cessna spin
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The 25% rule of thumb is just that. So far, no scientific evidence of it, just some personal experience. I also noticed that if you do the standard navigation planning exercise of flying between two points 100 nautical miles apart at 100 KIAS and start adding a wind that is directly along the track (to show how things don't always cancel out), that the effect really starts to show when you get to 25 knots of wind. Got me thinking and observing more closely and it appears to be a reasonable rule of thumb.
The folks who really know about this effect are ultra-light pilots - ask one of them what height he would climb to if taking off with a left cross wind of lets say 10 knots crosswind component and wants to do a right turn after takeoff. It's quite high, because when the right turn starts the aircraft will probably descend (their airspeeds are pretty low).
The folks who absolutely know and understand this are the model airplane folks. Their airspeeds are very low and even the slightest wind affects them when they turn.
Who knows, might be a good master's thesis subject...
The folks who really know about this effect are ultra-light pilots - ask one of them what height he would climb to if taking off with a left cross wind of lets say 10 knots crosswind component and wants to do a right turn after takeoff. It's quite high, because when the right turn starts the aircraft will probably descend (their airspeeds are pretty low).
The folks who absolutely know and understand this are the model airplane folks. Their airspeeds are very low and even the slightest wind affects them when they turn.
Who knows, might be a good master's thesis subject...
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Shawn,
Thanks for clearing that up, the visual of the ultra-light turning downwind from a crosswind really exemplified the problem, it doesn’t appear so vividly when using bigger airplanes as a model. A 25% loss of airspeed would be 44% degradation in lift. Another thing to put in my bag of tricks. That would be a fun thesis subject... (I thought it may have been Doolittle’s dissertation)
Thanks for clearing that up, the visual of the ultra-light turning downwind from a crosswind really exemplified the problem, it doesn’t appear so vividly when using bigger airplanes as a model. A 25% loss of airspeed would be 44% degradation in lift. Another thing to put in my bag of tricks. That would be a fun thesis subject... (I thought it may have been Doolittle’s dissertation)
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Loss of kinetic energy when you turn downwind, very interressant subject Shawn!
let's see the case of a downwind turn just after takeoff when the wind is more than 25% of your airspeed
I can found the following factors: (I maybe wrong!)
There is a variation of wind velocity and direction near the ground which affect lift, drag and moment of the aircraft. This horizontal gust or shear vary with altitude and are due to the friction of the air over the ground (sort of boundary layer).
In my view this ground boundary layer can have different profiles:
LAMINAR: low thickness, low velocity near the surface, gradual velocity change. (Mojave desert early in the morning!)
TURBULENT: greater thickness, higher velocity near the surface, sharp velocity change.
We also have to think about the effect of leaving ground effect:
— increase in induced drag and thrust required
— increase in angle of attack to maintain the same lift
— reduction in climbing performance (more drag with the same thrust)
I have experienced this problem while towing big gliders in summer: (180 Hp tug with a 750+ Kg glider), we had to do a turn just after takeoff (200 feet max) to avoid some house (noise). We always used to turn upwind and One day I decide to turn downwind towards a great cumulus and I never made it......As I was turning , I lost around 10 Kt of airspeed and start to descent.....unable to recover.......I had to release the glider in a crop field (pilot and glider OK). Wind speed was around 30% of towing speed and turbulent.
stardust
let's see the case of a downwind turn just after takeoff when the wind is more than 25% of your airspeed
I can found the following factors: (I maybe wrong!)
There is a variation of wind velocity and direction near the ground which affect lift, drag and moment of the aircraft. This horizontal gust or shear vary with altitude and are due to the friction of the air over the ground (sort of boundary layer).
In my view this ground boundary layer can have different profiles:
LAMINAR: low thickness, low velocity near the surface, gradual velocity change. (Mojave desert early in the morning!)
TURBULENT: greater thickness, higher velocity near the surface, sharp velocity change.
We also have to think about the effect of leaving ground effect:
— increase in induced drag and thrust required
— increase in angle of attack to maintain the same lift
— reduction in climbing performance (more drag with the same thrust)
I have experienced this problem while towing big gliders in summer: (180 Hp tug with a 750+ Kg glider), we had to do a turn just after takeoff (200 feet max) to avoid some house (noise). We always used to turn upwind and One day I decide to turn downwind towards a great cumulus and I never made it......As I was turning , I lost around 10 Kt of airspeed and start to descent.....unable to recover.......I had to release the glider in a crop field (pilot and glider OK). Wind speed was around 30% of towing speed and turbulent.
stardust
I'm not sure I totally agree with you Shawn about ultralights.
Microlights tend to have very little inertia and huge amounts of wing, so steep turns tend almost entirely to remain in the wind axes, rather than intertia keeping them in earth axes.
But, ultralights tend to operate not much above the stall. Typically at a cruise speed of 40-50 kn, compared to a typical Vs on a US FAR-103 ultralight of 20-24 kn, or 30-35kn for a European Microlight or ULM. They also don't generally have much excess power to balance the turn with.
So, standard practice is to do steep turns in a shallow descent, providing energy to keep the speed on in the turn.
Still a valid handling issue, but I think not entirely related to this Cessna case - where the ratio of inertia to wing area will be much higher.
However, ultralights have another issue - in a 60° banked turn at ultralight speeds, a 360° turn can be done in about 8 seconds - so the risk of flying through your own wake turbulence is very real. For which reason, such turns are (or should be) flown climbing or descending - and not level.
G
Microlights tend to have very little inertia and huge amounts of wing, so steep turns tend almost entirely to remain in the wind axes, rather than intertia keeping them in earth axes.
But, ultralights tend to operate not much above the stall. Typically at a cruise speed of 40-50 kn, compared to a typical Vs on a US FAR-103 ultralight of 20-24 kn, or 30-35kn for a European Microlight or ULM. They also don't generally have much excess power to balance the turn with.
So, standard practice is to do steep turns in a shallow descent, providing energy to keep the speed on in the turn.
Still a valid handling issue, but I think not entirely related to this Cessna case - where the ratio of inertia to wing area will be much higher.
However, ultralights have another issue - in a 60° banked turn at ultralight speeds, a 360° turn can be done in about 8 seconds - so the risk of flying through your own wake turbulence is very real. For which reason, such turns are (or should be) flown climbing or descending - and not level.
G
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Sideslip?
Sorry for being so late to this topic (I've just now read it), but it seems to me we are talking about the airspeed drop that occurs when you turn downwind from a sideslip.
GTE, I think the discussion that Shawne brought up may be valid for the Cessna spin that was the original subject of this thread.
Look at what airball posted regarding his incident:
Then look at what djpil posted:
What both incidents have in common is using a C150 to take pictures. What djpil points out is that he had to use bank and sideslip while taking the pictures. One would assume the reason for this was driven by the need to place the aircraft on the proper ground track in order the get the desired photo.
When an aircraft is sitting at the end of the runway ready for takeoff with a 20kn crosswind, it is already in a 20 kn sideslip when it leaves the runway. When you land in a 20 kn crosswind, you are sidesliping 20 kn just to keep the aircraft on the centerline.
Rule of thumb (Shawne or GTE check me on this)
If you're sideslipping into the wind and then quickly turn downwind, expect a temporary loss of IAS roughly equal to your sideslip speed.
Remember, you are flying the aircraft with IAS speeds relative to a moving airmass. It's usually when you are referencing a fixed point on the ground (whether taking pictures, taking off, or landing) that you enter a sideslip relative to the moving airmass you are flying in, because you are concentrating on a fixed point on the ground.
The 2 Cessna C150 photo examples, the glider tug example, and the ultralight examples all seem to demonstrate this. Inertia comes into play when you consider how long it takes for the aircraft to recover the airspeed downwind that you gave up during the sideslip.
GTE, I think the discussion that Shawne brought up may be valid for the Cessna spin that was the original subject of this thread.
Look at what airball posted regarding his incident:
We were circling while taking pictures and had done so many times previously in the same configuration (C150) often with less speed more bank and less stable winds. Suddenly with out stall warning buffet or any other symptom the a/c flicked over in to a full left-hand spin.
Only last week I was out in a C150 with a photographer circling at 1000 ft with various combinations of bank and sideslip (flaps up) thinking that an unwary pilot could get a shock doing things like that.
When an aircraft is sitting at the end of the runway ready for takeoff with a 20kn crosswind, it is already in a 20 kn sideslip when it leaves the runway. When you land in a 20 kn crosswind, you are sidesliping 20 kn just to keep the aircraft on the centerline.
Rule of thumb (Shawne or GTE check me on this)
If you're sideslipping into the wind and then quickly turn downwind, expect a temporary loss of IAS roughly equal to your sideslip speed.
Remember, you are flying the aircraft with IAS speeds relative to a moving airmass. It's usually when you are referencing a fixed point on the ground (whether taking pictures, taking off, or landing) that you enter a sideslip relative to the moving airmass you are flying in, because you are concentrating on a fixed point on the ground.
The 2 Cessna C150 photo examples, the glider tug example, and the ultralight examples all seem to demonstrate this. Inertia comes into play when you consider how long it takes for the aircraft to recover the airspeed downwind that you gave up during the sideslip.
Last edited by Flight Safety; 14th Jul 2002 at 06:54.
Without deep thought, I'm inclined to believe that anybody making a habit of initiating a turn from sideslip is likely to spend a certain amount of their life spinning. Turns in a FW aircraft need to be entered from balanced flight, and continue to be balanced.
A tiny amount of sideslip is inevitable in entering a turn, but I mean tiny.
G
A tiny amount of sideslip is inevitable in entering a turn, but I mean tiny.
G
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Genghis, the sideslipping finals turn (top rudder) is much in favour in long-nosed tail draggers; you get a better view of the landing environment and the degree of sideslip acts as a variable extra drag inducer.
The pilot must know his(her) vehicle, of course, and be comfortable with the margins over the stall/departure to avoid the scenario you hint at!
The sideslip has to be got rid of before arrival, of course, and there is some satisfaction in the co-ordinated elimination of sideslip and bank as the threshold is reached at the button speed.
[Edit to spell Genghis properly; sorry!]
The pilot must know his(her) vehicle, of course, and be comfortable with the margins over the stall/departure to avoid the scenario you hint at!
The sideslip has to be got rid of before arrival, of course, and there is some satisfaction in the co-ordinated elimination of sideslip and bank as the threshold is reached at the button speed.
[Edit to spell Genghis properly; sorry!]
Last edited by fobotcso; 17th Jul 2002 at 12:42.
I agree, but it's flown at approach speed (1.3Vs or higher) and manoeuvring turns (as opposed to a controlled descending finals turn) would not be initiated at these conditions without first returning to balanced flight. The poster discussed use of sideslip in aerial photography at relatively low levels, which if reported correctly, certainly sounds to me a little foolhardy.
G
G
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Indeed; but it can be safe if the general rule is always applied:
Never do something at low level that you have not thoroughly practised and understood at a safe height.
I've seen it go wrong, and it was the photog who paid the price for too low/late an ejection.
Never do something at low level that you have not thoroughly practised and understood at a safe height.
I've seen it go wrong, and it was the photog who paid the price for too low/late an ejection.
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> The folks who absolutely know and understand this are the
> model airplane folks. Their airspeeds are very low and even the
> slightest wind affects them when they turn. Who knows, might
> be a good master's thesis subject...
As a model flyer...
I don't think many model pilots do understand the 'downwind turn' issue. The debate surfaces regularly in the magazines and newsgroups. Usually we're too worried about the effect a downwind turn has on ground speed to notice if the sink rate is any higher/lower. The downwind turn has a bad reputation because the high ground speed fools the pilot into thinking he's going too fast. Pilot tries to slow up which causes a higher sink rate or worse a stall and spin in.
As a early solo glider pilot (lapsed)..
I never noticed the wind while circling in a thermal drifting downwind. Sure if you look down you can see your track isn't a circle on the ground but I can't say I noticed any "up's and downs" are you go round and round. Not even when you are sharing a thermal with another glider.
However it's clear that gliders have innertia or wind shear wouldn't be a problem, the plane would instantly accelerate to maintain it's flying speed.
Jury still out.
> model airplane folks. Their airspeeds are very low and even the
> slightest wind affects them when they turn. Who knows, might
> be a good master's thesis subject...
As a model flyer...
I don't think many model pilots do understand the 'downwind turn' issue. The debate surfaces regularly in the magazines and newsgroups. Usually we're too worried about the effect a downwind turn has on ground speed to notice if the sink rate is any higher/lower. The downwind turn has a bad reputation because the high ground speed fools the pilot into thinking he's going too fast. Pilot tries to slow up which causes a higher sink rate or worse a stall and spin in.
As a early solo glider pilot (lapsed)..
I never noticed the wind while circling in a thermal drifting downwind. Sure if you look down you can see your track isn't a circle on the ground but I can't say I noticed any "up's and downs" are you go round and round. Not even when you are sharing a thermal with another glider.
However it's clear that gliders have innertia or wind shear wouldn't be a problem, the plane would instantly accelerate to maintain it's flying speed.
Jury still out.
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Cwatters illustrates the stall/spin problem with model flyers who focus on ground speed instead of airspeed. Imagine what would happen to the glider if the pilot DID try to make the ground track circular? If there was a good wind, he might stall/spin on the downwind-to-upwind turn in the thermal.
The glider tug pilot who wants to make a left downwind turn after taking off with a right crosswind, should first turn right to negate the sideslip, then perform a balanced turn to the left. It will take longer to get to the desired heading, but at least you'd arrive at the final heading with normal airspeed. However, if the tug pilot insists on maintaining a ground track (AND heading) along the extended centerline of the runway, he will be in a right sideslip due to the crosswind just prior to making the left turn. If he turns left now, he will experience a loss of airspeed because the turn is not balanced.
The glider tug pilot who wants to make a left downwind turn after taking off with a right crosswind, should first turn right to negate the sideslip, then perform a balanced turn to the left. It will take longer to get to the desired heading, but at least you'd arrive at the final heading with normal airspeed. However, if the tug pilot insists on maintaining a ground track (AND heading) along the extended centerline of the runway, he will be in a right sideslip due to the crosswind just prior to making the left turn. If he turns left now, he will experience a loss of airspeed because the turn is not balanced.
Moderator
A long time since I played at being a tug pilot.... (and I say that with not the slightest flicker of a wry smile on my face)
A glider does its very best to drive the tug and pull it into line with the result that the tug pilot has two options .. either
(a) slip (which I had never thought anyone would permit due to the reduced climb performance ... nobody like a glider pilot to keep an eye on which tug pilot gets to height quickest ... and crucify the pilot who costs him/her an extra dollar or two) or
(b) use the leg to make it fly nicely and properly.
.. or am I just being old fashioned ?
A glider does its very best to drive the tug and pull it into line with the result that the tug pilot has two options .. either
(a) slip (which I had never thought anyone would permit due to the reduced climb performance ... nobody like a glider pilot to keep an eye on which tug pilot gets to height quickest ... and crucify the pilot who costs him/her an extra dollar or two) or
(b) use the leg to make it fly nicely and properly.
.. or am I just being old fashioned ?
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(c) Request the glider pilot to fly a half wing span (tug) out to the side opposite to the leg you are using and which is gradually beginning to get the shakes. On a long tow that's worth a beer in the bar afterwards.
Nothing to do with the thread, of course, but a happy thought on a glorious soaring week-end.
Nothing to do with the thread, of course, but a happy thought on a glorious soaring week-end.
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Of course, the 150 WAS designed for it. And spin vs. no spin training has always been a big debate. I've always found it interesting how easy it is to spin a 150, especially with power applied. I did my flight training in Canada where spin training is still mandatory, and the 1974 vintage 150 training aircraft I flew probably had more spins in its history than I could ever count. My instructor liked doing spin entry with power, since that would likely be the scenario in an approach gone wrong. The stall horn rarely got a chance to go off and there was very little buffet before the aircraft would roll over into a very well developed spin (very pretty over the fall colors of the Ottawa Valley). Also, it's no wonder you didn't get horn. If memory serves, the stall horn is on the left leading edge, and you were likely making a turn to the right. so your right wing would have stalled first. Worst "almost incident" we had was when a student did the preflight and filled the tanks prior to his lesson. The student was 230lbs + and the instructor was at least 200 (putting the little 150 over gross.) The instructor had been flying spins all day (the class was all at that point in the training) and so the instructor was a bit tired to notice the over gross situation. The instructor entered a demonstration spin and the aircraft would not recover using the standard method. Faced with no other choice the instructor threw in power - knowing the results could go either way. We suspect a combination of the yaw input from the prop and the increased airflow over the tail allowed them to recover from the spin. Stupid thing to get in to. Good thing he was familiar enough with spins to know his options.
