Jump to content
Sign in to follow this  
martynk

Cheshire Cat 100S-E

Recommended Posts

Jef Ott
2 hours ago, Gary B said:

I have a Flair Sunrise, straight inner panels, dihedralled tip panels. It will lock into a turn to the point I think I've had radio failure but can clearly see the rudder hard over out of the turn trying to right it!

In both directions - ie left turns AND right turns?

Did you try moving the CG back a bit?

 

 

Share this post


Link to post
Share on other sites
Jef Ott
8 hours ago, Gary B said:

A rule of thumb that I can't remember where it came from is one degree dihedral per foot of span, which works out at 8.3 for the Centi-Phase.

I remembered something similar for the 100" and Open models... 1" of total dihedral per foot of span.

Only ever saw a 12ft soarer with 12 degrees of dihedral after the towman gave it too much oomph on launch!

  • Like 1

Share this post


Link to post
Share on other sites
martynk

Hi all

trying to respond using the iPhone so apologies for typos including those introduced by the spell corrector.  

Jef - before I started this particular venture, I was just about to start on an F5J Bubble Dancer. I have redrawn the wing using a lighter spar in the centre panels. The outer panels will be the same except for a smaller diameter carbon wing panel joiner. I have even got a full set of ribs laser cut. I did consider (briefly) just making an F5J fus for the existing flying surfaces but I don't like compromise. I'd much rather build a full model from scratch that's designed for the job.  So my build plan for the summer will probably involve another BD :)

Gary - thanks for your thoughts I did consider the BD dihedral to be excessive when I built it. It has far more than I seem to remember from watching the thermal soarers in Richmond park back in the early 70's. The trend seems to be more dihedral with much bigger fins and rudders. I suspect that this improves spiral stability but I really don't know enough about the subject to make an authoritative   statement. I am very much a beginner here. I think for this model I'll go for something similar to what I imagine Mark Drela would do and try and get some experience. I have decided to go for the polyhedral wing even though it locks me into the built in dihedral with an 8mm centre carbon joiner. I guess time will tell if that was a Stooopid idea or not. 

Keep your thoughts coming please. It's all very helpful and making me think. 

Best wishes

  • Like 1

Share this post


Link to post
Share on other sites
martynk

hi Jef

sorry. Just spotted your last post. I hadn't realised there was a page 2. 

Have to admit that the dihedral bothers me most.  However your rule of thumb of 1" per foot of wingspan may not be a million miles out. A 100S is just over 8 ft span. I am planning on a total of 8.5" dihedral under each tip. Is this what you meant? I would rather have a little too much dihedral than too little. I just wonder whether these multi panel wings give the optical illusion of more apparent dihedral?

anyway - thanks again 

Share this post


Link to post
Share on other sites
Jef Ott

 

Hi Martyn,

The total dihedral on the better penetrating RES models is half of what you think I said, ie not 10" under each tip, 5" or 6" under each tip on a 10 footer.

Some of the big floaters do have stacks of dihedral, but their role in the average UK competition is merely to indicate where the lift is coming from, for the benefit of the more sleek machines. They sit into the wind and go up when good air comes through. The eventual winners then follow the good air downwind, but the floaters have to ignore it and hope some more good air comes along soon. The reason for this is that they cannot penetrate back upwind again if they do follow the good air for any distance.

This is obviously a generalisation, and every now and again the floaters can also follow the good air as it slowly drifts through.

Don't get me wrong, I am not against people building and flying floaters, such as the Ava, like I said, they prove very useful, to other competitors, but I am just trying to provide Martyn with honest thoughts for the design of models that will perform well in the UK F5J scene. 

The preloved market is not short of floaters that are built with heaps of dihedral. 

My advice would be buy a preloved model with heaps of dihedral, try it in a few comps, then make a decision. Selling it again will put you back to square one, but with better insight of what makes a great design.

 

  • Like 1

Share this post


Link to post
Share on other sites
martynk

Thanks for the insight Jef. I'll go back to the calculator again and do some more number crunching. 

Share this post


Link to post
Share on other sites
Jef Ott

Just in case I had remembered it wrong, I asked my 82 year old Dad how much dihedral he thought the 100" models (that we used to compete with) had. I did not load the question in any way. His answer, straight away, was... 4" to 5".

 

Share this post


Link to post
Share on other sites
evad

if you was to make a small centre section of the wing fixed say 2 or 3 ribs wide , then have the main wings plug into this centre section , you can then make different sections with more or less dihedral without remaking the wings themselves , also this way the joiners into the main wings can be straight with only the center section with any angle built in 

 

dave   

Share this post


Link to post
Share on other sites
pete beadle

Hi all

Interesting discussion

I seem to remember that, back in the day, most home builders were modifying their previously free-flight models for radio, and the dihedral of the free-flight model was usual starting point. Also, wing joiners were usually 6 gauge and could be bent after the first flights to give more(or less) dihedral whichever it was found to need.

Then again, the introduction of ailerons meant immediate reductions in dihedral and soon became the norm - with less

As you all know we had no computer programmes to guide (confuse?) us and EVERYTHING was suck-it-and-see, people didn't ask forums, they just went out and "modded" their designs depending on how well, or badly, they flew

As you said, the Flair kitted "Sunrise" was polyhedral but the inboard panels had no dihedral at all, and even looked anhedral at rest, so no-one was surprised at its poor turn authority - and poor penetration and poor etc, etc because it was designed with "min sink" in mind. I seem to remember that, in those days 100" models had 10" overall dihedral and 120" ones had 12" and so on, the factor of polyhedral or straight didn't come into it

Could you fit bendable wires in your design? You'd only have to build one set of "adjustable dihedral" centre panels for initial flight testing - then rebuild the inner panels stronger and with non-bendable joiners for the "Mark 2" - yes, we all used to call them "Mark 2's" or even "XYZ modified" until we got it right at (hopefully) the Mark 3......

Good luck

Pete

BARCS1702

Share this post


Link to post
Share on other sites
martynk

Thanks Pete and Jef and Dave

The bit I am puzzled about is why the amount of dihedral affects penetration. I always thought that penetration was primarily a function of (lack of) drag, wing loading and choice of a suitable wing section. I know that F/F models do not penetrate well - but nor do they need to. Can someone enlighten me as to why dihedral affects penetration please? I am aware that excessive dihedral increases drag and that the move to multi panel polyhedral wings helps reduce the parasitic drag, but the amount of dihedral we are talking about, this isn't huge.

Assuming we are operating with a CL of 0.6 to 0.7.. 

 

the difference in drag (the lower two lines) between 0 and 15 degrees

56f25c424b0b1_DihedralDrag.thumb.jpg.7fd

Although the graph is steep, the change in value in CD is marginal.  There are effectively 2 graphs here - so 2 sets of lines and refer to full size aviation.. 

(the graph is stolen from https://www.researchgate.net/publication/275844991_Computational_and_Experimental_Validation_of_the_Active_Morphing_Wing)

Thanks again

Share this post


Link to post
Share on other sites
oipigface

Try this:  Because a wing with dihedral can never be horizontal throughout its length, it produces less lift than a flat wing of the same (non-projected) area. In equilibrium, it therefore must reduce drag, and improve penetration. This argument, of course, ignores the (correct) points you make about parasitic drag. The two effects offset each other, and which dominates (if any one does) depends on more detailed points of the design.

  • Like 1

Share this post


Link to post
Share on other sites
martynk

I have a confession to make. I didn't do the 'R' bit in RTFI on the EDA spreadsheet while inputting the dihedral angles. I input the value as a panel value when it should have been the total angle (from the horizontal datum) for the panel.. (If that makes sense.).

Therefore with revised values, the plan now looks a bit like this:

CC-100S-E-v4-poly.jpg

I think that you will agree it looks rather more in line with the very polite way that you were telling me that I was totally wrong..

A minor tweak to the EDA spreadsheet now that I have read the instructions :frantics: and v4 now looks like this.

CC-100S-E-V4-POLY-EDA.JPG

The EDA is a little less that MD recommends but the Blaine Rawdons Parameter indicates the model will still be spirally stable

A2 plan also attached. Are you all happy/happier now? Happy for further encouragement. If you think I need to issue a bigger correction stick please let me know :poke:

Many thanks again

 

Cheshire Cat Poly Wing Plan-v4.pdf

  • Like 1

Share this post


Link to post
Share on other sites
martynk
5 hours ago, oipigface said:

Try this:  Because a wing with dihedral can never be horizontal throughout its length, it produces less lift than a flat wing of the same (non-projected) area. In equilibrium, it therefore must reduce drag, and improve penetration. This argument, of course, ignores the (correct) points you make about parasitic drag. The two effects offset each other, and which dominates (if any one does) depends on more detailed points of the design.

Ah yes, that makes sense and even more so with a large dihedral wing.

Thanks

 

Martyn

Share this post


Link to post
Share on other sites
Jef Ott
8 hours ago, martynk said:

 I am aware that excessive dihedral increases drag

You know the answer then.

Share this post


Link to post
Share on other sites
Jef Ott

Martyn,

If it is possible, think of it as the dihedral endowed wing is generating lift that is in the wrong direction ie sideways in equal amounts that cancel each other out in straight level flight in calm conditions. The drag that accompanies each of the opposing lifts however, is not cancelled out, so even in flat calm conditions the model has more drag. 

In reality on a windy day, when we are flying back to the field from downwind, we are actually flying through air which is generally flowing towards the machine, but the flow is actually made up of many component directions, so the model is flying across wind one way then the other as lift and turbulence from trees, roads, changes of crop, buildings, etc. in the surrounding areas all play their part. We don't have the luxury of flying in wind tunnel conditions!

Now imagine a playing card flicked across a room. How fast and far it goes. Now put a crease in the card and see how much it slows in comparison.

The model with big dihedral is affected far more by the changes in wind direction, like the bent playing card, preventing the distance achieved by the straight card.

Good luck with your much sleeker looking design, the front projection looks much better, and as long as you have four to five inches of dihedral measured to the floor in the middle of the inverted wing, it will be very competitive. It will require a smaller vertical stabiliser volume to prevent spiral instability. I am viewing the drawing on a small screen and cannot see dimensions, so can't see if you have reduced the fin and rudder areas already.

So, now after posting fifteen times in this thread, I'm bored with it and finished on this one! :D

 

 

  • Like 1

Share this post


Link to post
Share on other sites
martynk

Thanks Jef. 

Thats a great explanation. I understand and can visualise that perfectly. Thanks for explaining it so well. 

I haven't  reduced the fin and rudder size. I don't thing that it is necessary as my error was transposing the output from the EDA spreadsheet to CAD. the number crunching has produced output that looks right and now I have transferred it correctly looks ok on the plan as well. 

Now I feel much more comfortable, I have started work on the tailplane tonight.   I am looking forward to see this one fly :)

 

  • Like 1

Share this post


Link to post
Share on other sites
martynk

I have had a couple of off-topic chats with Jef and he has suggested a couple of more tweaks.

1. The inner and middle panel lengths have been increased at the expense at the tip panel

2. The middle panel dihedral has been reduced marginally and the outer increased - EDA is now 9.29

3. Nose length increased by 50mm

4, Spoilers brought inboard

Version 5 of the diagrams are attached for further comment. If Jef gives this the green light then I am also happy with the changes.

CC-100S-E-v5-poly.jpg

CC-100S-E-v5-poly-EDA.jpg

Vh-Vv-B-v5.jpg

A2 plan also attached.

Nothing quite like designing a model in a community :)  Thanks Jef

 

Cheshire Cat Poly Wing Plan-v5.pdf

Share this post


Link to post
Share on other sites
Gary B

All good stuff. Your tailplanes will be in the turbulent spoiler wash with that version, they're usually seen further outboard (but there are centre spoiler designs). Only one way to find out!

The Ava type floaters have a lot of dihedral because they have no ailerons, they are also very lightly loaded with their open structures, it's probably more a product of the low wing loading that they struggle penetrating into wind though I don't doubt that dihedral has a drag effect as explained by others.

There is a trend for optional increased dihedral tip joiners, I have them for my Ray X and Cluster 801/802. I've never seen the benefits of these fully explained though I've been told by a very well known F5J flyer (and joiner supplier!) that turns can be made flatter in weak conditions by just using the rudder, saving drag from wing control surface movements. 

Re:The Flair Sunrise, I haven't flown it for a long time, can't remember where the CG was (middle of the main spar I think) and misbehaved in both directions! It does have anhedral on the centre panels at rest as Pete says. It rolled off the bungee and crashed once though I was probably pushing the wind limit for flying it. Luckily not too much damage and lived to fight again, getting rid of its Fibafilm covering at the same time.

Must give it an airing this summer.

GB

  • Like 1

Share this post


Link to post
Share on other sites
martynk

Cheers Gary

Glad that you are finding this interesting. One of things that I love about this hobby/sport/pastime is the ability to experiment and do things your own way (or in this case Jef's way :) - credit where credit is due) and then prove your results. A proper mix of science and engineering. TLAR (that looks about right) is OK if you cant make a qualitative assessment however its nice to use tools if they are available. 

I just spotted an error in the Vv calculations above. I had entered the wring value for the Ver Tail area. Now corrected

CC-100S-E-v5-poly-EDA.jpg

Vv was too high on the earlier version, Model would probably have made a very good weathercock if I used a fin that big. The drawing is correct though.

  • Like 2

Share this post


Link to post
Share on other sites
martynk

I have had a couple of offline chats with Jef and he has suggested a couple more amendments regarding the length of the wing panels and the associated dihedral. So here is v6 of the model and the usual A2 plan for your comments and perusal. Feel free to add your thoughts to this. One final change may be the extension of the spoilers, its a balance between effectiveness and not blanking the tailplane.

 Magic numbers are:

EDA - 9.6 degrees

Blaine Rawdons - 5.45

Just waiting for a set of wing ribs to materialise

CC-100S-E-v6-poly.jpg

Cheshire Cat Poly Wing Plan-v6.pdf

Share this post


Link to post
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.
Note: Your post will require moderator approval before it will be visible.

Guest
Reply to this topic...

×   Pasted as rich text.   Restore formatting

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

Sign in to follow this  

  • Recently Browsing   0 members

    No registered users viewing this page.

×
×
  • Create New...

Important Information

We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue.