Jump to content
martynk

Converting decalage to A0A

Recommended Posts

martynk

Went out to fly F3-RES last night. The sky had clouded over and it was absolutely flat calm and the air had gone cold. I used the opportunity to try to optimise the glide and this got me thinking about what the wing is actually presenting to the oncoming air stream.

Some basic and approximated maths. I never managed to get the model to the top of the bungee.  I did manage to get some nice floating launches  from an estimated 30m launch height, I was gliding for typically 90 seconds. (Some slightly better). Judging model speed in the flat calm, I would guess it was going a bit faster than I could run - say 8 mph (I only have short dumpy legs) - 3.6 m/s. So that means it flew for about 324 m as the idiot at the sticks tries to keep it flying as smoothly as possible.

Launching from 30m  gives tan-1(30/324) = a glide angle of about 5.3 degrees. (That seems steeper than I would have expected so my maths may be poo)

If the model is set up with 4 degrees decalage, does that mean that the AoA is 9.3 degrees,? If not how do you calculate what AoA the wing  is actually flying at?

 

 

 

Share this post


Link to post
Share on other sites
SilentPilot

AoA is variable depending on the airspeed and is controlled by the Elevator. 

How to actually measure it I'm not sure! 

Share this post


Link to post
Share on other sites
martynk

Yes I appreciate that, but if the model is trimmed for a flat stable low sink glide, the model attitude will not change (significantly).Its impossible to guess what attitude the model has when flying, it may be nose up or down within 1 or 2 degrees and you couldn't really tell. Thinking about it there may be an attitude telemetry sensor that could be fitted which would be very helpful..

 

 

Share this post


Link to post
Share on other sites
SilentPilot

Fit an FPV camera and a protractor :D 

Share this post


Link to post
Share on other sites
martynk

Thanks Phil

I'll need to go away and carefully read that

 

Share this post


Link to post
Share on other sites
mikef
2 hours ago, martynk said:

......     Launching from 30m  gives tan-1(30/324) = a glide angle of about 5.3 degrees. (That seems steeper than I would have expected so my maths may be poo)

 If the model is set up with 4 degrees decalage, does that mean that the AoA is 9.3 degrees,? If not how do you calculate what AoA the wing  is actually flying at?

“.....does that mean I AoA is 9.3 degrees?”      No.

Assuming the 5.3 degrees is correct, Phil's reference lets you calculate the L/D.  You are no nearer knowing the AoA.  You need to know what angle the fuselage datum is at relative to the horizon in a smooth glide in still air and combine that with the glide angle and the rigging angle of the wing relative to the fuselage datum.

Why do you want to know the AoA?  What will you do with the information?

 

  • Like 1

Share this post


Link to post
Share on other sites
martynk
Posted (edited)

Thanks Mike

I suspected that my 9.3 degrees was incorrect.

Why? Well, If my understanding is correct, if you know the in flight AoA, you can accurately calculate the Power factor for the wing section and then set the CG accordingly so that the  model is optimised (or very nearly) before it gets in the air. I appreciate that its an iterative process and conditions to do proper testing are few and far between.

When I was playing last night, I was just tweaking the elevator trim until the model stalled and then back it off a couple of clicks, much in the way that I used to trim FF models. This does not mean that the wing is optimised of course, what I really needed to do was to move the CG as well to alter the decalage.  I am not convinced that modern skinny wing sections are optimal when flown near the stall, I suspect that flying at a higher Re will make them more efficient which means moving the CG back (at the risk of losing longitudinal stability).

Please shoot me down in flames if I am on the wrong track! :)

Edited by martynk
Clarified
  • Like 1

Share this post


Link to post
Share on other sites
mikef

Are you referring to this info :-  https://www.hippocketaeronautics.com/hpa_forum/index.php?topic=5364.0

You'll need a polar plot for the whole model to determine the best trim speed for min sink.  You could base this on airfoil data and some assumptions.  That's the problem, you have no real-world airfoil data to use with your AoA figures.

But you have a complete model and could measure sink rate directly with a recording altimeter in still air.  I can send you one to play with if you like FOC.  With that, you can try different trim speeds and check the effect on sink rate.   You can also vary the cg and other variables to see the effect.  Warning - you will have trouble getting consistent 'still air' and might be better off just putting more lift picking practise in....

I have attached a .pdf file about ballast that explains the whole model polar.  The second half shows practical ballast solutions.  It's from a PowerPoint file and you need the notes (in my head) to fully follow it all, sorry.

Ballast.pdf

  • Thanks 1

Share this post


Link to post
Share on other sites
martynk

Thanks Mike

I hadn't seen that post and I'll take a look.

That is very kind of you. I have a little HK altimeter that sort of works and should be OK with the 1S LiPo that I use for power. What model do you use?

Last night was probably the best dead/still air I had ever witnessed, (I am not a 4am riser to fly)  even when flying over my usual 'hot spots' - a tarmac farm track, there was no perceivable lift. Which was very frustrating.

I am certainly inspired to try and understand this better - especially as I am about to start a new incarnation of my F3-RES glider

 

Share this post


Link to post
Share on other sites
mikef

I have used Ram 2 and Ram 3 from soaring circuits.

Share this post


Link to post
Share on other sites
martynk

Thanks 

Just reading through your PDF.  It's well written. I need time to assimilate that as well. A quick scan through shows that many of the problems of flying a DLG apply to lightweight F3-RES as well with the added complication of a (predominantly) wooden structure..

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.


  • 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.