3/4 Panther build

[Erfolg]

I intend building a 3/4 Panther, which is to be published very shortly, less than a month from now.

You may wonder why a 3/4 version. There are two reasons

1. I bought a rotor head from HK for a C30. Thinking it would be fine for a Panther
2. I prefer smaller models these days, for the following reasons:

• Storage
• The cost of motors and Lipos
• I tend to shy away from powerful propellers and motors for the reason of potential damage to me.

I am already grateful for the help of Richard Harris, for help. Fortunately he has also offered help and guidance with the project.

I have been studying RH's Firefly 2 plan. The reason for this is that I have no shame in ripping of other peoples knowledge and designs. I have done this throughout my professional life as an engineer. Although BEB disapproves, I feel I am in good company, as all successful F1 teams do the same, as an aid to build there own designs. It is very much the reason that all commercial aircraft have a very similar look and feel.

There is one big difference, they, mostly know what they are doing. In my case, I need to learn a lot.

There is to date four sources of my knowledge as follows

1. Thread "Panther Trainer"
2. the "Firefly" thread
3. the "what makes autogyros work" thread
4. The RH Firefly 2 drawing.

I have learnt a lot from these sources, more than I had thought, as some of questions arise from these sources and a little understanding of issues and has generated more questions. These questions I hope will be answered as I go along.

[Richard Harris]

Erflolg,

Looking forward to your creation, any questions just fire away!

Rich

[Barryorbik]

Erflolg, My thinking is the same as yours. Itoo have been looking at autogyro plans from Jochen K for some time and recently at both the pdf's from Tom Wright but like yourself I fancy something a little more transportable and that uses the HK C30 head parts. Looking forward to hearing how you get on with the 3/4 build.

Barry

[Tom Wright 2]

Hi All.

Small Autogyros do have a major draw back .........disorientation ! .

Keeping in mind that most recent sports and trainer designs allow the blades to be folded back , IMHO a decent size model like the Panther, built to plan, is not difficult to transport , but likely to be more practical to fly .

Tom.

[Barryorbik]

Thanks for that input Tom. I saw a gentleman flying a fairly compact RC one on the free flight lines at last years BMFA nationals and he had painted one side fluorescent green and the other fluorescent orange, so hard to loose that in the long grass!!

Does anyone know the size of the blades on the HK one as I fancy trying to scale the Crane Fly MK2, so it can use HK blades too.

Barry

[Erfolg]

The HK blades are 450mm long, which equates closely to the Firefly 2. The chord appears to be narrower however. The other problem they are out of stock at the moment. On Sunday, 11 sets were available, Monday morning all had gone. It seems you have to be quick.

As you suggest Tom, there will be downsides with smaller models, there always is, a debit as well as benefit.

One of the most obvious is that excess weight build up is harder to control on a small model, any weight having a disproportionately detrimental impact on performance.

I personally believe there is room for both the smaller and larger model. Emotionally I do not find large models exciting, I do acknowledge that some do.

I do have real concerns when models reach the stage of being 1/4 scale as an example. The issue of space, weight and other factors become significant. Yet when operated responsibly, acknowledging and managing risks, why not? Just not for me!

 

Edited By Erfolg on 31/01/2013 11:14:03

[Tom Wright 2]

Posted by Barryorbik on 31/01/2013 09:12:28:

Does anyone know the size of the blades on the HK one as I fancy trying to scale the Crane Fly MK2, so it can use HK blades to.

Barry

Apologies all for the thread diversion ,but the CF MK 2 flies very well on the C-30 blades , I have two in current service. Great care is needed when modifying existing designs though ,as stuff like mast angle,plate compliance, blade design ,and control movement settings / trim settings need to be considered .Also as Erfolg suggests the weight translation can effect the rotor loading.

Tom.

[Barryorbik]

Thanks Tom, Your Crane Fly MK2 is definately winning the race now, as I just did not fancy making and balancing the blades after my feeble attempts at making 325mm wooden helicopter blades, as my wood working skills and tool set are poor to say the least!!

Barry

[Erfolg]

Studying RH Firefly, together with the drawings and explanation given by RH is quite informative.

I had wondered why typically masts were raked, from the drawing you are made aware it is a setting tool, for setting the CG. The model being hung from the mast top. The Firefly is a 6 degree rake. Flowing from this observation when the model is horizontal (which you are told is not the normal condition, flying nose slightly down) that the CG is in the region of 2" forward of the rotor head CL.

Typically we know that aerofoils will stall at about +15 degrees and generate no lift at something like -5 degrees, measured on a line from the nose radius to the trailing edge.

Where for me it gets really interesting is when looking at the sit of the model when sat on the ground. If we had the model pointing into the wind, which I will call 0 degree, the angle of the blades to the ground at the 90 & 270 degrees is 24 degrees. Now this surprised me as I came to the conclusion that the airfoil on the blades is either stalled or generating maybe a slight down wards force. So what lifts the model I thought, It must be from the blades at the 0 and 180 degree regions. I came to a tentative conclusion, that when the rotor generates enough lift to raise the model, quite quickly the regions that were doing little will quickly start to increase the rate of generating an up wards force rapidly as the rotor moves towards the horizontal. At the 0 & 180 position the wind has little effect, as the AOA is that at which the disc of rotor and blade makes as it rotates The driving force for rotation coming from the difference in CD of the advancing and retreating blades at the 90 and the 270 positions,

It is also apparent that the tailplane plays quite an important part in generating the balancing force. I guess it needs to be in the draught from the propeller, particularly when on the ground. As this is how it generates the forces necessary for control, when static.

Of course I do not know that I am correct

The other thing that has surprised me, from previous discussion Fire Fly drawing. the blades have a sharp LE entry. Conventional airfoils have a well rounded LE. This helps in the stagnation point, moving in a gradual and predictable manner. The sharp LE encourages a sudden breakdown in flow over the airfoil. The question I have, why is this helpful>

Again I do not know I am correct.

[Andy Nash]

Hi Erfolg

I will try to explain the blade shape question, as I understand.

Looking at the end of a flat bottom autogyro blade…. There are two lines in a blade 'the line of lift' and 'the line of gravity'.

If we have a flat bottom blade that has a large curve at the leading edge (LE) the 'line of lift' is from the middle of the LE to the middle of the TE. It is at a positive attack angle to the bottom of the blade. What I mean, if you place the blade on a flat surface and put your steel rule onto this line then it would be at a positive angle to the surface. To counter act this problem the designers have added a 'shim' under the TE. Not ideal. This then puts the blade 'line of lift' negative to give an autorotation.

If we have a very sharp LE on the blade the line is almost parallel with the bottom of the blade. We need very little, or no shim in order to get a negative angle on the autogyro blade. Much more efficient, we get more lift.

I hope I have correctly explained the difference.

Regards

Andy

Edited By Andy Nash on 31/01/2013 20:59:31

[Tom Wright 2]

Posted by Erfolg on 31/01/2013 19:42:22:

Typically we know that aerofoils will stall at about +15 degrees and generate no lift at something like -5 degrees, measured on a line from the nose radius to the trailing edge.

CD of the advancing and retreating blades at the 90 and the 270 positions,

It is also apparent that the tailplane plays quite an important part in generating the balancing force. I guess it needs to be in the draught from the propeller, particularly when on the ground. As this is how it generates the forces necessary for control, when static.

Of course I do not know that I am correct

The other thing that has surprised me, from previous discussion Fire Fly drawing. the blades have a sharp LE entry. Conventional airfoils have a well rounded LE. This helps in the stagnation point, moving in a gradual and predictable manner. The sharp LE encourages a sudden breakdown in flow over the airfoil. The question I have, why is this helpful>

Again I do not know I am correct.

Erfolg

The  wisdom as applied to rotors, suggests that they do not stall in the accepted sense .It's more a case of drag increasing with AOA until forward motion stops and the model sinks at a rate that is a function of the rotor loading . I suppose this could be described as a stable condition , but open to debate.

A tail plane theoretically should not be necessary , and some Autogyros fly without , but they can serve useful functions that justify the weight and additional structure.

Damping pitch response and compensating for any set up or design errors are the two functions that come mind .

A sharp l/e does seem to improve the spin up rate ,and generates good lift . The predictable high drag surprisingly is not a problem .

Tom.

 

 

 

Edited By Tom Wright 2 on 31/01/2013 21:13:46

[Erfolg]

Tom, when I wrote about being stalled that was in relationship to the model sat on the ground. I was not suggesting the whole of the rotor swept area would have stalled blades, the stalling would be in regions.

Whilst flying, that is with the rotor more or less horizontal, I can easily believe that to be true. That is the rotor does not normally stall.

I was principally considering the rotor at take of, with the rotor set at a very high angle of attack to the prevailing wind.

I cannot see how a tailplane can be avoided with the CG being so far in front of the the rotor CL.

With respect to high drag on a very sharp entry airfoil. I was not considering that aspect, and no nothing about that aspect. I was considering that sharp LE tend to have a very sudden transition from a controlled laminar/attached flow, to total disorder, a stall. I do not know the impact on a rotor blade. Other than that is what I would expect.

I do not know I am correct, but welcome alternative scenarios of what happens and why.

With respect to your post Andy, I will need some time to take in and try to understand. At the moment I just see that the lift angle has been reduced by the shimming plate relative to the rotor plane. I still need some time to consider though.

Edited By Erfolg on 31/01/2013 21:35:51

Edited By Erfolg on 31/01/2013 21:38:41

[Richard Harris]

Erfolg,

Take a look at this video of me flying one of my older models, notice the angle of attack as it is sat on the ground. Then as the blades reach full autorotation there is enough lift/drag and airspeed for the tail to rise off the grund.

It will not balloon in the air because if you look carefully you can see blade disc above the nose flapping up more than the rear of the disc, this is dampening the upward force at that point. The more the flexibilty is stiffened up the more likely it will nose up on take off.

 

 

 

This version has a high angle of attack becuase the only pitch control is on an elevator rather than DC control to the head.

 

Rich

 

 

Edited By Richard Harris on 31/01/2013 22:12:13

[Tom Wright 2]

Cross posted with Andy. But his explanation with regard to blade l/e profile is very much in line with my perception . The principle thing to note is a sharp l/e allows a lower or even zero shim angle resulting in the combination of easy spin up and near optimum lift for the section.

As for cog in front of c/l then the effect of a tail, can be emulated by altering the rotor pitch trim , but getting rid of the tail plane would also be dependant on the pitch follow rate characteristics. A high following rate, in pitch, without damping could be impossible to control .Clearly the mechanical advantage value of the pitch arm and placement of the control rod clevis's on the servo arms also effect the pitch sensitivity and the ability of the pilot to avoid over controlling,or even finding his reactions are not quick enough to provide the appropriate input.

The high rotor back angle employed during the take off roll is simply to achieve rotor speed .As lift develops and acceleration is still increasing, the rotor pitch angle is lowered by the pilot ,so the model levitates at close to the cruise pitch angle.

Sorry if my comments are simplistic .

Tom.

Edit, cross posted with Rich this time. 

Edited By Tom Wright 2 on 31/01/2013 23:02:03

[Erfolg]

Tom, What is following rate?

I think I have written this before, that on a conventional wing, a sharp entry does not cause a drag problem perse, it is the sudden stall that is unpleasant. Normally although the neutral line is normally drawn from the centre of the radius of the LE, to the TE. The stagnation point will not be where this line intersects the curve/arc of the radius, other than intermittently. What normally happens is that the stagnation point, wanders along the arc, dependant on the AoA. The Higher the AoA, the nearer to the undersurface the point goes. Where there there is a sharp edge, it suddenly encounters a point which it cannot get around easily. Suddenly the flow breaks down as point tries to go further round. I am assuming that in some way, that this is beneficial in some way. Just like sticking a sharp nose was on early supersonic aircraft, It concentrated the point of where the shock waves are generated. On our models sharp noses generate turbulence in a yaw situation, They may look smart on a Jart, but normally do little for drag reduction, other than when perfectly aligned with air stream.

Richard

All the data I have sen relative to L/D plots for airfoil sections does indicate that some regions of the rotor swept disc/area will be stalled relative to the wind, that is initially.

As the rotor speed/revs/radians per sec (I am on a roll now) increases, the wind speed becomes less important  to the lift generated and where. This is with the model pointing into the wind, sat on the ground with the undercarriage angles which appear to be typical. Or the rotor plane would possibly be initially tilted back instead. Although I do not know the speed, typically of a typical rotor, I can envisage that the tip speed is far higher than the wind speed, at and near take of speed. The resulting airspeed (velocity is a better description, as it includes direction) that the blade tips are subjected to increasingly generating an upward force. On reflection that is why we do not see a ballooning tendency.

Returning to the airfoil. Starting with the premise that there is a difference between a sharp entry blade and a normal entry blade. How does it show in performance terms.

I will have another look at your video now, as there appears to be differences between what I have read and what I seem to have seen. Then again, I may have interpreted the text and or what I have seen incorrectly.

Edited By Erfolg on 01/02/2013 11:09:57

[Tom Wright 2]

Erfolg.

My interpretation of the the following rate , is the speed at which the rotor responds to displacement from gyroscopic equilibrium .

Light blades with the span wise cog located in board are likely to respond so quickly that the pilots reaction speed is insufficient to keep up . A low following rate produces a control response lag time . Blades with excessive tip weighting tend to result in control lag.

Tom.

[Erfolg]

I had intended using HK C30 blades. This was on the basis that they should work, probably better made than I can achieve and are reasonably prices, especially where success is a priced commodity. However they are out of stock, the day after I had thought, I can think carefully if buying them makes sense, then there were 11 in stock, the day after back order.

Since then I have considered the RH Firefly blades as drawn, and considered the information contained in the Panther thread etc.

It appears that the important aspect is that CG is in front of the pivot. The pivot appears to be at the 30 percent cord.

I just thought why not make the LE, a little wider, to make it easier to ensure that the CG is infront of the pivot line? What would the problems be?

I also seem to remebmer that someone had considered using heat shrink as a covering, which was thought not to be a good idea. Why not, to jog my memory?

[Tom Wright 2]

Erfolg.

With respect ,your deliberations to some extent , are in effect covering old ground , or if you like re inventing the wheel . As you know  I went though a process of designing Autogyros based on a myriad of ideas mixed with some sound advice from those that had already been through the process.

My experiments did result in designs that flew ,and yet employed unconventional ideas that were frowned on by those who followed convention,. In the end convention became more appealing and practical simply as an aid to interact, and exchange information with others.

This now connects with your observations with regard to blade cog, and rotor bolt position ,which is roughly accepted as conventional ,however I have several models with 50% chord mounting, no chord wise balancing ,but these models fly very well despite the departure from the norm .

IMHO when contemplating a first Autogyro the best approach is to follow convention by building a simple design known to fly well ,and then delve into putting your own ideas into practise ...but ! all this can be a fruitless exercise until good experience is gained flying such machines . Some get to involved with side issues that detract from the primary requirements often leading to unsuccessful attempts ,lots of air frame damage ,and disillusionment terminating in giving up .

As for replica C-30 blades , making a set is easy , as there is no hard wood or extensive sanding involved . Or to get round the exasperating HK stock situation I would be happy to send you a set irrespective of what model you might build.

Of course the FF blades are also viable, as I  guess the model you end up with will not weigh more than a couple of pounds .The FF blades have more area than the C-30 so pitch trim will be different , But ! as you seem to have discounted the CF , the FF plan is available why not build one? as it's a smaller , lighter, model than the Panther? and only requires a small motor

Tom.

 

 

Edited By Tom Wright 2 on 02/02/2013 17:09:12

[Erfolg]

Tom, I am not inventing anything.

When starting on something new, success is easier if you understand why things are done a certain way, rather than just doing, then faffing about, until it works.

I am all in favour of copying things, as long as I know what is important and what is not. In fact that is what I have done in my professional live. I have stood on the shoulders of others. In my case not very high, but higher than if I had relied on myself

I have only one drawing. I never built a an autogyro. I have seen a I think, a Chipiawata, or some such name, which was an accident waiting to happen, when I saw it fly.

I am building a conventional design, the Panther, allbeit at 3/4.

I remember that my very early models were all kits, as were my friends. We rarely had many really successful flights, primarily because we had little idea, with respect as to what mattered. We had no real idea of what each bit of packing was actually doing, nor the effect of moving the CG, as followed a set of instructions. Understanding hopefully will remove mystique, that was our handicap.

I remember many years ago, a lecturer and lecture which had a profound affect on me. The jist was, "In the past the Japanese monks made superb swords, quite outstanding". The process was involved, incorporating religious ceremony as an intrinsic component of production. Many believed that without the mystique of ceremony, the swords would be poor. Although some will try and tell you otherwise, metallurgists today, both understand the science and how to apply the science, what is important, what is critical. and what is of little importance. My job is to provide you with the tools to be able to operate in a professional environment, using science to your advantage. Make no mistake, all those television programmes which say we cannot understand how the swords were made or replicate the metallurgy are very wrong.

All I want to do is make autogyro that should work, understanding what matters, so that my piloting skills are the weak link, not the airframe.

[Richard Harris]

Erfog,

l admire your questioning and to answer your question regarding the position of the blade mounting point is pretty simple, it makes it more stable and less pitch sensitive.

If you can imagine 2 blades mounted as per my drawing and a blade that has its bolt mounted forwards of the cord wise CG position. As the blades rotate they flap up, the odd mounted blade will be more pitch sensitive can rise higher. This blade is out of track with the others and causes vibrations and unwanted stresses to the air frame and controls.

Because the Fire Fly uses 1/32" / 0.8mm thick fibre glass sheet which is pretty flexible this would exaggerate bad blade tracking. So moving the mount centre slightly rearwards is just a bit of a safety factor.

There are a few other things that can cause bad tracking, bent/twisted blades, bent spindle missing negative shim , aerofoil differences and one that has happened to me on more than one occasion covering film coming loose!

Rich

Rich

[Tom Wright 2]

Erflog..

Please do not misinterpret my comments as derisory ,they are simply an account of practical experience, but as this forum has a good reputation for courtesy, I was surprised it was not extended within your response to my offer.

I wish you every success with the scaled down Panther ,Its just my opinion of course but I still maintain the FF would have been an obvious choice for a first timer ,wanting a smaller model.

Tom.

[Erfolg]

I am really sorry that i have unintentionally caused offence Tom.

I am asking questions, with a view to understanding what does what, and how. I have no solutions, particularly as I am grappling with what are the issues.

When there are fixed views such as blades benefit from a sharp entry, particularly when this is contrary to conventional thinking with airfoils, you have to ask why, what is the benefit, how is it seen, are there any downsides.

Other questions come to mind, why not have the blade pivot on the TE or the LE etc. Why not have a broad blade. Is a heavy blade advantageous. Does it matter where the centre of mass is located, across the blade?

The questions may appear to go on and on. But if even tentative views are placed against each question, it is a step to moving towards knowing why and being able to quantify what will work in various situations.

If I seemed to be offensive, my sincere apologies, particularly as I am looking for help, assistance and information. At the same time if I appear to challenge, it is only to seek answers. An example, is my first stab at what happens when the rotor spins up. Feed back from you guys, helped me clarify in my own mind what does happen, why, and when. I am sure it will be the same with much else, I will change my mind. or at least review what happens when.

Sorry Tom

[Tom Wright 2]

Erflog.

Thank you for taking time to address my concern, it's very much appreciated ,and accepted.

Sharp edged banter is par for the course , but without the benefit of a face to face conversation perceived inflections in words can ,as you know lead to misconceptions on both sides ,as can the manner in which information is exchanged .

I am probably just as guilty as many others who fail to read posts carefully enough to frame an appropriate reply , but I have always been willing to give my answers to the many questions that always arise from Autogyro projects .

At this point my words should be in a pm but I feel sure we are both man enough to show sides of our personalities and opinions that are normally kept suppressed.

As Rich has been up to ears in work on both fronts I have endeavoured to keep the responses going ,and answering your questions but if you don't mind me saying you do seem to often largely ignore my response and go on to ask more questions .Mountains of questions are not a problem at all ,the more the merrier,it's just the nature of the continuity between posts that can lead to the wrong emotional response ,vague yes, but I know you know what I mean.

We all learn from each other , and we all have our opinions, differences will arise,but if any of the comments or information I have posted in reply to your questions are incorrectly framed or in your view inaccurate please just say so , I will not be offended . None of us are ultimate authorities on anything... well BEB possibly lol.

All your questions have been absolutely justified and welcome , answers should be sought so a good understanding of how and why is indeed beneficial , but keep in mind my responses are a mixture of experience and basic theory , not necessarily carrying a CE or kite mark but born from hands on experience. I also endeavour to frame replies in a straight forward manner for the benefit of others looking in ,and without giving the impression that Autogyros are impossibly complex , and to much brain work is needed to succeed.

Well there we are that's me! and I look forward to continuing our exchanges ,and hearing how the build goes.

Tom.

[Erfolg]

It is the nature of asking questions, that more questions are generated.

There is probably a body of work on autogyros, which are more orientated to professional engineers, with a view of quantifying specific issues. This level of detail is not generally required or much benefit with our models. I am seeking a broad understanding of all the aspects of autogyros. This is even the starting point for engineers, before they reach for a calculator, when a critical issue is suspected.

I am a little surprised that you think I am ignoring any replies. It could be that i am challenging if the conventional wisdom has a foundation in fact, fashion, or observation of what appears to happen. The process is not meant to be confrontational, but informatively revealing, about what is known, and is yet to be fully understood, at least as a concept.

It does seem that RH could easily get another article out of just defining the principles and how they interact, for the RCM&E. I am suggesting that all the information which is spread out in all the threads is drawn together. Such an article would be of interest and help to me, perhaps many others

Just the thought on using heavy and light blades raises issues with respect to the forces on the blade (tension), and the model. Questions of inertia in slowing or increasing rotor speed . Are gyroscopic forces significant and how do they show up, if they are. This could cover, starting the rotor head, climbing, descending and of course direction (turning, precession).

Again going back to my youth, I managed to harden and temper metal, by heating and cooling steel. I came to better understand what was happening, when i understood about, thermo equilibrium diagrams, phase change, the atomic structure, slip planes. Which brings in alloying elements, how they fit in the structures. How cooling rates affects the structures. Of course you do not necessarily need to know the why, to obtain acceptable results. It is when things go wrong the deeper understanding can often pay dividends.

So todays parting question is: what about gyroscopic effects!

 

 

Edited By Erfolg on 03/02/2013 11:57:51

[Tom Wright 2]

As in life few things are straight forward ,this also applies to rotor characteristics. The mass , hinge compliance, rotor loading ,shim angle, blade section ,motor thrust line, and the centre of lift relative to the cog, are to some extent inter active in terms of controllability, stability , and most certainly effect the perception of how easy or difficult the model is to fly.

The effect of blade mass is best expressed in practical terms as in reality blades and the associated components need to withstand a good deal of abuse, which must be factored into the design to avoid excessive repair work . Heavy blades on tall masts,subjected to blade strikes, clearly result in high loads transmitted to dynamic components and supporting structures. Strikes will happen regardless of pilot ability or airframe design , so the strength to weight aspect of the design has a direct effect on the level of durability required for the type.

As blade weight is increased for a given rpm the tension forces in the blade attachment components will obviously be greater ,with force values of 8-16lbs per blade possible for a 1 M disc. These loads in flight can can easily be accommodated using conventional constructional techniques, standard materials and components.

As previously mentioned heavy blades take longer to spin up , and greater shim angles may be required to reduce the take off distance distance in lower wind speeds ,but more shim will also reduce the lift , so if the rotor loading is on the light side it may be acceptable to sacrifice some lift for more reliable spin up.

Light blades will accelerate and decelerate at a higher rate than heavy blades , but I think all of my observations would already be quite obvious to you Erfolg, but maybe not to other first timers looking in which leads me to think my reply is of little value to you , however the analysis of principles attributed to bodies in gyroscopic motion,can be found on line within numerous papers suited to the academically mined , but may not directly assist in getting a model to fly successfully , unless, the information is fully understood and translated in the correct way to implement the data into hardware for our models . This may a little to much for many who just want to build a model already proven to fly , but if a deeper knowledge is regarded as a essential I would say it is commendable, but possibly beyond the scope of of a forum thread short of pasting such a document directly into a dialogue box.

Tom.