If this is your first visit, be sure to check out the FAQ by clicking the link above. You may have to register before you can post: click the register link above to proceed. To start viewing messages, select the forum that you want to visit from the selection below. |
|
|
|
Thread Tools | Display Modes |
#11
|
|||
|
|||
|
#12
|
|||
|
|||
With regard to the original message,
I don't think Derek is arguing specifically that the effectiveness of the elevator depends on the angle of bank, rather that pulling back on the stick with the wings level or in a shallow bank leads to a nose high attitude from which the glider will slow down and stall. In a steeply banked turn, pulling back on the stick tightens the turn more than it raises the nose so you won't slow the glider down very much, any pre-stall buffet is more likely to be the result of stall speed increasing with higher g. Its easier to reach that pre-stall buffet (inadvertently or otherwise) by slowing down than by pulling g, thus it is easier to use the elevator to stall a glider from straight flight or in a gentle turn than in a steep bank. Either way, the recovery is the same, ease the stick forward. If your instructor asks you why its harder to stall a glider in a steep bank and you reply that the extra g makes the glider nose heavy, you are liable to get a few demonstrations as to why that isn't true. The reason why its hard to stall in a 60 degree bank is not because the glider is nose heavy but because you are using virtually all the elevator authority to make it nose heavy. I'm sure its not what Bill meant to suggest, but its important that you understand that just because you're pulling 2g, the glider is not too nose heavy to stall. Ed |
#13
|
|||
|
|||
That's why I gave a very specific example and also mentioned proper rigging,
which includes that the elevator actually moves within the design deflections at annual time. There's also no accounting for pilots that can't coordinate a 60deg bank or think a 45deg bank is 60degs. Of course there are gliders that will flick into a spin with little or no warning from this attitude, so try it at altitude, away from the crowd, and with your instructor if necessary. Frank "Dave Martin" wrote in message ... The danger here is that we are talking theory where we may start to confuse pilots. It is harder to stall with 60 degrees of bank. Gliders like the K13, by design run out of elevator in straight and level flight. They are difficult when flown with heavy pilots to develop more than a mushing stall in sraight and level flight. Put light -- bottom weigh pilots in and they become a different glider. The Puchacz on the other hand has plenty of rear elevator even when banked, quite steeply. There can be some dangerous assumptions that gliders will not spin. The pilot must know the limitations and characteristics of the glider he/she is flying. This can only be achieved by carefully experimenting with different configurations and different flight situations. Gliders with reputations that they will not spin, can catch pilots out who load them wrongly, fly them badly or worse combine both. Dave Martin Get some empirical experience. Hop in a G-103, circle at 60deg bank and bring the stick back to the stop. If properly rigged, it will not stall. Do the same in straight and level flight. It will stall, in a mushy sort of way depending on loading. Also, in a G-103, you will get more elevator authority in tight turns by moving the trim forward. This is not true of all gliders, but clearly in a 60deg bank, the G-103 is stall proof by design. Frank Whiteley |
#14
|
|||
|
|||
Hi Eric
I am pretty well convinced that the CG causes the majority of the effect of keeping airplanes form stalling in a steep turn. The CG will determine the AOA of the stabilizer at 1G and the stabilizer size will determine the ratio of how the AOA varies with G loading. A small stabilizer will reach the critical angle of attack quickly and large one will not. For Example if the CG is at the Center of lift on the wing then a symetrical stablizer would be flying at an AOA of zero in level steady flight. Obviously in the AOA of the Stablizer has to increase (in the down direction) to increase the AOA of the wing. The size of the stabilzer will determine how much the AOA increases. On a side note this is actually one method that is used by test pilots determine the aft CG limit for aircraft. They incremently move the CG aft and put the aircraft into a steep turn at low airspeed. As long as the aircraft exibits positive control pressure through out the turn the CG limit is adequate. As the control pressures go neutral the aircraft is at it's aft limit. If the CG goes any further back the pilot would have to put forward stick into the turn to keep the the airplane from increasing the AOA to the point that the airplane stalled. I know this because it is how we determined the AFTCG limit for the Thunder Mustang. My HP16T had the opposite problem when I bought it, the CG was so far forward that I could not fly at less than 60kts in a 60deg bank. The stabilizer would stall and the nose would pitch down with the stick all the way back. I would have to roll out of the bank to bring the nose back up. After a Weight and Balance and 4lbs in the tail and I can turn at 45kts an a steep bank and the nose has no tendance to drop. Brian Case CFIIG/ASEL |
#16
|
|||
|
|||
"Dave Martin" wrote in message ... The danger here is that we are talking theory where we may start to confuse pilots. It is harder to stall with 60 degrees of bank. Gliders like the K13, by design run out of elevator in straight and level flight. They are difficult when flown with heavy pilots to develop more than a mushing stall in sraight and level flight. Put light -- bottom weigh pilots in and they become a different glider. The Puchacz on the other hand has plenty of rear elevator even when banked, quite steeply. There can be some dangerous assumptions that gliders will not spin. The pilot must know the limitations and characteristics of the glider he/she is flying. This can only be achieved by carefully experimenting with different configurations and different flight situations. Gliders with reputations that they will not spin, can catch pilots out who load them wrongly, fly them badly or worse combine both. Dave Martin You make a good point. Some gliders are very resistant to stalls and others will stall readily - especially with light pilots. It seems that trainers made in Eastern Europe come equipped with large, effective elevators that can stall the wing in any attitude. On the other hand, many German single place glass gliders often have small elevators with limited up authority. For example the Blanik L-23, IS28 b2 Lark and, as another poster pointed out the Puchacz, can be stalled from a steep bank easily. For this reason, they make good trainers since the student must learn to be constantly aware of pre-stall buffet. However, the point the Derek was making is that it is more difficult, but not impossible, to stall in a steep turn. I've had this discussion with pilots who feared steep banks. I suggest that thermalling is steep banks is easier in that the glider is more difficult to stall and fewer corrections are needed to stay in the thermal since the turn diameter is smaller. Bill Daniels |
#17
|
|||
|
|||
On Tue, 29 Jul 2003 08:05:21 -0600, "Bill Daniels"
wrote: "Dave Martin" wrote in message ... The danger here is that we are talking theory where we may start to confuse pilots. It is harder to stall with 60 degrees of bank. Gliders like the K13, by design run out of elevator in straight and level flight. They are difficult when flown with heavy pilots to develop more than a mushing stall in sraight and level flight. Put light -- bottom weigh pilots in and they become a different glider. The Puchacz on the other hand has plenty of rear elevator even when banked, quite steeply. There can be some dangerous assumptions that gliders will not spin. The pilot must know the limitations and characteristics of the glider he/she is flying. This can only be achieved by carefully experimenting with different configurations and different flight situations. Gliders with reputations that they will not spin, can catch pilots out who load them wrongly, fly them badly or worse combine both. Dave Martin You make a good point. Some gliders are very resistant to stalls and others will stall readily - especially with light pilots. It seems that trainers made in Eastern Europe come equipped with large, effective elevators that can stall the wing in any attitude. On the other hand, many German single place glass gliders often have small elevators with limited up authority. For example the Blanik L-23, IS28 b2 Lark and, as another poster pointed out the Puchacz, can be stalled from a steep bank easily. For this reason, they make good trainers since the student must learn to be constantly aware of pre-stall buffet. However, the point the Derek was making is that it is more difficult, but not impossible, to stall in a steep turn. I've had this discussion with pilots who feared steep banks. I suggest that thermalling is steep banks is easier in that the glider is more difficult to stall and fewer corrections are needed to stay in the thermal since the turn diameter is smaller. Bill Daniels This is an extremely important caution. Gliders do not all have the same behavior. Along this line, I have been cautioned that in a turn, the inside wing, even in a coordinated turn, is flying at a higher angle of attack than the outside wing. The degree of difference would vary with the bank angle. Thus, some gliders may not only stall in a steep turn, they can flick into a spin "out the bottom" in the blink of an eye. This may not be the case with many (most?) "modern" ships, but none-the-less is worth keeping in mind I guess. |
#18
|
|||
|
|||
I don't think the point was that a glider won't stall in a steep turn.
I think the point was that in some gliders with the CG far enough forward and a small enough stabilizer, the stabilizer can not produce enough down force when in high G turn to get the wing to the critical angle of attack and thus stall. Limiting the stabilizer/elevator is a well known trick to prevent (or least make it more difficult to stall) stalls if power aircraft, both Ercoupe and Stinson used this trick. Flying glider with an aft CG and/or large stabilizer will definently stall in a steep turn. Other might be made to, but it are more difficult to stall in a turn. Brian CFIG/ASEL "Jose M. Alvarez" wrote in message ... Don't agree to the idea that the glider won't stall in a steep turn. First, the stalling speed at 60 degrees of bank is higer due to G forces, and second, if you point your nose up (with left rudder in a right handed turn, for example) the speed will drop and cause a stall, even a spin. This could be very dangerous in a crowded thermal. You can also pull up your nose, as when entering a thermal at hight speed and pull up, and then bank hard to center the thermal. Nose up, speed drops, steep bank. Then stall, maybe spin. Stall is only a function of AoA, wich is in turn dependant of speed, wing load and G-load (hope I don't miss anything). Nothing about bank angle there, IMHO. Good flights, Jose M. Alvarez. "Bill Daniels" escribió en el mensaje ... "Dave Martin" wrote in message ... The danger here is that we are talking theory where we may start to confuse pilots. It is harder to stall with 60 degrees of bank. Gliders like the K13, by design run out of elevator in straight and level flight. They are difficult when flown with heavy pilots to develop more than a mushing stall in sraight and level flight. |
#19
|
|||
|
|||
Brian Case wrote:
I don't think the point was that a glider won't stall in a steep turn. I think the point was that in some gliders with the CG far enough forward and a small enough stabilizer, the stabilizer can not produce enough down force when in high G turn to get the wing to the critical angle of attack and thus stall. Limiting the stabilizer/elevator is a well known trick to prevent (or least make it more difficult to stall) stalls if power aircraft, both Ercoupe and Stinson used this trick. Flying glider with an aft CG and/or large stabilizer will definently stall in a steep turn. Other might be made to, but it are more difficult to stall in a turn. I completely disagree with the idea that you need more down force (at any given bank angle, including zero) in order to get a more nose up attitude. This would mean that the wing airfoil is stable, which is not the case except for flying wings. What you need is more elevator deflection, but not because you need more force, rather because the airflow at the tail plane has a different direction when the attitude is more nose up, needing a higher deflection even to produce a lower force. If this would not be the case, the whole aircraft (wing + taiplane) would be unstable. |
#20
|
|||
|
|||
A finite wing is one that ends (in a wing tip).
Looking at the evolution from ASW17 to ASW22 to ASW22BL to Eta, the idea of a finite wing seems to be repellant to many soaring pilots :-) -- Bert Willing ASW20 "TW" "Mark James Boyd" a écrit dans le message de ... http://www.geocities.com/CapeCanaver...pectratio.html A picture is worth a thousand words. I have no idea what a "finite wing" is, but apparently it's important. I also recall something about aspect ratios and AOA. I think there was something about a higher aspect ratio wing stalling at a lower AOA than a lower aspect ratio wing with the same wing area and loading. This is not the case. Stall AoA on relatively high-aspect ratio wings such as on gliders is dependent on airfoil only. Delta-wings with very low aspect-ratio indeed have a higher stall AoA. P.S. Also, it's apparently not important for the FAA to put an index in the back of the Pilot's Handbook of Aeronautical Knowledge (1997). And no index for the FAR/AIM? No wonder people buy the commercial products instead of the gov't ones. |
|
Thread Tools | |
Display Modes | |
|
|
Similar Threads | ||||
Thread | Thread Starter | Forum | Replies | Last Post |
AOPA Stall/Spin Study -- Stowell's Review (8,000 words) | Rich Stowell | Aerobatics | 28 | January 2nd 09 02:26 PM |
Parachute fails to save SR-22 | Capt.Doug | Piloting | 72 | February 10th 05 05:14 AM |
Procedure Turn | Bravo8500 | Instrument Flight Rules | 65 | April 22nd 04 03:27 AM |
Calculating vertical time and distance in a stall turn (US Hammerhead) | Dave | Aerobatics | 3 | November 20th 03 10:48 AM |
AOPA Stall/Spin Study -- Stowell's Review (8,000 words) | Rich Stowell | Piloting | 25 | September 11th 03 01:27 PM |