Logo
Builders' Plan Gallery  |  Hip Pocket Web Site  |  Contact Forum Admin  |  Contact Global Moderator
December 12, 2019, 09:56:23 PM *
Welcome, Guest. Please login or register.
Did you miss your activation email?

Login with email, password and session length
 
Home Help Search Login Register
Pages: [1]   Go Down
Print
Author Topic: Angle of Attack  (Read 607 times)
0 Members and 1 Guest are viewing this topic.
Larry R.
Bronze Member
***

Kudos: 3
Offline Offline

United States United States

Posts: 67

Topic starter


Ignore
« on: August 08, 2019, 07:08:47 PM »

I'm building (again) Bill Hannan's Flying Funtique, an all balsa sheet, rubber free flight model of 18" wingspan.  The wing has a bent flat plate airfoil, with an inch of dihedral at each wingtip, and has a 7 degree angle of attack.  The horizontal tail is also placed with about 7 degrees (or a bit less) angle of attack.  The trim flights were at first a bit squirrelly, but eventually the model settled down and flew nicely.

Now I'm building another Funtique with a few small changes.  I think the model would benefit from a taller vertical tail.  I'm keeping the wing and horizontal tail as per the original design.  I'm curious, though, how the unusually steep angle of attack affects the model's fight characteristics.  Does it produce increased lift via a kite effect?  I'm supposing the rather steep angle of attack of the horizontal tail is intended to counter the model's tendency to nose up and stall, but I'm just guessing here.

Logged
piecost
Gold Member
*****

Kudos: 7
Offline Offline

United Kingdom United Kingdom

Posts: 431



Ignore
« Reply #1 on: August 08, 2019, 08:41:50 PM »

If both surfaces are angled as you mention then i expect that it would fly with the fuselage pointing down. In effect it has built in down thrust. It is the difference in angle between the wing and tail that is important rather than relative to the fuselage.
Logged
Crabby
Titanium Member
*******

Kudos: 134
Online Online

United States United States

Posts: 2,191


I never met a modeler I didn't like



Ignore
« Reply #2 on: August 08, 2019, 11:24:32 PM »

Well explained piecost! I never thought of it that way!
Logged

The Threadkiller!
ZK-AUD
Platinum Member
******

Kudos: 50
Offline Offline

New Zealand New Zealand

Posts: 1,152



Ignore
« Reply #3 on: August 09, 2019, 03:07:46 AM »

Yes that's why it's done.  Check out the Skeeta (an old NZ design) on Outerzone to see the same approach
Logged
TimWescott
Platinum Member
******

Kudos: 15
Offline Offline

United States United States

Posts: 1,031



Ignore
« Reply #4 on: August 09, 2019, 03:16:25 PM »

OK, I just put on my nerd hat, firmly.

It's angle of incidence, with respect to the fuselage centerline.

The angle of attack is the angle of the airfoil with respect to it's motion in the air (it's the angle the airfoil attacks the air -- them airfoils should chill, and maybe take some anger management classes!).

What matters (roughly) for trimming is the angle of the tail with respect to the wing.  Tilt the tail down with respect to the wing and you have more "up" trim.  Tilt them both up, and the nose points more down. (Bill Hannan may have done it that way to make the thing fly level 'cuz the wing needs that angle of attack?).

The Vought F-8 (and maybe others) had a variable-incidence wing for landing on carriers, so the airplane could go slow while the pilot could still keep his eyeballs on the landing safety officer.
Logged
Larry R.
Bronze Member
***

Kudos: 3
Offline Offline

United States United States

Posts: 67

Topic starter


Ignore
« Reply #5 on: August 09, 2019, 04:33:53 PM »

Yes that's why it's done.  Check out the Skeeta (an old NZ design) on Outerzone to see the same approach

A very attractive model!
Logged
charlieman
Silver Member
****

Kudos: 5
Offline Offline

United States United States

Posts: 232



Ignore
« Reply #6 on: August 10, 2019, 03:12:34 PM »

One thing I've noticed about some Hannan designs is that he often shows stab wing incidence angles going somewhat contrary to well entrenched notions of proper model wisdom. For example, and IIRC, his profile all sheet balsa Citabria plan seems to show only a smige (hardly any) of down thrust , with wing set possibly at 1 deg positive and 0 deg at stab. I've built several and all flew excellently, right off the board.



 
Logged
lincoln
Titanium Member
*******

Kudos: 32
Offline Offline

United States United States

Posts: 2,074



Ignore
« Reply #7 on: August 11, 2019, 05:17:47 PM »

Charlieman:
If the tail is flat, and the wing is cambered, that's like having a bit of downthrust and decalage added already.
Logged
charlieman
Silver Member
****

Kudos: 5
Offline Offline

United States United States

Posts: 232



Ignore
« Reply #8 on: August 11, 2019, 06:04:27 PM »

lincoln,

Not gonna challenge you on that. Grin  Just noting Hannan's somewhat different design style. IIRC, his Avro G plan  shows similar sheet wings and tail with no incidence at wing nor tail. BTW, I believe  this plan appeared in a recent posting of an older  Aeromodeller, up in the Builder's Plan Gallery.

Logged
billdennis747
Titanium Member
*******

Kudos: 54
Offline Offline

United Kingdom United Kingdom

Posts: 3,715



Ignore
« Reply #9 on: August 13, 2019, 04:41:24 AM »

Not gonna challenge you on that. Grin  Just noting Hannan's somewhat different design style. IIRC, his Avro G plan  shows similar sheet wings and tail with no incidence at wing nor tail. BTW, I believe  this plan appeared in a recent posting of an older  Aeromodeller, up in the Builder's Plan Gallery.
The brain is fascinating - I remember very little of lots of the stuff I did at school (I'm not saying it hasn't been useful) but knew instantly that Hannan's Avro G was in that 1966 Aeromodeller, even though I never built it. Was it one of the 'cardboard' Aeromodellers?
Logged
billdennis747
Titanium Member
*******

Kudos: 54
Offline Offline

United Kingdom United Kingdom

Posts: 3,715



Ignore
« Reply #10 on: August 13, 2019, 05:41:29 AM »

Not gonna challenge you on that. Grin  Just noting Hannan's somewhat different design style. IIRC, his Avro G plan  shows similar sheet wings and tail with no incidence at wing nor tail. BTW, I believe  this plan appeared in a recent posting of an older  Aeromodeller, up in the Builder's Plan Gallery.
The brain is fascinating - I remember very little of lots of the stuff I did at school (I'm not saying it hasn't been useful) but knew instantly that Hannan's Avro G was in that 1966 Aeromodeller, even though I never built it. Was it one of the 'cardboard' Aeromodellers?
Oh, and sorry for the thread diversion, but compare that cover - in fact any of the covers -  with what we get now.
Logged
charlieman
Silver Member
****

Kudos: 5
Offline Offline

United States United States

Posts: 232



Ignore
« Reply #11 on: August 13, 2019, 06:45:08 AM »

Another of Hannan's "fun" designs appears in just posted  Feb.'67 Aeromodeller (again see Builder's Plan gallery).  Seems to be based upon pre-WWI Antionette(sp?) . This one has zero down thrust, noticeable positive wing incidence with curved sheet airfoil, and no incidence at stab.

I get that the set up has down thrust effect "built in", given the relative angles. However, I'm having trouble accepting that the airfoil  also contributes to that effect. With all the same wts , angles and moments, wouldn't such a model, but with a different airfoil (say a Clark Y or even a flat plate) be just as stable longitudinally, as curved sheet airfoil model?






Logged
Yak 52
Titanium Member
*******

Kudos: 67
Offline Offline

United Kingdom United Kingdom

Posts: 2,484


Free Flight Vagrant



Ignore
« Reply #12 on: August 13, 2019, 07:21:59 AM »

As Tim said, the angle of attack is the wing's angle relative to the airflow. This varies with speed, slower flight requiring a higher angle of attack. Aircraft flying slow look like they have their nose in the air. Go faster and the model trims nose down a little more. Something like 3-5 degrees AOA is normal for our small models. Full size can go to more like 15 degrees before stalling.

The angle of incidence drawn on the plan just sets the fuselage attitude in flight. If it's 3-5 degrees negative (leading edge tilted up a little), the model will fly with the fuselage roughly parallel to the ground. In the case of the 7 degrees mentioned by the OP the model will likely fly a degree or two nose down.

The decalage or angle between the tailplane and wing is what sets the AOA in flight (providing you leave the CG alone and don't add noseweight.) Giving more 'up elevator' ie a more positive angle on the tailplane will increase the AOA of the wing, make the model fly slower and pitch the nose up a little.


I get that the set up has down thrust effect "built in", given the relative angles. However, I'm having trouble accepting that the airfoil  also contributes to that effect. With all the same wts , angles and moments, wouldn't such a model, but with a different airfoil (say a Clark Y or even a flat plate) be just as stable longitudinally, as curved sheet airfoil model?

The airfoil does make a difference because the 'zero lift line' changes with camber.  Clark Y will make lift at zero AOA - it needs to be a couple of degrees negative to make zero lift. A flat plate or symmetrical has no camber and requires some AOA to make lift. At zero AOA it will make zero lift. A cambered airfoil will make the same lift with a degree or two less geometric angle of attack than a symmetrical airfoil.

The angles that matter are measured relative to the zero lift line of the wing - any thing else we measure, such as geometric AOA, decalage, incidence, are all arbitrary datums (data Huh), albeit useful approximations. The short answer is that if you change the camber eg. a curved plate to a flat sheet, you will need to tweak the incidences slightly to achieve the same trim.

Specifically: change a curved plate to a flat plate airfoil and you'll need a degree or two more negative wing incidence on the plan.

so:

With all the same wts , angles and moments...

This is the problem - changing the camber changes the wing pitching moment and requires some tail adjustment.

Specifically: change a curved plate to a flat plate airfoil and you'll need some more decalage.
Logged
Indoorflyer
Platinum Member
******

Kudos: 15
Offline Offline

United States United States

Posts: 1,112




Ignore
« Reply #13 on: August 13, 2019, 09:15:49 AM »

Add enough horsepower and you can make a barn door fly.
Logged

Make the same mistake on both sides; nobody will notice...
Crabby
Titanium Member
*******

Kudos: 134
Online Online

United States United States

Posts: 2,191


I never met a modeler I didn't like



Ignore
« Reply #14 on: August 23, 2019, 09:04:08 PM »

Great thread going on here! Well done Jon. Great explanations guys! I think that after all these years building nice models that either flew or didn't I am finally beginning to "get it"
Logged

The Threadkiller!
Pages: [1]   Go Up
Print
Jump to:  

Powered by MySQL Powered by PHP Powered by SMF 1.1.21 | SMF © 2015, Simple Machines Valid XHTML 1.0! Valid CSS!