Stall speed of a wet wing

[Bluey]

Just talked to airborne about an unrelated issue and the conversation turned to flying in the wet or with wet wings. While we may have all taken off with a damp wing without any problem at least one crash has been attributed to a wet wing stalling on take off. Apparently, the build up of beads of water on a leading edge especially will significantly disrupt airflow enough to increase stalling speed by several knots. The problem arises when taking off at low air speeds. Many of us use the take off technique of simply holding the bar against the down tube and letting the trike get airborne on its own. At least this is the way some instructors I've come across teach their students to do it.

 

My question is: Has anyone noticed the increase in stall speed of a wet wing. Given that most of us don't try to stall a trike wing too often, then many may be oblivious to the increased risk of flying in the wet, or taking off early in the morning with dew forming on the wing.

 

It is a good idea to wipe the leading edge dry before take off early on those cold winter mornings when dew forms so readily. I know I will now add this to my pre flight check list from now on.

 

Bluey.

 

 

[facthunter]

A small amount of ice or frost can cause problems , so why wouldn't beads of water? Nev

 

 

[skeptic36]

Just talked to airborne about an unrelated issue and the conversation turned to flying in the wet or with wet wings. While we may have all taken off with a damp wing without any problem at least one crash has been attributed to a wet wing stalling on take off. Apparently, the build up of beads of water on a leading edge especially will significantly disrupt airflow enough to increase stalling speed by several knots. The problem arises when taking off at low air speeds. Many of us use the take off technique of simply holding the bar against the down tube and letting the trike get airborne on its own. At least this is the way some instructors I've come across teach their students to do it.My question is: Has anyone noticed the increase in stall speed of a wet wing. Given that most of us don't try to stall a trike wing too often, then many may be oblivious to the increased risk of flying in the wet, or taking off early in the morning with dew forming on the wing.

It is a good idea to wipe the leading edge dry before take off early on those cold winter mornings when dew forms so readily. I know I will now add this to my pre flight check list from now on.

 

Bluey.

Hi Bluey,

I'm not arguing the point that a wet wing has an increased stall speed, but I can't swallow the bit about holding the bar out on take off being the problem.

 

If the stall speed is increased the wing will not produce enough lift to become airborne until you are traveling faster so that can't be the problem.

 

Two potential recipes for disaster I see, 1) Takeoff distance is increased which obviously could cause trouble if operating from a short field and the second is something I've seen a few times is incorrect inputs after takeoff ie. you must let the bar return to trim immediately you leave mother earth and the second is maintain full power. The nervous pilot is prone to not doing the later because the steep climb that trikes have causes them to s**t their pants so they back off. Most times they get away with it but if the stall speed is increased or they are at a strange airfield that is at a higher elevation than normal or both.........

 

Regards Bill

 

 

[Guest Crezzi]

A wet wing does indeed change the flying characteristics though, in my experience, not so significantly that most pilots would notice the difference.

 

Ice, as Nev points out, is much more serious - a couple of guys were badly injured / killed in a UK trike a few years ago after attempting to take off with ice still on the wing.

 

I'm not sure why so many Aussi trike pilots have been taught to take-off with the control bar fully forward - maybe just because it is easier to "teach".

 

It's not the recommended technique in any trike that I'm aware of and its use really should be restricted to soft / rough field take-offs (where the drawback of lifting off at the minimum possible airspeed may be outweighed by the advatanges)

 

Cheers

 

John

 

 

[Bluey]

According to airborne, the beading of water as airspeed increases can cause an issue just after take off resulting in a change in stall speed once airborne. So the trike gets off the ground but may then stall moments later. However, if you maintain power and reduce the angle of attack there shouldn't be a problem since airspeed should quickly increase. As stated above, ice is a much bigger problem.

 

Bluey.

 

 

[Guernsey]

I've seen a Trike take off with wet wings against the advise of a more experienced trike pilot, take off seemed ok but the aircraft stalled just after being airborn. Not a pretty sight, as trike was badly damaged.

 

Alan.

 

 

[Bluey]

Hi Alan, do you recall what the pilot did as they took aff that may have exacerbated the problem and cause the crash? Did the trike come down nose wheel first? Or did it drop a wing making control difficult.

 

Bluey.

 

 

[Guernsey]

I am not a Trike pilot but it seemed to me that he had reached take off speed, pushed the bar forward, started to climb then sank heavily onto the runway.

 

Alan.

 

 

[Bluey]

Thanks Alan, that supports the theory that beads of water forming on the wing after lift off cause the problem. The pilot must have been getting airborne at the lowest possible flying speed before the airflow over the wing was altered by the developing water droplets.

 

It seems that the only way to alleviate the problem is to ensure the leading edge of the wing is dry before liftoff.

 

Bluey.

 

 

[alf jessup]

My question is: Has anyone noticed the increase in stall speed of a wet wing. Given that most of us don't try to stall a trike wing too often, then many may be oblivious to the increased risk of flying in the wet, or taking off early in the morning with dew forming on the wing.

Bluey.

I practice stalls on a regular basis in my trike reason being when you carry different weighted pax's your stall speed could be higher, so depending on your AUW, temp, humidity ect could just save your trike some serious damage when rounding out and holding off during the landing phase after taking you pax for a nice flight+ it is just good practice, lots of dead sticks, sim forced landings ect.

 

Alf

 

 

[Tex]

To further compound the stalling problem is the beads of water move around on the Mylar (of course Dacron is no problem as it absorbs the water) ... a term often thrown around is 'divergence'... and in turns they will tend to feel like they want to wind in... though it depends on the wing model.

 

I'm not arguing the point that a wet wing has an increased stall speed, but I can't swallow the bit about holding the bar out on take off being the problem.

If the stall speed is increased the wing will not produce enough lift to become airborne until you are travelling faster so that can't be the problem.

 

Bill

Bill the stall characteristics will not necessarily be equal for each wing: we are talking about a random amount of water and beading which is quite unquantifiable in amount and effect. One might stall while the other does not.

 

Worth noting also that it is not just airspeed that stalls the wing it is angle of attack and airspeed combined... I too have always wondered about the high AoA take off technique.

 

 

[Bluey]

Just wondering if anyone out there watches their ASI to determine when the wing reaches flying speed. personally, I do it all by feel.

 

Bluey.

 

 

[Ultralights]

Stalling is a function of AOA, not speed, could the issue be the carrying of extra weight in the form of water be the problem? more weight requires higher speed to fly, and the pilot tries to get airborne at the usual speed, and in failing to do so, increases AOA to get more lift. increased AOA= more drag, then you find yourself on the wrong side of the drag curve at a critical phase in flight?

 

 

[Bluey]

I doubt the added weight of a a small mass of water sitting on the wing makes that much of a difference. The problem is the disruption to the airflow over the wing the beads of water or ice produce. The same disruption can occur if the wing is too dirty. Ie, covered in bugs splatter. It pays to wash your wings from time to time.

 

Bluey.

 

 

[facthunter]

U/L, I think it is more of a question of boundary layer control, (separation) the actual weight of water could be a consideration. but if the aerofoil is "degenerated" the lift may be less at any angle of attack than would be normal, and expected.. Nev

 

 

[Guest davidh10]

I've only seen the "bar full out" technique used by HGFA trained pilots. It is an incorrect technique, except for soft field and guarantees the trike will be unstable on lift-off. To do this is inviting trouble with gusts, wind shear, etc..

 

I never push full out unless intending to stall, and don't take-off at full throttle either. I always check the ASI for rotate speed It's airspeed you need for lift, not ground speed.

 

I haven't noticed any difference with a wet Wing, but I haven't done any stalls in the rain.

 

 

[alf jessup]

I've only seen the "bar full out" technique used by HGFA trained pilots. It is an incorrect technique, except for soft field and guarantees the trike will be unstable on lift-off. To do this is inviting trouble with gusts, wind shear, etc..I never push full out unless intending to stall, and don't take-off at full throttle either. I always check the ASI for rotate speed It's airspeed you need for lift, not ground speed.

 

I haven't noticed any difference with a wet Wing, but I haven't done any stalls in the rain.

Well David,

 

I always use full throttle for every take off, if my machine doesn't make it's max 5500 rpm i won't take off I will abort as I would hate to be needing that extra rpm later on and and finding out I don't have it.

 

I get to about 200ft then throttle back to about 5000 rpm and cruise climb out.

 

My belief is you should know if your engine is making max available power before you leave the ground.

 

I also disagree that full out is an incorrect technique"except for soft field " as you say, it is sort of a contradiction what your saying, it's no good for any other takeoff other than soft field? as the trike is guarenteed unstable on lift off ?, maybe on some earlier underpowered trikes but definetly not on a 582/912 trike they have far more power than they need, so as as long as you return to trim at lift off the trike will never be unstable. (if your using full power)

 

BTW, I'm HGFA trained and I use both (Bar out or in neutral) depending on the mood i'm in and have never been unstable at takeoff with either in 730hrs of flying trikes.

 

Cheers

 

Alf

 

 

[Guest Andys@coffs]

So to me the issue is that with the wing fully at the bar in the event of an EFATO you have nothing you can do to trade energy for altitude/time because thatg profile has the least stored energy (in airspeed )that you can call on. if its in the more normal trim position you can trade off energy into a flair if needed, or pull back on the bar to get the nose down much quicker with less loss of airspeed if using the 912 and you are deadset on approximating a columbus climb profile....

 

Aimanship is always about having options available to you, bar fully out deprives you of options.

 

Not to say there arent valid reasons for doing so, but to me they are in specific circumstances where the trade off is concidered and acceptable

 

Andy

 

 

[alf jessup]

So to me the issue is that with the wing fully at the bar in the event of an EFATO you have nothing you can do to trade energy for altitude/time because thatg profile has the least stored energy (in airspeed )that you can call on. if its in the more normal trim position you can trade off energy into a flair if needed, or pull back on the bar to get the nose down much quicker with less loss of airspeed if using the 912 and you are deadset on approximating a columbus climb profile....Aimanship is always about having options available to you, bar fully out deprives you of options.

 

Not to say there arent valid reasons for doing so, but to me they are in specific circumstances where the trade off is concidered and acceptable

 

Andy

Andy,

 

As soon as the trikes wheels leave the ground the bar is back to neutral and normal accelaration is attained, there is no chance in hell on takeoff i would keep the bar right out as it would climb like the space shuttle right until it stalled at say 100-200 ft and fell back on the ground.

 

I cannot see a problem with it, not that i have tried but i would presume if you had a EFATO with the wheels just leaving the ground the trike would just mush back down.

 

I see your point on the use of stored energy going that bit faster.

 

I have never ran into any problems using either technique, mainly use the bar out on shorter private grass strips i fly in and out of.

 

Cheers

 

Alf

 

 

[Guest Crezzi]

With all due respect Alf, I don't think the fact that you have been using the bar out technique without any issues makes it the correct technique.

 

May I ask - are there any circumstances where you wouldn't take off like that ?

 

Cheers

 

John

 

PS - short field take off should be another different technique

 

 

[alf jessup]

With all due respect Alf, I don't think the fact that you have been using the bar out technique without any issues makes it the correct technique.May I ask - are there any circumstances where you wouldn't take off like that ?

Cheers

 

John

 

PS - short field take off should be another different technique

G'day John,

 

Most times i use neutral controls bar and push out, the main time i use bar out is on a mates strip up in the hills which is a little undulating and not perfectly smooth, and another mates 500 mtr strip with trees at one end.

 

I definetly wouldn't use it out of these strips if there was a tailwind factor or if the wing was wet, not that i fly in the rain.

 

What is your technique for a short feild.

 

Alf

 

 

[facthunter]

Short field, rough or soft surface, or near obstacle clearance? Different considerations. You can fly slow in ground effect if you don't climb out of it. If you do , then you determine your margin above stall by the technique you use. Best rate of climb wouldn't be right on stall speed, and there's no margin of safety there. Nev

 

 

[Guest Crezzi]

What is your technique for a short feild.

1. T/O into wind as much as possible

 

2. Determine a point to abort T/O if you're not in the air

 

2. Start as close to end as you can get even if it means pushing the plane. You waste metres of potential rwy by taxiing & turning.

 

3. As much power as you can hold on the brakes

 

4. Bar neutral in pitch & release brakes

 

5. Keep bar neutral

 

6. Rotate at min safe flying speed then pitch to best climb speed

 

7. Normal climb once able.

 

Hope that helps

 

John

 

 

[alf jessup]

1. T/O into wind as much as possible2. Determine a point to abort T/O if you're not in the air

2. Start as close to end as you can get even if it means pushing the plane. You waste metres of potential rwy by taxiing & turning.

 

3. As much power as you can hold on the brakes

 

4. Bar neutral in pitch & release brakes

 

5. Keep bar neutral

 

6. Rotate at min safe flying speed then pitch to best climb speed

 

7. Normal climb once able.

 

Hope that helps

 

John

Pretty much what i do bar No4 & 5 sometimes and I also dont land in real short paddocks, probably 300meters would be the shortest, I know a bloke who used to fly landed in a real short paddock with his mate cause they needed a leak and couldn't get back out, had to de-rig the trike and trailer it out, he only done it once, his ambitions exceeded his and his trikes ability.

 

But he made a good decision that day.

 

Alf

 

 

[skeptic36]

1. T/O into wind as much as possible2. Determine a point to abort T/O if you're not in the air

2. Start as close to end as you can get even if it means pushing the plane. You waste metres of potential rwy by taxiing & turning.

 

3. As much power as you can hold on the brakes

 

4. Bar neutral in pitch & release brakes

 

5. Keep bar neutral

 

6. Rotate at min safe flying speed then pitch to best climb speed

 

7. Normal climb once able.

 

Hope that helps

 

John

Hi John,

 

How do you recognize min safe flying speed?

 

I guess from watching the ASI

 

There is quite a lot to going on when taking off from a short field and my eyes are always straight ahead, making sure I am still on track and not going past the point previously determined as the abort takeoff if not airborne mark.

 

If when using your technique my attention is distracted from the ASI at the point of reaching takeoff speed making me a bit slow recognizing exactly when to push out that would be a disadvantage I believe.

 

If you have the bar full out rotation happens automatically, giving me one less thing to worry about and getting me off the ground at the earliest possible moment. I see nothing wrong with this given, as Alf said, if efato occurs early your only going to mush back to the ground and anything after that I don't see much difference in either technique.

 

The main thing is getting the bar back to trim immediately after liftoff and maintaining full power.

 

Having said all the above, I did most of my training in the instructors 912 using the full out technique then I purchased a 582 powered trike. The first time out in that we took off as per normal, on returning to trim we didn't climb at all for a second or two. Discussing this with the instructor he suggested I may need to delay pushing the bar out until I had a bit of speed built up, no mention of any particular air speed.

 

The airfield is at 935ft AMSL and the wing is a Pegasus Q2. I don't have this problem at home, (about 72 ft AMSL.)

 

In a previous post you mentioned avoiding the problems associated with the bar out technique. Could you explain what those problems are?

 

Regards Bill