Aerobatic Design

[Apivat J]

Hi Guys,

I'm planning to scratch built an Extra 300s (yeah..yeah...it has been done till death but the purpose is to learn how to put different design parameters together so they work as intended) and got confused at the design stage, especially, its lateral stability.

First let's start from its spec:

Wing Area = 682 sq inch
Span = 60"
Wing Root = 14.31"
Wing Tip = 7.7"
Taper Ratio = 0.54
Sweep Angle = 0 (measured at 25% cord)
Airfoil = Not yet decided
Weight = Not yet determined but we can work out from cruising speed, Cl, hence lift
Engine = OS 70FS -4stroke
Prop = Master Airscrew 12x8
Stab Area = 162 sq inch
TMA = 28.52"
Vertical Tail Area = 93 sq inch
VTMA = 30.51

Design objectives:
1. Capable of all aerobatic maneuvers
2. 3D capable

From the above spec and design objectives I have a few questions:
1. Looking at full size Extra 300s, the wing has no diheral of any kind. From the kits available in the market, there seems to be no diheral either. How does this aircraft gain it's lateral stability?

2. The full size Extra 300s wing seems to be aerodynamically twisted having MA 15s airfoil at the root and MA 12s at the tip. These airfoils look pretty much like 3D/funfly type of airfoil. However, most kits seem to use different types of airfoil, perhaps, from NACA00xx family.

2.1 Why can't we use similar airfoil such as NACA0016 mod (0% camber,
max thickness = 20%)?
2.2 Airfoil thickness has the effect on drag and Cl Max. Generally thicker
airfoil has higher Cl Max, higher Cd but wider drag bucket. Is higher
drag desireable for Aerobatic design?
2.3 The purpose of aerodynamic twist at the tip is to prevent tip stall.
Would it be a good idea to incorporate aerodynamic twist in Extra 300
wing? If so, would that effect how th plane snap roll?
2.4 What would be the appropriate airfoil to use considering the objectives
sought for?

3. How do we calculate static thrust of the above engine and prop? Is this a
neccessary thing to do during the design?

It would be very thankful if any expert out there could shed some light into my confused mind.

Sorry, this is a bit long but I hope we'll learn something from it.

Thanks,
AJ

[Andy W]

1. It doesn't, that's what makes it fly so well.
2. Full-scale airfoils do not translate well to models.
2.x - it's usually a matter of "build one, fly it, see what works, try another if you'd like it to be better"
3. Prop RPM can be used to determine thrust. There are plenty of calculatores online that can calculate that for you.
..a

[Apivat J]

Andy,

I have been down that "trial and error" road before when I scratch built a P-40. It took me a year and a half to make 7 models before I get it right. I learned something from it though:

1. My wife got mad 'cause I spend countless weekends building and re-building.
2. All my friends thought I will be the second Eistien.
3. Model behaviour can be predicted quite accurately by those "dreaded" formulas way before it is completed (provided they are being used appropriately).

I'm interested in getting real technical on #3. I believe it will save me time so I can take care of both #1 and #2.....and, oh yeah, my wife is still mad at me. I think she's gonna ask me to marry a plane someday.

AJ

[Ollie]

E474 airfoil is for Aerobatic Design.

http://www.nasg.com/afdb/show-airfoil-e.phtml?id=184

Aerobatic Design is main idea with next to zero stability.

Good roll rate is fast with tip stall!!!!

You can't have cake and eat it too.

Thrust over comes weight. Make low weight and high thrust ratio.

Drag is high (good) but the thrust be much bigger than drag.

Streamlining is for style, not for reducing drag.

Think 3D for function.

[Apivat J]

Aerobatic Design is main idea with next to zero stability.


Wow, this must be the easiest of all design, in this case. I tend to think the otherwise.

Any advice on the proportion of the control surface area? What would be a good percetage to start with?

Any other considerations to be made beside strength to withstand the G?

BTW, thanks for the airfoil recommendation.

Thanks,
AJ

[Apivat J]

I thought it would be nice to show some work. Here the top and side view of Extra 300s. I thought I would reposition the wing so that it is in line with the thrust line.

[Ollie]

The nice plan view is proportional.

The side view is not so good. The vertical tail area could be shifted down so the thrust splits the fifty-fifty % area. Knife-edge will fly better with the thrust splitting the vertical area. Function over style.

The structure should have high strength to weight ratio. Keep the mass close to the CG. The strength at least 7 to 10 G's. Keep the airspeed down with the weight low.

[Apivat J]

That's a very tough decision to make. But I see your point. I tried tweaking the lines today but it was quite tough to balance the tail area over thrust line without significantly change the look of the aircraft altogether.

Does this means that both full size and scaled down Extra300 don't Knife-edge very well? If so, can we conclude that CAP 232 and Edge 540, for example, have the same
problem too?

I heard that some models doesn't track very well during Knife-edge. They either have tendency to roll, or pitch side way (need either up or down elevator input to keep it straight). Are these the kind of symptoms associated with 50-50% tail area split?...I'm so curious....???

[Ollie]

You can substitute flying skill for bad handling qualities. Function vs. style.

An aircraft is a design that has many, many conflicts flying in formation.

[Apivat J]

Alright, it's all clear to me now. Thanks a lot.

AJ