How did that happen?

[OZ e flyer]

I’ve read about it in reviews and seen it with my own eyes, a model enters into a loop and at the top of the loop goes in to a spin without input from the pilot.

My question is why does it happen? Not all models do it and I know it’s not what the pilot desired.

I’m guessing it’s something to do with power and possibly the control surfaces or throws of the model but it’s never been explained to me.

Can anyone help in nice easy layman’s terms for us less learned enthusiasts please?

Many thanks

TP

[Dave Cunnington]

Happens in real life too.....if you let the speed get too slow and controls are not properly centred

[Ace]

As Dave C mentioned with both full size and models, if you let the speed bleed off to much over the top a stall occurs (inverted), wing drops followed by a spin.

[Biggles' Elder Brother - Moderator]

It happens because the aircraft stalls at the top oif the loop. Usually too much elevator on, too high an angle of attack (AoA), and bingo, inverted stall, wing drops, into the spin you go.

It should not be assumed this is linked causually to low speed - it isn't. Its caused because the AoA to high and that is governed by the elevator. In fact you could argue that it is even more likely to happen at high speed. You enter the loop very fast and try to pull tight loop. Due to the centripedtal forces you need to pull a lot of elevator to keep the loop tight - classic cause of a stall.

Remember - aeroplanes don't stall because they are flying too slowly - they stall because the AoA is too high!

BEB

[OZ e flyer]

Thankyou Gents.

BEB I think your description is particularly enlightening. The ones I’ve seen and that I was questioning occur during attempts at a high speed loop.

Good to high horizontal speed, apply a fist full of up elevator, model enters the loop, hits the top of the loop and spirals out continuing in a vertical ascending spin.

I’ve never had it described to me as a too obtuse angle of attack causing a stall. Totally makes sense.

Thanks again.

TP

[cymaz]

You could be trying to make your loops too big...BEB explanation then takes over

[Anthony Scott 2]

Ascending spin?

[Geoff S]

Posted by Biggles' Elder Brother - Moderator on 17/04/2018 13:35:58:

Remember - aeroplanes don't stall because they are flying too slowly - they stall because the AoA is too high!

BEB

Isn't it actually a combination of the two? Too low a speed for the AoA? eg if you slow a plane up too much even in straight and level flight (ie virtually zero AoA) it will stall but will recover (hopefully) if you apply a negative AoA.

Geoff

[Alan Hilton]

BEB is correct ,too much elevator at the top of a loop is a problem and can result in a stall and spin or a flick roll. A loop should be round so elevator input should be reduced towards the top and increased again to achieve a smooth transition to level flight .

Alan

[Martin Harris - Moderator]

 

Posted by Geoff Sleath on 17/04/2018 17:30:10:

Posted by Biggles' Elder Brother - Moderator on 17/04/2018 13:35:58:

Remember - aeroplanes don't stall because they are flying too slowly - they stall because the AoA is too high!

BEB

Isn't it actually a combination of the two? Too low a speed for the AoA? eg if you slow a plane up too much even in straight and level flight (ie virtually zero AoA) it will stall but will recover (hopefully) if you apply a negative AoA.

Geoff

How do you slow an aircraft by reducing the angle of attack? The only way it would slow down is when it hit the ground at high speed!

I suspect that BEB's point was trying to emphasise that angle of attack governs speed rather than airspeed having any direct relationship to stalling. If the elevator is powerful enough, stalling is possible at any speed up to the point that the airframe fails.

Edited By Martin Harris on 17/04/2018 19:30:45

[Biggles' Elder Brother - Moderator]

I think the reason speed gets confounded up as a cause for stalls is this:

Leaving aside geometric factors how much lift an aeroplane is generating is dependent primarily on airspeed and AoA. Suppose our model is happily flying along S&L so generating exactly as much lift as it weighs. All is well,.

Then we slow down. What's going to happen? Well the lift will decrease and the model will start to descend. But suppose we don't want to descend? We pull back on the elevator, increase the AoA, get more lift back that way and we're OK.

Now we slow down even more, same happens again. Now the AoA is starting to get big!

Now we slow one more time, in order to compensate for the loss of speed-generated-lift we pull back even further on the elevator to increase the AoA; result? We exceed the critical value of AoA, the airflow separates off the back of the wing and we stall.

Conclusion? We were going too slow, so we stalled?

No. We stalled because we were going so slow that we had to increase the AoA beyond the critical angle. Its the excessive AoA that was the primary physical cause of the stall. We were goaded into making the AoA too large by the need for extra lift as the speed dropped, true, but the speed was not itself a physical cause of the stall.

Why is it important to undrestand this? Because it explains why stalls can happen a very high airspeeds. If you exceed the critical AoA it really doesn't matter how high your airspeed is - you're going to stall! This is ultimately what limits one fighter's ability to turn inside an another, the point at which, as you pull very hard in the turn, the buffeting that warns of an impending stall starts!

BEB

[The Wright Stuff]

Posted by Geoff Sleath on 17/04/2018 17:30:10:

Posted by Biggles' Elder Brother - Moderator on 17/04/2018 13:35:58:

Remember - aeroplanes don't stall because they are flying too slowly - they stall because the AoA is too high!

BEB

Isn't it actually a combination of the two? Too low a speed for the AoA? eg if you slow a plane up too much even in straight and level flight (ie virtually zero AoA) it will stall but will recover (hopefully) if you apply a negative AoA.

Geoff

Geoff, it's not possible to "slow a plane up too much even in straight and level flight". It's important to realise that AoA is measured relative to the direction of travel, not the horizontal necessarily. If you slow a plane in straight and level flight with zero AoA, it will descent rapidly because the weight exceeds the lift, but it has not stalled - it is still flying. Of course, the trajectory of the aeroplane is now downward, and so the AoA increases if the plane attitude is kept level relative to the horizontal. Eventually, this will lead to a true stall. But the stall itself was not caused by slowing down, it was caused by the pilot's inability to accept that the aircraft 'wanted' to pitch down, and correct it with up elevator.

If I'm honest (and I've had these discussions with BEB before), I don't think there is anything fundamentally wrong with citing a relationship between speed and stalls, under controlled conditions (e.g. quasi straight and level flight) But it's important to recognise that correlation is not causation. Depart from these controlled conditions (e.g. inverted at the top of a loop) and the relationship breaks down.

[J D 8]

I once had a model that was a "bitsa" Flair Fledgling wing,drainpipe fuzz,tail from something else.

Build up some speed in a dive,pull full up and it would perform a lovely cloverleaf maneuver with no other input. The one piece wing had a tad more incidence on one side.

[Rob Ashley]

Posted by Anthony Scott 2 on 17/04/2018 17:24:49:

Ascending spin?

Most definitely Anthony - a spin is an autorotation so can be ascending or descending. Getting an aircraft to spin upwards is quite satisfying.

Speed is most often related and attributed to stalling as, historically, in full size flight there was no way of reading AoA, but airspeed is easily obtainable. So correlations were found that at certain speeds in certain flight regimes speed could be used to determine the stall point. Problem is the stall occurs at an AOA not a speed. The faster you go the more lift the wing generates but the critical AoA doesn't really change until we get toward high speed problems such as compressibility (even then not much), which not generally a problem with models - but it means that irrespective of speed you can exceed the critical AoA and therefore stall at any speed.

As for entry to a loop, you can pull hard on the elevator and get the aircraft to depart quite easily as the critical AoA has been exceeded (even with rudder and ailerons central) - you would feel this buffet in a full size aircraft but for a model you have to feel this point visually from the ground. Easing off the elevator input on entry to the loop as the G comes on helps here then backing off further to neutral as the model is at the top, then reapplying on the way down - this is a gradual movement of the elevator and should scribe a circle instead of an egg shaped loop.

Rob

[The Wright Stuff]

Posted by Rob Ashley on 18/04/2018 11:24:35:

Posted by Anthony Scott 2 on 17/04/2018 17:24:49:

Ascending spin?

Most definitely Anthony - a spin is an autorotation so can be ascending or descending. Getting an aircraft to spin upwards is quite satisfying.

Indeed. And in this case, the wing's lift is working in the same direction as gravity, to provide the centripetal force. When the wing stalls, the loss of lift actually causes the wing to rise!