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Ben_Chiu Certificates/Ratings/Crew Stations: "Attitude at Altitude?" 12-07-02, 06:17z  Greetings: Carlos and I were discussing high altitude ops the other day and somehow we got onto the subject of attitudes and altitude. I thought this topic might generate some interesting comments. Here's the situation: If an aircraft maintains the same flight configuration and TAS (True Airspeed) at various altitudes, does AOA (Angle Of Attack) change or remain the same? How about if you maintain the same IAS (Indicated Air Speed)? Of course if you say either changes, explaining why will get you an extra 10 points! Ben -= VPC OffLine Reader 2.1 =- Registered to: Ben Chiu -OLR.PL v1.80- Alert Edit | Reply | Reply With Quote | Top

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Subject      Author      Message Date     ID      RE: Attitude at Altitude? John_M 12-08-02 1    RE: Attitude at Altitude? Ben_Chiu 12-08-02 2         RE: Attitude at Altitude? PeteDowson 12-08-02 3              RE: Attitude at Altitude? andrewluck 12-08-02 4                   RE: Attitude at Altitude? Ben_Chiu 12-12-02 6                        RE: Attitude at Altitude? andrewluck 03-24-03 12                             RE: Attitude at Altitude? Ben_Chiu 03-25-03 13                                  RE: Attitude at Altitude? andrewluck 04-05-03 14              RE: Attitude at Altitude? Ben_Chiu 12-12-02 5                   RE: Attitude at Altitude? PeteDowson 12-12-02 7                        RE: Attitude at Altitude? p51mstg 12-13-02 8                             RE: Attitude at Altitude? Ben_Chiu 12-13-02 10                        RE: Attitude at Altitude? Ben_Chiu 12-13-02 9   RE: Attitude at Altitude? Ben_Chiu 03-22-03 11

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John_M Certificates/Ratings/Crew Stations: 1. "RE: Attitude at Altitude?" 12-08-02, 00:00z  Ben, I'm of the position that a plan with a bad atitude will not be able to maintain altitude <g>. As one gets higher, the air gets thinner. To maintain the same lift, the AOA must be increased. At some point, the aircraft reaches it's service ceiling ( ROC = 100 fpm ). If the pilot forces it further, a stall can be expected. Regards, John Remove | Alert Edit | Reply | Reply With Quote | Top

Ben_Chiu Certificates/Ratings/Crew Stations: 2. "RE: Attitude at Altitude?" 12-08-02, 02:40z  Greetings John: > I'm of the position that a plan with a bad atitude will not be able to > maintain altitude <g>. Rgr that. I've also thought that pilots with bad attitudes often run out of altitude. > As one gets higher, the air gets thinner. To maintain the same lift, the > AOA must be increased. OK, so what if TAS or IAS remains constant? > At some point, the aircraft reaches it's service ceiling ( ROC = 100 fpm ). If the > pilot forces it further, a stall can be expected. That's a part/factor for one answer. We'll give you partial credit for that one. Best! Ben -= VPC OffLine Reader 2.1 =- Registered to: Ben Chiu -OLR.PL v1.80- Remove | Alert Edit | Reply | Reply With Quote | Top

PeteDowson Certificates/Ratings/Crew Stations: 3. "RE: Attitude at Altitude?" 12-08-02, 13:01z  Hi Ben, > OK, so what if TAS or IAS remains constant? If IAS remains constant so does the AofA, as you are getting the same pressure on the wings -- that's what IAS is measuring, the effective pressure of the air due to your movement through it. Less air at higher altitude means that at the same IAS you are going faster -- a higher TAS. If TAS remains constant you are losing lift at the same AofA, so it must increase to stay at that altitude -- up until the point where more AofA tends to stall part of all of the wing, then you cannot stay at that TAS or that altitude, at least one has to change. Regards, Pete -= VPC OffLine Reader 2.1 =- Registered to: Peter L Dowson -OLR.PL v1.82- Remove | Alert Edit | Reply | Reply With Quote | Top

andrewluck Certificates/Ratings/Crew Stations: 4. "RE: Attitude at Altitude?" 12-08-02, 13:43z  Pete I concur. For a given configuration an aircraft will always stall at the same CAS no matter what the altitude. TAS will differ from CAS. The higher you fly the bigger that difference will get. In the F4, only the back seater got a TAS indicator. Can't remember if this was a direct feed from the Air Data Computer or if it was derived from the Nav Computer (part of the Inertial Nav setup). Andrew Luck 18 SW EGSH Remove | Alert Edit | Reply | Reply With Quote | Top

Ben_Chiu Certificates/Ratings/Crew Stations: 6. "RE: Attitude at Altitude?" 12-12-02, 19:18z  Greetings Andrew: > For a given configuration an aircraft will always stall at the > same CAS no matter what the altitude. This is a common mispreception. Stalls, as you know, are caused by exceeding the critical angle of attack. Stalls can occur at any airspeed, attitude, or altitude. Configuration (flaps, gear, spoilers, etc. extension or retraction) is only one factor that influences stalls speeds (indicated and calibrated). The others factors are weight, bank angle, CG (Center of Gravity), and in reference to this thread, altitude when referencing IAS (please see my not to Pete regarding EAS). So assuming level flight (just to complicate things!), weight must change to climb to say FL450 due to fuel burn. So does AOA remains the same at the same IAS at high altitude? OK, a bonus question, if AOA does change, which way does it change at high altitude? > In the F4, only the back seater got a TAS indicator. Can't remember if > this was a direct feed from the Air Data Computer or if it was derived > from the Nav Computer (part of the Inertial Nav setup). Did you fly in Phantoms? I didn't know that. Fantastic if you did! Best! Ben -= VPC OffLine Reader 2.1 =- Registered to: Ben Chiu -OLR.PL v1.80- Remove | Alert Edit | Reply | Reply With Quote | Top

andrewluck Certificates/Ratings/Crew Stations: 12. "RE: Attitude at Altitude?" 03-24-03, 21:59z  Ben Missed your post when it originally came out. Must have been something to do with Christmas bearing down at terrifying speed . No, I didn't fly Phantoms. I was an avionics maintainer on them. Not too many people still flying them, pity. A truely great airplane. Andrew Luck 18 SW EGSH Remove | Alert Edit | Reply | Reply With Quote | Top

Ben_Chiu Certificates/Ratings/Crew Stations: 13. "RE: Attitude at Altitude?" 03-25-03, 19:45z  Greetings Andrew: > No, I didn't fly Phantoms. I was an avionics maintainer on them. Not too > many people still flying them, pity. A truely great airplane. The last time I was out to Mohave I saw a bunch of Phantoms on the ramp. (Sit down before you read the next part.) I asked a pilot what they were doing with them. He replied that they were rigging them for remote control to fly as target drones. Ben -= VPC OffLine Reader 2.1 =- Registered to: Ben Chiu -OLR.PL v1.80- Remove | Alert Edit | Reply | Reply With Quote | Top

andrewluck Certificates/Ratings/Crew Stations: 14. "RE: Attitude at Altitude?" 04-05-03, 10:00z  Jeez, you guys know how to do target practice. All our pilots used to do was fire at flares dropped from a drone. Used to be real trouble if they hit the drone; they cost money On one occasion a drone got lucky and hit a chasing Phantom on the stabilator with a flare case. Made a nice neat rectangular dent. Andrew Luck 18 SW EGSH Remove | Alert Edit | Reply | Reply With Quote | Top

Ben_Chiu Certificates/Ratings/Crew Stations: 5. "RE: Attitude at Altitude?" 12-12-02, 19:17z  Greetings Pete: I guess no one else is going to bite on this one, so maybe I should stir the pot a bit. > If IAS remains constant so does the AofA, as you are getting the same > pressure on the wings -- that's what IAS is measuring, the effective > pressure of the air due to your movement through it. At low altitudes (below FL220) this is essentially true. However, at high altitudes (above FL220) one needs to account for compressibility and use EAS (Equivalent Air Speed, which is CAS - Calibrated Air Speed - corrected for compressibility). EAS can be relied upon to maintain the same AOA. IAS can vary by as much as 30+ knots in relation to EAS at high altitude. > If TAS remains constant you are losing lift at the same AofA, so it must > increase to stay at that altitude -- up until the point where more AofA > tends to stall part of all of the wing, then you cannot stay at that TAS > or that altitude, at least one has to change. I'll buy that. Very good! 5 points!! Ben -= VPC OffLine Reader 2.1 =- Registered to: Ben Chiu -OLR.PL v1.80- Remove | Alert Edit | Reply | Reply With Quote | Top

PeteDowson Certificates/Ratings/Crew Stations: 7. "RE: Attitude at Altitude?" 12-12-02, 19:58z  Hi Ben, > At low altitudes (below FL220) this is essentially true. However, at > high altitudes (above FL220) one needs to account for compressibility > and use EAS (Equivalent Air Speed, which is CAS - Calibrated Air Speed - > corrected for compressibility). EAS can be relied upon to maintain the > same AOA. IAS can vary by as much as 30+ knots in relation to EAS at > high altitude. Yes, I did actually know all that too, sorry. I was thinking in GA light aircraft terms -- I can't get to 22000 in my Piper! <G> It also gets all very different around Mach 1 and above, but you are going to start needing pretty large essays if we are to go into all the ramifications! <G> Regards, Pete -= VPC OffLine Reader 2.1 =- Registered to: Peter L Dowson -OLR.PL v1.82- Remove | Alert Edit | Reply | Reply With Quote | Top

p51mstg Certificates/Ratings/Crew Stations: 8. "RE: Attitude at Altitude?" 12-13-02, 17:28z  Can I still drink my coffee if EAS, CAS, IAS and TAS are all equal?????? I'll take that last dougnut if nobody wants it, too..... *shakes head violently, hears rattle* This is probably the wrong place to post this, but hey, I'm a newbie and that's what newbies do: With all of this air density, AoA and airspeed talk, what happens to that stick spinning around in front of the nose when all this is taking place (assuming we're talking about an airplane with a propeller that can keep level flight at the altitudes you people are talking about)? I was just reading a press release about Hartzell coming up with a new propeller design - actually, several - which debuted at Reno this year. Something about addressing compressibility... or maybe not.... Will these designs have any affect on the service ceilings of propeller-driven aircraft? --Bob Remove | Alert Edit | Reply | Reply With Quote | Top

Ben_Chiu Certificates/Ratings/Crew Stations: 10. "RE: Attitude at Altitude?" 12-13-02, 23:30z  Greetings Bob: > Can I still drink my coffee if EAS, CAS, IAS and TAS are all > equal?????? I know of one situation that'll net all to be the same in the real world--when you're airplane is standing still! However, whether or not you'd want to drink that old FBO coffee is a completely other matter. > I'll take that last dougnut if nobody wants it, too..... > *shakes head violently, hears rattle* LOL! > This is probably the wrong place to post this, but hey, I'm a newbie and > that's what newbies do: > > With all of this air density, AoA and airspeed talk, what happens to > that stick spinning around in front of the nose when all this is taking > place (assuming we're talking about an airplane with a propeller that > can keep level flight at the altitudes you people are talking about)? Actually Carlos and I were talking about the Lear 35/36 (which prompted this thread), however, that's an interesting question. From what I can gather, flying level at a constant airspeed requires thrust generation, so unless you're talking about near supersonic/transonic airspeeds, a prop will have little effect on our general AOA at various altitude discussion comparted to jets. > I was just reading a press release about Hartzell coming up with a new > propeller design - actually, several - which debuted at Reno this year. > Something about addressing compressibility... or maybe not.... I seem to recall seeing something about that, but didn't really pay much attention at the time. I suspect what they're working on is creating new tip designs to reduce shockwaves at high RPM's and airspeeds. I don't think anyone is threatening to break the sound barrier in level flight in a prop-driven airplane, but it'd be really neat if it could be done. > Will > these designs have any affect on the service ceilings of > propeller-driven aircraft? Possibly. But the limiting factors for service ceilings are numerous, so it depends on the aircraft. If thrust generation is the limiting factor (thrust or more accurately, excess thrust can augment the lack of aerodynamic lift at very high altitudes), then a prop that can spin faster/produce more thrust) could be beneficial. Welcome to the VPC! Ben -= VPC OffLine Reader 2.1 =- Registered to: Ben Chiu -OLR.PL v1.80- Remove | Alert Edit | Reply | Reply With Quote | Top

Ben_Chiu Certificates/Ratings/Crew Stations: 9. "RE: Attitude at Altitude?" 12-13-02, 23:29z  Greetings Pete: > Yes, I did actually know all that too, sorry. I was thinking in GA light > aircraft terms -- I can't get to 22000 in my Piper! <G> I suppose the term "high altitude" is relative. > It also gets all very different around Mach 1 and above, but you are > going to start needing pretty large essays if we are to go into all the > ramifications! <G> Well, let's keep it subsonic. We can talk about supersonic matters in another thread. However, it'll be from my perspective all theoretical since I've never flown faster than sound. Best! Ben -= VPC OffLine Reader 2.1 =- Registered to: Ben Chiu -OLR.PL v1.80- Remove | Alert Edit | Reply | Reply With Quote | Top

Ben_Chiu Certificates/Ratings/Crew Stations: 11. "RE: Attitude at Altitude?" 03-22-03, 21:55z  Greetings: I guess no one else is going to bite on this one. The answer is your AOA will need to increase to maintain level flight as you fly at high altitudes even if you maintain TAS or IAS. As is well known, at altitude TAS will increase even though IAS remains the same due to lower air density (fewer air molecules). While it's true that the airplane only cares about how many air molecules fly under the wings and into the engine intakes regardless of altitude and TAS, the relationship between IAS and the velocity and density of air molecules flying around the wings isn't constant. Enter EAS or Equivalent Air Speed. The formula to convert IAS to TAS is: IAS --> CAS --> EAS --> TAS IAS = Indicated Air Speed - What the ASI indicates CAS = Calibrated Air Speed - IAS corrected for instrument and position errors EAS = Equivalent Air Speed - CAS corrected for compressibility (more in a minute) TAS = True Air Speed - EAS corrected for density altitude Compressibility is the term that describes the compressible characteristics of air. What this describes is how air can be compressed (in the simplest terms air molecules can bunch up under certain conditions). Most folks relate compressibility to supersonic and transonic flight, however compressibility is also a factor in subsonic high altitude flight. The chart below illustrates how compressibility accounts for a loss in CAS as altitude increases. We know that lift can be generated by AOA and/or increase in airspeed. So if we lose airspeed and wish to maintain level flight, we need to increase AOA as we fly higher even if IAS remains the same. Other related questions that commonly come up are why do we fly Mach instead of EAS at high altitudes, and what happens to AOA in level flight if you fly a constant Mach and at high altitudes? Here are a couple of points to consider: - Mach can be thought of as your airspeed in relation to the speed of sound. The speed of sound varies with temperature. - TAS is EAS corrected for density altitude--where density altitude is pressure altitude corrected for non-standard temperature. Ben -= VPC OffLine Reader 2.1 =- Registered to: Ben Chiu -OLR.PL v1.80- Remove | Alert Edit | Reply | Reply With Quote | Top

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