Flying speeds.

[Eddy Small]

Quite often when flying in competition and practice  i find my model either speeds up or slows right down.

Iv`e tried circling in these conditions with no positive results.

Can anyone explain why this happens and wot i should do?

Eddy.

 

[Eamon]

Eddie, I Hope this finds you well.

I don't know enough about this, as I am still relatively new to all this, but for my two pennies worth, I imagine it can only be down to one of two things:

1.  The model is altering its position of attack, because of the stab moving (faulty servo, faulty linkage); or

2.  It is due to lift and sink in the relative air, lift slowing the model and sink causing it to speed up?

I am probably wrong, but I would imagine the second option is most likely, as you build you models too well for the first to be possible.

I have found, often when my model slows that taking out camber helps to increase the speed over the wing and then you can often climb.  I leant quite early on, when competing with all of you guys, that I flew far too slowly at times, thinking slow meant less sink.... Watching some of our better pilots showed me speed is our friend, well some speed - not too much 

As I say I am a newby so look forward to seeing what the more experienced members post, as this will be a learning curve for me too.

Kind regards

Eamon

[Gary B]

Hi Eddy,

It could be a combination of factors. Some suggestions:

• Glider is not trimmed accurately for thermal/cruise/speed/landing, if you use flight modes take time to trim accurately in each mode.
• Glider is trimmed but you are pushing/pulling on the stick, this overrides the natural stability and the trim.  You can get out of sync  with the glider's natural stability by applying corrections in the wrong sense, this is known as a Pilot Induced Oscillation (PIO) and is common in full size gliders at higher airspeeds (i.e. aerotow, final glides).
• Centre of gravity is too far aft, close to the neutral point, this makes the glider twitchy in pitch.
• Range of movement on the elevator too large, use flight modes to set the ROM in each mode.
• Overdone snap flap mix if you have it (elevator to flap/wing trailing edge camber).
• Free play in the elevator linkage.
• Flying too slowly, semi-stalling with the glider self correcting.
• Moving flaps up or down (camber/reflex) causes a trim and pitch change, this is due to the centre of pressure moving forward or back applying a twisting force on the wing, we Have Cd (coefficient of drag), Cl (coefficient of lift) and Cmo (twisting MOment).
• Flying through lift and sink.

  Cheers

       Gary

[mikef]

Assuming there's some wind and that you mean it changes speed relative to the ground, it could just be a change in the wind speed.  An increase (gust)  will  appear to slow the model down and a reduction (lull) makes it appear to speed up.  If the model is stable in pitch, you may also  see a pitch change, nose up in a gust, nose down in a lull.

[PeteMitchell]

Assuming your model  is not being noticeably influenced by gusts of wind and you are happy with it's cg 

Speeding up is often a sign that you are in lift .

When this happens the  tail can be seen to rise  which automatically points the nose towards the ground, and gravity takes over making the model fly faster. 

If I am lucky enough to notice this going on, I try to circle in the thermal.

In sink the reverse happens, the tail drops. it looks like your model is tail heavy, the nose points up and gravity takes a hand.

If this happens, don't hang about, move away from the area asap.

At least, that's how I understand it

 

[Eamon]

1 hour ago, PeteMitchell said:

Assuming your model  is not being noticeably influenced by gusts of wind and you are happy with it's cg 

Speeding up is often a sign that you are in lift .

When this happens the  tail can be seen to rise  which automatically points the nose towards the ground, and gravity takes over making the model fly faster. 

If I am lucky enough to notice this going on, I try to circle in the thermal.

In sink the reverse happens, the tail drops. it looks like your model is tail heavy, the nose points up and gravity takes a hand.

If this happens, don't hang about, move away from the area asap.

At least, that's how I understand it

 

I always thought I slowed in lift and sped up in sink.  Clearly why I am so bad at comps

[Eddy Small]

Thanks guys, great help.

Eddy.

[Jef Ott]

Eddy.

A more rearward CG may stop it, if it is on just one model, and the change in speed is cyclic.
If it is on all your models this is unlikely. 
Maybe worth checking for wing and tail surface wander with temperature change - could be Tx or servos causing it if you do get fluctuations. On the models you fly, a change of a quarter of a mm (10 thou in old money) will make a noticeable difference.

 

[SilentPilot]

2 hours ago, Buster46 said:

I always thought I slowed in lift and sped up in sink.  Clearly why I am so bad at comps

It can do if you hit the thermal head on. The wing lifts and slows you down while the tail is still in non rising air. 

Often what is more noticeable is being ‘tipped’ out of a thermal at quite an alarming angle. The wing leaves the rising air and goes into still or sinking air while the tail is still being lifted. 

I’ve even sensed this in full size gliders!

 

Tony

[Kikapu]

At the heights we fly at, Eddy, topographically disturbed wind is likely to play a part in the model's stability, as well as rising and sinking air.   Might be worth adding a lump of lead to the model in such conditions and see if that helps.  Peter Mitchell's comments were helpful, though I never seem to pick up lift when my model suddenly picks up speed.  Mind you I'll pay more attention next time.

[Mr_SMO]

There was mention of this on rcgroups. The consensus was that there was no fixed result on any model entering a thermal. Whether it sped up or slowed down depended on the design of the model, trim, and settings. I've always thought mine slowed down on entering, but then coming last, I'm definitely no expert.

 

[SilentPilot]

I’m thinking the speeding up is because you’ve flown right through the thermal and got tipped on the way out. Turning round has you heading back into the lift again. It doesn’t really matter what I, You or Fred Bloggs thinks is going on though, if it works for you then there’s absolutely no reason to change anything!

 

Tony 

[SilentPilot]

If it works it works!

When I’m flying in my glider I sometimes turn off the audio vario for some peace and quiet. When in strong thermals I think you can “feel” the air better if it’s quiet and fly literally by the seat of your pants  

Other people may think I’m crazy but... if it works it works!!!

 

Tony

[isoaritfirst]

The difference between peoples views on fast or slow in lift is simply down to what they are doing with their models.

Lift is energy.

It will give your glider more energy.

If trims and sticks are left alone and the model is trimmed for level flight it will go faster.

In areas of lift you can of course pull back harder on the sticks without causing a stall. result model flys slower but climbs.

If that happens without you touching the sticks then you were flying with too much up trim. 

Flying with too much up trim will make you climb in lift and fall out of the sky in sink.

Falling out of the sky may appear as flying faster, but its nowhere near as fast as diving down in rising air.

[paulj]

I don't have enough of a population to be firm about the performance of all gliders, however, my Eliminator 134 lifts the tail visibly and speeds up  when in good air, and the opposite is evident in bad (sinking) air. Whether that's a result of having flown through and out the other side of the thermal, I can't comment. However, turning brings it back into good air.

Regarding full size Tony - the varios in the club K8's that I used to fly were agricultural to say the least, but as you say you get used to the "feel" of the glider when in rising air. I was always surprised at how I found it difficult to spot before becoming proficient at using thermals considering how obvious it was once the skill was mastered!

[Richard Swindells]

The tail-up \ tail-down perception of models flying in lift is due to one of, or both of the following....

1 in sink, our mind automaticly interprates the model sinking, as needing a bit of up elevator. We often apply this as an involuntary action on the sticks and interprates the result as a dragging tail. I do this a lot, even though I know it's the wrong thing to do, I sometimes have to force myself to recognise and react correctly.

2 an optical illusion. Similar to the downwind turn perception that many of us have. If we are flying in a football stadium sized lift travelling downwards at 1ms, the model would not appear to be nose up or down. However in the real world, sink means that our vertical frame of reference is moving in relation to the mass of air the model is flying in. This can make our mind play tricks around the actual attitude of the model. 

In reality aircraft can have momentary pitch up when directly entering lift, and a pitch down when leaving it.

With lift upwind, wind drops, giving the perception the model is flying faster.. and vice versa if the lift is downwind. 

 

[Jef Ott]

I had often wondered why the tail droops in sink. I think Richard has nailed this.

When the airframe is moving in air that is moving downwards, the airframe simply adjusts its attitude until equilibrium is reached (without any adjustment of the sticks in our hands).
- ie the airflow over the airframe must generate sufficient lift to keep the weight of the airframe supported,
the airflow is essentially at the same angle over the airframe, but the slightly downward vector of the airflow dictates that the tail has to drop (from the ground based pilot's perspective) to keep the air flowing in the right direction from the section's perspective. 

And yes, the tail really does drop, in relation to the observer, when the model is flying in sink, of that I am certain.
Because the vector angle (from horizontal) will be increased for lower airspeed aircraft, the effect will be more visually apparent the lower the flying speed of the airframe - hence why it is not as pronounced in bigger airframes (I'm thinking full size here) with higher airspeeds. 

Thanks for solving the mystery for me.   

Jef  

[Richard Swindells]

Jef. I think you misunderstood my post, I stated the opposite. 

There is a Drela post on it somewhere. 

It's like the downwind/upwind turn. We all know that there is no difference between them, but our perception is always telling us something different 

 

[Jef Ott]

25 minutes ago, Richard Swindells said:

Jef. I think you misunderstood my post, I stated the opposite. 

There is a Drela post on it somewhere. 

It's like the downwind/upwind turn. We all know that there is no difference between them, but our perception is always telling us something different 

 

Richard,

I didn't realise you were stating the opposite. We will have to agree to disagree then. 

Thanks for solving the mystery for me, anyway. I know what I have seen and know that it wasn't caused by my stick inputs, the tail actually drops in sink, and I am convinced that this is because of the direction of flow of the air over the airframe not being horizontal, but moving closer to the earth. 

Thanks for putting me out of my misery though - I thought it was strange that we agreed on something  LOL 

Jef