (as you point out). At sea level, on a standard day, your airspeed indicator will indicate your true airspeed, because that is what it is calibrated to. As you climb, temperatures change, etc, the amount of pressure your AI records changes due to the changing density of the atmosphere. This is where the equation comes in. Because we know how temperature, humidity, & elevation affect the atmosphere, we can use math to adjust the indication of AI to get rid of these variances. What you get is True Airspeed, or, the speed at which the molecules (though there are less of them) of air are passing by.

cjensen · Post by **cjensen** » Mon May 15, 2006 4:13 pm

I think we may be arguing some of the same points here Spike.

The statement that I made bold is incorrect technically. Indicated airspeed is just a measure of pressure, not the airspeed itself.

Forgot the technical word in front of airspeed.

Indicated is, in fact, what I was implying. Sorry for the confusion.

I'm not even sure where I was going with the statement that "it has nothing to do with how fast the air is moving past the airplane." Strike it from the record!

captain_john · Post by **captain_john** » Mon May 15, 2006 4:41 pm

Okay...

So the difference is the pitot sees larger molecules and can't effectively compress them and the wing sees a volume of them and the size doesn't matter because they are still moving past the wing at a certaion clip... whether they are compressed in the pitot and displayed on the AS indicator or not.

Is this the correct layman's take on the subject or what?

CJ

Post by **Spike** » Mon May 15, 2006 4:43 pm

Dont shoot me John, but I believe the wing and the pitot see the exact same thing

Dan A · Post by **Dan A** » Mon May 15, 2006 7:05 pm

I received my new RVATOR today and Van has a good article about ignoring factory recommendations you all will want to read.
Cheers!

Dan N742DA 94+ hrs. closing in on the magic 100 number -- then it's inspection time!

jim_geo · Post by **jim_geo** » Mon May 15, 2006 9:31 pm

Chad appears to be right on every count as I see it. Lets all remember though that the airframe is moving through the air and not the air blowing past. It all comes down to compressibility you have to move faster past fewer molecules to compress fewer molecules to a pressure great enough to indicate, in this case, say 200. 200 indicated may be 215* on the ol GPS.

Then there was mention of flutter. I'm not sure I got the drift right but if you're airflow causes an indication of 200 MPH then your control surfaces are going to act like 200 MPH even if you're velocity through air space is 280 MPH.

*totally made up number and again dead calm for simplicity.

captain_john · Post by **captain_john** » Tue May 16, 2006 5:59 am

Jim, you said:

Lets all remember though that the airframe is moving through the air and not the air blowing past.

This is totally relative and this is why we call it relative wind.

Yes, agreed, we do have less particulate aloft and therefore, less resistance.

I think THIS is a contributor to flutter and consequently a factor in determining Vne.

What confuses me is, how does this work in a fashion that appears totally opposite to rationale?

If the plane has less resistance, wouldn't it be tricked into faster flight at a lower indicated like the pitot?

Still confused...

CJ

cjensen · Post by **cjensen** » Tue May 16, 2006 9:16 am

Flutter can happen at any altitude, therefore any density (but is more likely at higher altitudes). It's more of a function of airframe design and negative harmonics/resonance in the control surfaces. Here's a good, but brief discussion on flutter:

13.3 Aerodynamic effects of flight at excessive speed
Flutter
When aerodynamic forces applied to the wing or a control surface alter the aoa, the dynamic pressure distribution changes. These changes plus the structure's elastic reactions may combine as an oscillation or vibration (probably initially noticed as a buzz in the airframe) which will either damp itself or, as the airspeed is increased, may begin to resonate at the natural frequency of the structure and thus rapidly increase in amplitude if the phase relationships are right. (Pushing a child on a swing is an example of phase relationships and amplification). This latter condition is flutter and, unless airspeed is very quickly reduced, will cause control surface separation within a very few seconds.

Inertia has a role in flutter development requiring that control surfaces – ailerons, elevators, rudder – be mass balanced (i.e. the centre of gravity of the control surface coincides with the hinge line) to limit the mass moment of inertia; and also to prevent them becoming heavier as airspeed increases. It may be acceptable for the control surface to be over-balanced, i.e. the cg is slightly forward of the hinge line.

The critical flutter airspeed [or something akin to it] may eventuate well below Vd or Vdf if wear in control surface hinges, slop in actuating rods/cables/cranks/torque tubes, water or ice inside control surfaces or absorbed within a foam core, mud outside, faulty trim tabs or other system weaknesses exist which alter the structure's reactions.

The following paragraph is an extract from an article by William P. Rodden appearing in the McGraw-Hill Dictionary of Science and Technology; it provides a succinct description of flutter:

"Flutter (aeronautics) – An aeroelastic self-excited vibration with a sustained or divergent amplitude, which occurs when a structure is placed in a flow of sufficiently high velocity. Flutter is an instability that can be extremely violent. At low speeds, in the presence of an airstream, the vibration modes of an aircraft are stable; that is, if the aircraft is disturbed, the ensuing motion will be damped. At higher speeds, the effect of the airstream is to couple two or more vibration modes such that the vibrating structure will extract energy from the airstream. The coupled vibration modes will remain stable as long as the extracted energy is dissipated by the internal damping or friction of the structure. However a critical speed is reached when the extracted energy equals the amount of energy that the structure is capable of dissipating, and a neutrally stable vibration will persist. This is called the flutter speed. At a higher speed, the vibration amplitude will diverge, and a structural failure will result."

Less resistance means the airplane FEELS less, like the pitot. There is no tricking the airframe. In less dense air, there is obviously less resistance on the airframe. Flutter can develop here (as you stated CJ) if airspeeds are increased to the point where the air becomes unstuck from a control surface, and can no longer control balance of that surface.

The airplane is TRUING faster than indicated, but the airframe does not FEEL like it is traveling faster through the thinner air (no trickery here). Less stuff impacting and compressing on the airframe.

Did that BS help at all???

N200PF · Post by **N200PF** » Tue May 16, 2006 12:49 pm

Oh my gosh I think I just got it!!!

The airframe is able to move FASTER through the air becuase it's thiner but the molecules that are used to indicate are fewer so the indicated airspeed shows lower than "True". Thus the speed of the air moving over the airframe (regardless of density) is higher which brings you closer to flutter speed (Vne) with out showing it on your airspeed indicator.

The density of the air has NO bearing on Vne. Flutter will happen when the air moves over the control serfaces at a certain rate (Vne) regardless of how thin or thick the air is. Thin air allows you to get there faster as it provides less resistance on the airframe.

...did I get it? Maybe not. SHEESH!!!

- Peter

PS - I'm going back to my vixen file and sandpaper now...

Lorin Dueck · Post by **Lorin Dueck** » Tue May 16, 2006 8:36 pm

Peter -

Yup, you got it.
Flutter is induced by harmonics not force.
All it takes is a wind at the right frequency (i.e. speed)

Ever seen the video of the Tacoma Narrows bridge (Gallping Gertie)?

http://www.enm.bris.ac.uk/research/nonl ... acoma.html

Lorin D
9A Wings
(N194LD reserved)

captain_john · Post by **captain_john** » Tue May 16, 2006 10:12 pm

Sooooooo, due to the fact that the air is thinner, the boundary layer has the ability to become "unstuck" more easily and provide less pressure on the control surfaces depriving them of a normal "neutral" position and causing them to flutter?

Am I far off the mark?

CJ

jim_geo · Post by **jim_geo** » Wed May 17, 2006 12:52 am

What say's it's comming unstuck easier?

captain_john · Post by **captain_john** » Wed May 17, 2006 5:44 am

Chad's excerpt says:

At low speeds, in the presence of an airstream, the vibration modes of an aircraft are stable; that is, if the aircraft is disturbed, the ensuing motion will be damped. At higher speeds, the effect of the airstream is to couple two or more vibration modes such that the vibrating structure will extract energy from the airstream.

The airstream. That is what I assume to be the boundary layer?

CJ

Post by **Spike** » Wed May 17, 2006 10:18 am

N200PF wrote: The airframe is able to move FASTER through the air becuase it's thiner but the molecules that are used to indicate are fewer so the indicated airspeed shows lower than "True". Thus the speed of the air moving over the airframe (regardless of density) is higher which brings you closer to flutter speed (Vne) with out showing it on your airspeed indicator.

The density of the air has NO bearing on Vne. Flutter will happen when the air moves over the control serfaces at a certain rate (Vne) regardless of how thin or thick the air is. Thin air allows you to get there faster as it provides less resistance on the airframe

On the money I believe. Obviously it would be great if one of the Van's guys would explain a bit more.

Post by **Spike** » Wed May 17, 2006 10:20 am

John, I believe airstream to just be the air moving past the control surfaces. I wouldnt extrapulate that to boundry layer personally cause Im not sure exactly what that entails. But I believe that the excitation that they are referring to is directly related to the velocity of the air, not the pressure that the air is applying.

-- John