What Goes Up

[Tony Richardson]

David loved your articles, though a pea-brain, I had to read some of them more than once to get my head around it, just watched the vids on this site and can relate to the Beaver I have the same model! and like the Piper cub it is a stable aircraft but, and it is a big "but" you cannot afford to get low and slow with no power it will bite and bite hard.

 

There is a short video Here and another Here that show the results of getting caught in this situation.

[John Bunting]

Has anyone here downloaded the NASA 'FoilSim' program that David mentioned in the May issue? I tried, but am new to Zip files, and must have done something silly, as I didn't get the necessary interactive part.

[Tom Wright 2]

Yes John.

Try Foil Sim 111 student version 1.4 d beta Put that in google and it comes up.

Tom.

[David Burton 2]

Hi all,

 

I'm very sorry about the delay in getting back to you all - its been a busy period! I'll try to deal with questions and comments now in a series of posts.

 

Tom, your question about the flat plate is a very interesting one and not at all a trivial question. You'd think that flat plate aerodynamics would well boxed off, and to an extent at least it is, but only with rather theoretical solutions using a technique known as "potential flow" theory. They tell us that a flat plate wing will produce lift and interestingly suggest that the rate of increase of CL with respect to alpha would actually be higher than is the case for many well know aerodynamic aerofoil sections!

 

But to be honest practical information on flat plate lifting surfaces is thin on the ground - as John says above. There is the NASA report he refers to and there are a couple of papers by some German scientists in the 1920's but that's about it! So we are in pretty unknown territory here. What is certain is that John is correct in his view - you could make a cardboard box fly if you give it enough power!

 

You mention that you believe your wings are operating at low Re values - I think that they may not be as low as you suspect! Your wing is 24" chord - that's 0.6m - if your model is flying slowly - let's say 20mph, or roughly 10m/s, the Re value across your wing is 400,000! Only a little below the critical value for flow across a flat surface of 500,000. But is it below at that speed - so all things being equal your boundary layer flow should be laminar. IF it was that might explain the relative insensitivity to pitch you observe.

 

But - as you rightly say - the square cut off leading edge is very likely to act a turbulator - especially given that fact that you are quite close to the critical Re value anyway - so on balance I would be surprised if the actual boundary layer flow over your flat plate wing is not turbulent. I say I'd be surprised - but I must be honest I do not know!

 

I'm planning to do a few experiments in the wind tunnel this summer (quite time of year in Universities!) on flat plates as lifting surfaces - just for bit of fun. It seems to me to be an area few people have really looked at in any depth. I'm hoping I'll get some data I can use in a second series to talk about the aerodynamics of shock fliers. I'll let you know how I get on.

 

So you see, as I said, not a trivial question at all! Keep on experimenting. Real experimental data is king in this field - its not a question of "are your observations correct, can they be aligned with the theory?" Its the other way round! If practical experience says that happens then its the theory's job to try to explain it and if it can't then the fault is with the theory! Maybe if I do those tests in the summer and get some quantifiable measured data of lift on flat plates I might have some better answers for you!

 

David

[David Burton 2]

Hi Simon,

 

you are of course correct. The classical entry into a spin would indeed be via a tip stall. So I suppose its a case of "one man's meat is another man's poison"! If you deliberately stall the aeroplane and then push in rudder in the direction in which the first wing stalled (because 99 times out of a hundred one wing will stall before the other) then its a spin. If on the other hand you didn't intend to do it - then its just a tip stall.

 

There is I think one major advantage in your position though - if you recognise that "death by tip stall" is really a spin then provided you know, and have practiced, how to recover from deliberate spin then you know the correct recovery procedure for a tip stall as well!

 

David

[David Burton 2]

Tony - loved the videos - the second one in particular is very good showing as it does the increased lift being demanded as the turn tightens and of course the ineveitable stall when the critical AoA is reached.

 

David

[David Burton 2]

John - did you sort out the download of Foil Sim? If not and you need some help then PM me and I try to help you get hold of it.

 

David

[Tom Wright 2]

Posted by David Burton 2 on 22/05/2011 21:32:47:

. What is certain is that John is correct in his view - you could make a cardboard box fly if you give it enough power!

 

David.

Firstly thank you for the informative ,and comprehensive reply,investigations such as this can make one feel some what isolated within the popular modeling fraternity,a frequent comment would be ,"its not a proper wing ," .

However my views on the subject were formed 55 years ago when i was a child with not enough pocket money to buy a model aeroplane kit ,so as the proud owner of a Mills 0.75 i fashioned a fuselage with a simple sheet balsa tail and rudder,and a vee wing mount designed to accept one of my Christmas presents,a draughts board with an AR of 1. The model flew free flight very well indeed and was extremely stable in pitch and roll.

A cardboard box ,open ended, is a bit like a Voisin biplane and requires very little power to levitate, if the wing loading is kept low,which of course is the key to many flat plate model designs that have become popular again under the guise of shock fliers, which in my view is a bit like re inventing the wheel.

The practical application of "flat plates" within the modeling world has excelent potential once popular resistance has been over come ,to demonstrate this the video on the next post ,that i forgot to load on this one ,is my 5ft span flat wing design that weighs in at 4lbs Well not quite true its got a jedleski wing section but my original version with the same dimensions had a flat wing and generally behaved in a very similar way ,the differences although quite small would be another discussion.

This does of course introduce another dimension to wing section discussions the long forgotten jedleski section which is a flat plate with a single change in angle at about 30%of chord .The video demonstrates the very forgiving nature of this type of section, tip stalls incipient spins or developed spins are virtually impossible to demonstrate,and i should mention that the motor only consumes 220 watts which i would imagine could be considered efficient for this particular plan form.

 

Tom.

 

Edited By tom wright 2 on 23/05/2011 02:13:46

Edited By tom wright 2 on 23/05/2011 02:37:53

[Tom Wright 2]

[John Cole]

Tom: you say that your model seems to display a low increase in Cl with increasing Alpha. I think there is a reason for this that is not aerofoil-section related. It's the low aspect ratio of your wing: this behaviour is most evident in delta-wing aeroplanes.

 

If you look at the airflow close to the wing root on a wing with rectangular planform then it will be travelling front to back. If you look at the airflow near the tip then this is not the case, as pointed out by DB in his articles - read about the tip vortex. On the top (low pressure) surface the airflow direction is backwards-and-inwards. On the bottom (high pressure) surface the airflow is backwards-and-outwards. So in both cases the air travels further before it reaches the trailing edge. What this means is that Alpha near the tip is less than near the root. So the average alpha for the whole wing is less than you would expect.

[John Bunting]

Thanks, Tom and David, for your kind offers of help with FoilSim. No trouble finding and downloading it, but I'm new to zip files and I expect I'm doing something wrong in trying to unzip it. I'll consult a fellow club member, who will probably spot it immediately!

[Tom Wright 2]

Hello John.

Thank you for the replies,in particular the take on rectangular wing plan forms .The effects of the upper and lower pressure distribution would seem to apply to flat and cambered sections,which does appear to some degree question my intuitive flat wing theory in respect to reduced CL sensitivity at low alpha values .

Clearly the result of this in effect suggests that in aerodynamic terms the wing is washed out ,despite the fact that in reality it is perfectly flat, and this would indeed account for the characteristics observed during flight tests with the twin boom and the BFM.

Going back to my original theory regarding blunt edged flat plates ,the turbulence created will effect the laminar flow ,as David predicted , resulting in a CL reduction in favour of lower pitch sensitivity at lower alpha values,which could be can be used as a design feature ,if the blunt l/e progressively migrates to a contoured section towards the tips.

Maybe its the combined effects of regressive turbulent flow and the pressure distribution resultant that make the type of wings discussed so forgiving.

 

Tom.

 

[Shane Sunday]

I didn't know where to post this so I thought I would just say thanks here. I have always been into planes from a young age and use to build airfix models from the time I could control my motor skills. Once I stumbled across RC models I was hooked and wanted to fly all my favourite planes that I had built when I was a youngster. The problem was That I didn't understand exactly what made the aircraft fly as well as why it behaved the way it did once it was airborne. David Burtons Columns have helped me to understand what is happening with the wing and the AoA and all that good stuff so I can avoid things like the stall as well as build into my models things that will help to creat more lift and a smoother flying model. His latest article has hit a sonorous chord we me as I had a spiralling stall with my foamy mustang but had no idea what actually happened at the time. Sure it won't make my model come back now but now that I understand more the effects on a slim tapered dihedral wing I know now to make my turns wider and sweeping and not ask too much from the wing.

When I started flying models it was all trial and error, it still is for the most part but at least now I have more knowledge at my disposal and can make better decisions While taking off, flying and landing.

 

David I want to say thank you very much for the effort you put into your articles and that fact that you make it relatively easy for us normal folk to understand. I look forward to the final instalment in next months issue. having read the previous ones a couple times already I'm sure I will give the next a few just to get it to set in. The fog starts to clear.

 

Cheers

 

Shane Sunday

[fly boy3]

Hi all. the Glossary mentions the stall. There is nothing on a tip stall. Is this one and the same thing, and is it caused by the same effect. Cheers

[Ben B]

The tip stall is a situation where one tip of the wing stalls before the rest of the wing does. This creates a nasty situation where suddenly one half of the wing loses a lot of lift.

Edited By Ben B on 29/05/2011 14:45:14

[fly boy3]

Hi Ben, thanks for the info, so it looks like a tip stall can turn out to be "nastier" than a normal stall ,due to the position of the model in the air, after a tip stall. Cheers.

[Ben B]

In what we consider a "normal" stall the whole wing loses lift and the whole plane will lose height (usually with a nose-down nod (or worse) but with the wings level. In a tip stall one wing tip will suddenly drop. Tip stalls are more common in planes without wash-out, with tapered wing shapes and with out-board airelons.

[fly boy3]

Thanks Ben, that,s probably why I have always steered away from tapered wings etc. I have also read way back ,that there is such a thing as a fast speed stall. I assume this would only occur in a turn ?. Cheers.

[Ben B]

high speed stalls can happen in a turn but can just happen if someone is going really fast then tries to go vertical by pulling back on the elevator. If there's too much control throw it can de-laminate the flow over the wing resulting in a loss of lift. But yes, it's often seen when people try to do a tight turn at high speed.

[fly boy3]

Cheers Ben ,nice talking to you FB3

[Big Bandit]

Dave,

 

A very understandable tutorial of which I've read similar over the years, but yours hit's the sweet spot for me . Just read the last installment and am now going to read the series from start to finish. That's just the way I like to do it with these things.

 

Cheers,

 

Chris.

[Tony Richardson]

David in your article about stalling you describe having to fly through the stall to enter a Harrier take a look at the video of Dave Sullivan and his aircraft at around the 5.30 min to the 7.00 minute mark and watch how close this plane flys to the stall, without the power of that big engine and prop this would not be possible. By the way the Curtiss P6-E in the stills is mine and yes it does need rudder to turn properly.

[Jez Saunders]

Hi David, your articals have been enjoyable,very understandable and informative to me and have enthused a thirst for more imformation. Will be reading them again and more, thank you. To those who wish to further confuse the issues, slit hairs over basic theory and drag in the maths go and have a cold shower,calm down.Some of us only need and want to know the basics and if we want to know more we will go find out. More articals like this please,this is what the majority of us want to see in the magazine. Thanks again. Jez