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#11
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-- "Andrew Boyd" wrote in message m... wrote: So if I normally commence a loop at 100 knotts but get the entry speed wrong and start at 105 knots .... Anyone care to formulate what happens when speed ( or "G") are not constant? Your speed and G are NEVER constant during a loop. A vertical maneuver is always low and fast, then high and slow, then low and fast again, etc. You continually convert your kinetic energy at the bottom, to potential energy at the top, then back to kinetic energy on the downline. A hhead (aka stall turn) is a perfect example of this. You go straight up until you stop, then pivot, and fly down and gain airspeed again. Given a constant density altitude, additional entry speed implies additional G to make the same radius, assuming you fly at (or near) the stalling AOA which generates Clmax. Think of it this way: given that you fly at Clmax: 1) the radius of the vertical maneuver is a function of the aircraft stall speed (Vs), and 2) The G you must pull or push is a function of the entry speed. Does that make sense? It's not completely true - it will not withstand a rigorous proof, but practically speaking, it's what you really need to know to yank and bank down low. Thanks for the informative posts. I wish we had more discussion on this newsgroup about the physics of aerobatic flight. Here is an interesting article in Air Force Flying Safety regarding optimizing the pullout in an altitude-critical situation. I found it very interesting reading and perhaps relevant to this discussion. http://afsafety.af.mil/magazine/htdo...98/pullout.htm Cheers! -justin yak52 driver |
#12
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" An interesting fact, which is not obvious to many folks,
including some aerobatic pilots (judging by the number of loop into the ground accidents) is that the radius of any turn, up, down, sideways or whatever, is a function of the square of TRUE airspeed, which is of course a function of density altitude and calibrated airspeed. So, if the density altitude increases your true airspeed by 5mph, you get a 5mph^2 impact on your radius. This kind of change in radius can ruin your day if you are playing down near the dirt. This velocity^2 thing is also why the reverse cuban or loop down is a real killer. If you start the pull with X knots too many, you will use X^2 more radius for the 1/2 loop, throw in an increase in TAS of say Y due to density altitude and you are into (X+Y)^2 more radius ... not good. If you have not left margin either in terms of available G or altitude you are either gonna high speed stall on the way down (and hit the ground) or hit it on the arc. I think this may need a little more explaining even if only for my understanding. I am very new to aerobatics. So if I normally commence a loop at 100 knotts but get the entry speed wrong and start at 105 knots then my loop (assume horizontal plane and constant speed for simplicity) will be New_Loop_Diameter=Old_Loop_Diameter x New_Speed ^2 / Old_Speed ^2 i.e. a factor of 1.103. A bad entry of 15 knots over speed would have a factor of 1.323. But a target speed of 200 but entry of 215 would have a factor of 1.156. If my understanding is not correct then please explain why. I prefer to understand the physics/maths before I attempt some of these manoeuvres. Anyone care to formulate what happens when speed ( or "G") are not constant? Yes unfortunately while the radius is a function of velocity ^ 2, it is also a function of a great many other things. The main point is to recognize the non-linear relationship between speed and radius, and to then recognize that density altitude changes produce non-linear effects on radius. I don't recommend you do anything with the math except make a big 'ahhhhhhh' sound and let it cause a sensible sense of dread in you whenever you pull towards the ground. By the way, a minimum radius pull is at a bit above the stall speed so if you pull to just before the buffet you are pretty close regardless of airspeed. Cheers, Peter |
#13
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"Peter Ashwood-Smith C-GZRO" wrote in
message om... " An interesting fact, which is not obvious to many folks, including I don't recommend you do anything with the math except make a big 'ahhhhhhh' sound and let it cause a sensible sense of dread in you whenever you pull towards the ground. By the way, a minimum radius pull is at a bit above the stall speed so if you pull to just before the buffet you are pretty close regardless of airspeed. Hopefully someone isn't flying the bottom of a loop near stall speed. Isn't the minimum radius turn going to be accomplished at corner velocity and max-G pull? Al |
#14
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"DaBear" wrote in message ... "Peter Ashwood-Smith C-GZRO" wrote in message om... " An interesting fact, which is not obvious to many folks, including I don't recommend you do anything with the math except make a big 'ahhhhhhh' sound and let it cause a sensible sense of dread in you whenever you pull towards the ground. By the way, a minimum radius pull is at a bit above the stall speed so if you pull to just before the buffet you are pretty close regardless of airspeed. Hopefully someone isn't flying the bottom of a loop near stall speed. Isn't the minimum radius turn going to be accomplished at corner velocity and max-G pull? Yes, turn rate (or time in the pull) is also a factor and max turn rate and min turn radius are maximized if married with max available radial g at corner. You should actually throttle up if you're behind corner!!!! But you should never be in this situation going through a topside gate. The airspeed should be married to the known radial g profile for your airplane or a roll exit initiated IMMEDIATELY!! The trick with the gate apex is not to go through at a higher airspeed than the radial g profile you are using for the airplane will allow through the backside commit. If the airspeed is too high through the gate, roll should be used to exit instead of pull. In low altitude demonstration work, the time to do your math is on the ground. All you need to know in the air are your commit numbers. You have them at the top or you don't...period! If you are going through a top gate and ANY of the numbers are off and you don't execute a roll save, you are well on your way to becoming a statistic! I've attended more than a few funerals in the fifty years I've been associated with low altitude acro. It's a totally unforgiving environment! Dudley Henriques International Fighter Pilots Fellowship Commercial Pilot/ CFI Retired For personal email, please replace the at with what goes there and take out the Z's please! dhenriquesZatZearthZlinkZdotZnet |
#15
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#16
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Every aircraft is slightly different, and the Pitts is most surely different
from the Decathlon. Looking in the NTSB database from 1983 to 2001, I see19 fatal spin accidents in the Pitts and only 4 in the Decathlon. Source data: http://www.iac-chapter11.net/Fatal_A..._Accidents.htm The Decathlon POH states that both rudder and stick input are required for spin recovery. Can't remember the exact wording, but the basic gist was to apply opposite rudder and anti-stall elevator at the same time. Dave Swartz wrote an article on "unrecoverable spins", based on some spin modes he encountered in a Decathlon. He described several instances where he encountered rudder blanketing and was unable to slow the rotation. Both pedals were "soft". Moving the stick back to neutral increased his rudder effectiveness and allowed him to recover. As I recall, the spins he described were inverted, flat, and accelerated. The link to the article, and to Dave's website, doesn't work anymore; not sure why. In my own limited experience, I found my Decathlon to be a bit reluctant to spin inverted, and to quickly recover as soon as pro-spin inputs are relaxed. For upright spins, I use the standard opposite rudder to stop rotation, then stick to break stall, and it has worked every time so far. Obviously the original poster shouldn't be seeking inverted spin instruction over the internet. However, if he is just nervous about learning positive-G acro without having had inverted spin instruction, that's not really too big a worry. For any kind of blown maneuver in the Decathlon, just neutralize all controls and let the nose fall through. Neutral controls will stop most every incipient stall/spin in the D. "Rick Macklem" wrote in message om... (Guenther Eichhorn) wrote in message ... One important comment: DO NOT pull the stick back before the rotation has stopped. This is very dangerous since first you will accelerate the spin, and then it may get you in a cross-over spin, meaning you transition from an inverted spin to an upright spin, with the airplane rolling in the same direction, but yawing in the opposite direction. To recover from that, you now need to use the opposite rudder. If you don't notice that you have crossed over, you will more than likely not get out of the spin. A good friend of mine friend of mine died in exactly that situation. My only concern with the about statement is that I'm not sure all airplanes will stop spinning without pulling the stick aft. (I recall in an inverted spin in a Decathlon, I had to give it a little extra pull through neutral to get it to pop out. I've been told that Beggs-Mueller doesn't always work for the inverted spin in a Decathlon, which suggests some aft stick is required to stop rotation.) I believe Sammy Mason's version is: - Full rudder opposite to the yaw (he says opposite to the spin, which I've always found confusing for inverted spins) pause - then pull the stick smoothly back through neutral In the three types I've done them in (Pitts, Decathlon, Extra), I never needed full aft stick. Of the 3, the Decathlon seemed to need the most aft stick to get it to pop out. (It's been a long time since I flew a Pitts, but I recall it recovering almost instantly and the Extra seems to come out easily, although I haven't sat there will full opposite rudder and full forward stick to see if it stops before any aft stick movement.) ALWAYS first stop the yaw, then use the elevator however necessary. Yes, I think that is a better way to put it. (And I think you'd agree that it's not the same as waiting for the spin to stop before moving the elevator.) rick |
#17
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Hopefully someone isn't flying the bottom of a loop near stall
speed. Isn't the minimum radius turn going to be accomplished at corner velocity and max-G pull? Yes, turn rate (or time in the pull) is also a factor and max turn rate and min turn radius are maximized if married with max available radial g at corner. You should actually throttle up if you're behind corner!!!! Question: how does one compute or determine cornering velocity for a specific aircraft? Must that be done through flight testing, or is there a mathmatical relationship to the stall speed/etc? Specifically, does anyone know the cornering V for a Decathlon 160-CS? Thanks! |
#18
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Ed wrote:
Hopefully someone isn't flying the bottom of a loop near stall speed. Isn't the minimum radius turn going to be accomplished at corner velocity and max-G pull? Yes, turn rate (or time in the pull) is also a factor and max turn rate and min turn radius are maximized if married with max available radial g at corner. You should actually throttle up if you're behind corner!!!! Question: how does one compute or determine cornering velocity for a specific aircraft? Must that be done through flight testing, or is there a mathmatical relationship to the stall speed/etc? Do a Google search for 14cfr23.335.pdf Specifically, does anyone know the cornering V for a Decathlon 160-CS? With the above, you can calculate it for max gross weight. For low speed aircraft I think it is just Va. Look in the aircraft manual, it should have a V-n diagram. |
#19
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Thanks for getting this discussion pointed toward the mathmatics, Andrew.
A Google search of "V-n diagram" pulls up some really good articles on recovery from a vertical dive that explain in easy to understand terms. |
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