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Going really fast - 3/26/2002 6:45:59 AM   
Cometlover


 

Posts: 32
Joined: 3/25/2002
From: Europe
Status: offline 		I have been playing with the idea of building a very high speed, aerobatic and fun to fly plane. These three requirements combined make for an interesting project.
I was planning on using a OS .91 VR-DF engine for power. I don't even know if such an engine is suited for use with a prop, but that's a question for the jet forum. My guess is that it is...
This thing should be doing 125+ MPH! At least, that is the goal (more like 150 really )

So the question is: is it possible to combine the three requirements listed above in one airplane.

Does anyone have any ideas as to an airfoil that meets these requirements? Should I apply different airfoils for root and tip to shape up stall properties? Do I need to calculate the Reynolds number before someone can give me some advice?
How about the profiles for the tail section?

Remember: acceptable stall speed, unaffected by inverted flight (thus a symmetrical airfoil), capable of high airspeeds, resistent to very high G maneuvers...

I think you know what I mean!

Thanks
       Post #: 1

Going really fast - 3/26/2002 7:08:32 AM   
ChuckN



Posts: 482
Joined: 3/4/2002
From: tucson, AZ, USA
Status: offline 		So you basically want to design the best flying, coolest looking, neatest sounding pattern plane right here on this forum. Can't be done!

But! If you work at it through several itterations using all the know-how you've gleaned from books and magazines you could do it - FOR REAL! How do you think folks like Joe Bridi, Don Lowe, and Ivan Kristenson ended up with such good performing and nice looking designs?

OR:

Have you ever checked out the pattern designs of the late '70s? These planes can do what you want with a piped rear exhaust .60 (a .91DF will not spin an 11 in. prop!). Rear exhaust Rossi's and O.S.'s can still be found at reasonable prices because there isn't much demand for these propellor spinning 2-strokes anymore. RCModeler and Model Airplane News still sell plans for many of the old pattern planes because they are such great performers.

Dirty Birdy (RCM)(still available from BridiRadioControl)
UFO (RCM)
Escape (BridiRadioControl)
EU-1A (MAN)
Curare (MAN)
Atlas (MAN)
Arrow (MAN)
Saturn (MAN)

(in reply to Cometlover)
       Post #: 2

Fast Aerobat - 3/26/2002 9:10:55 PM   
Ollie


 

Posts: 958
Joined: 12/8/2001
From: Punta Gorda, FL
Status: offline 		You can have tip stall margin, both upright and inverted, at the expense of roll rate by using little or no taper in the planform.

You can have speed by using a thin airfoil, small crossection fuselage, retracts, a well cowled engine and, a long tail moment arm with small tail surfaces. The results will look much like an electric “hot liner.”

To get the high lift coefficients out of a thin airfoil so that you can do high G maneuvers, you will need full span flapperons coupled to the elevator channel. The flapperons will help lower landing and takeoff speeds.

You will need a unicarbon spar that can take 30 or 40 G’s.

If you want to keep your speed up in the turns, you will need a high aspect ratio to limit induced drag. Unfortunately, this will also limit roll rate.

BTW, dynamic soarers have hit 170 MPH while flying tight circles. You might look to them for some spar design ideas.

You will need a prop with low enough pitch to take off but high enough pitch that it gives a high top speed. The wider the range of RPM the easier it will be to prop for the desired performance.

Finding the right compromise between the conflicting objectives is the challenge.

_____________________________

Regards, Ollie

(in reply to Cometlover)
       Post #: 3

Consider a delta - 3/26/2002 11:41:53 PM   
Craig-RCU



Posts: 524
Joined: 1/10/2002
From: minneapolis, MN, USA
Status: offline 		Planform will effect flight characteristics too. My funnest, most aerobatic plane with the widest speed range is a modified Balsacraft Rapier (I added the Vstab and landing gear).


It goes around 120 straight and level and does wild hoverbatics too with control surface throws of 45 degrees. It will loop within its own span and the verticle is out of site since it weighs only 4.75 lbs and the Jett 50 puts out about 8 lbs static thrust. It will also do great flatspins like the Tuflight Enduro. Check out the "rotolanding" videos on the Tuflight website [URL=http://www.tufflight.com/videos.html]http://www.tufflight.com/videos.html[/URL]

Low aspect ratio flying wings have a couple of advantages over conventional planforms that will get you the flying qualities you are after. First, they are aerodynamicly cleaner for high speed flight. Second, they are very stable and controlable at high AoA which is great for flight perfomance all the way to hover-mode. Check out the "Flying Pizza Box" thread in the fun-fly forum.
[URL=http://www.rcuniverse.com/forum/fb.asp?m=66540&s=#66540]http://www.rcuniverse.com/forum/fb.asp?m=66540&s=#66540[/URL]

I believe in building on what works. Consider saving yourself the hassle of choosing the right airfoil (for now) and just modify an existing design for your goals. If you are now thinking "fast delta", the fastest delta prop plane that I have heard of is the Diamond dust (200 mph + with a Jett 50). Since the Diamond Dust doesn't have a controlable rudder needed for some aerobatics, I would suggest making similiar modifications to the Diamond dust that I have done with the Rapier and I think you will achieve your flight parameters. D

< Message edited by Craig-RCU -- Mar 26 2002 6:48PM >

(in reply to Cometlover)
       Post #: 4

Going really fast - 3/27/2002 12:26:04 AM   
Ollie


 

Posts: 958
Joined: 12/8/2001
From: Punta Gorda, FL
Status: offline 		The only thing a delta can't do well is maintain airspeed in a tight turn. If tight turns while maintaining high airspeed isn't a requirement, go with a delta. it will certainly roll much better than a high aspect ratio planform.

_____________________________

Regards, Ollie

(in reply to Cometlover)
       Post #: 5

fast... - 3/27/2002 4:15:10 AM   
Cometlover


 

Posts: 32
Joined: 3/25/2002
From: Europe
Status: offline 		Thanks for the input guys.

But everything in the above posts has already been taken into consideration.
I am not looking for a pattern ship. I am not looking for a delta. I am looking for something in between basically.

I was thinking about something like this: a modified Midwest Super Hots!
And by modified I mean something like this: approximately the some airplane size, but, lots of power in the nose, different extremely streamlined fuse form, high taper wing allowing the use of a thin airfoil at the root and still have enough thickness for strength and a thick airfoil at the tip to improve stall characteristics (maybe combined with some sort of a tip- or winglet...), but the same moment arms as in the Hots.

I know there will have to be made some compromises in such a design. Landigns will probably be terrible, for instance...

I was thinking about using full floating horizontal and vertical stabilizers, to minimize drag in high speed maneuvers, large ailerons and flaps for the same purpose.

Oh and Chuck, you're right, I am looking for the best flying, neatest looking and coolest sounding plane on this forum....

(in reply to Cometlover)
       Post #: 6

Going really fast - 3/27/2002 7:59:38 AM   
ChuckN



Posts: 482
Joined: 3/4/2002
From: tucson, AZ, USA
Status: offline 		Your best flying, neatest looking and coolest sounding plane on the forum won't last long with "full floating" (otherwise known as "flying" stab and rudder due to the fact they will flutter severely at high speed.

To be honest, if you want something more radical than one of the high speed pattern planes from the late '70s you will need to go with either a Diamond Dust or a quarter midget 40 pylon racer. If that's not what you want then design and draw your special plane and show it to us. I'd be curious to see what you got in your head because I have been thinking about a similar project over the last couple of years.

(in reply to Cometlover)
       Post #: 7

All Moving Stab. - 3/27/2002 2:27:28 PM   
Ollie


 

Posts: 958
Joined: 12/8/2001
From: Punta Gorda, FL
Status: offline 		An all moving stab doesn't flutter if the pivot line runs through the aerodynamic center.

_____________________________

Regards, Ollie

(in reply to Cometlover)
       Post #: 8

Going really fast - 3/27/2002 9:58:26 PM   
ChuckN



Posts: 482
Joined: 3/4/2002
From: tucson, AZ, USA
Status: offline 		Ahh, but the aerodynamic center moves with angle of attack, no?

(in reply to Cometlover)
       Post #: 9

AC movement - 3/28/2002 2:25:16 AM   
Ollie


 

Posts: 958
Joined: 12/8/2001
From: Punta Gorda, FL
Status: offline 		The Aerodynamic Center (AC) can move appreciably at very low reynolds numbers as the angle of attach varies. It won't move much ( probably only a percent or two) in this high speed application. Flutter can easily be avoided by loacting the pivot line at the forward limit of the AC's excursion.

BTW, at full scale sizes and speeds the AC doesn't move with angle of attack.

_____________________________

Regards, Ollie

(in reply to Cometlover)
       Post #: 10

Going really fast - 3/28/2002 4:07:12 AM   
gubbs3



Posts: 1403
Joined: 12/8/2001
From: Coon Rapids, MN, USA
Status: offline 		You should take a look at a Diamond Dust. They are capable of going 180+ with an O.S. 46 FX. They are a delta wing that is very inexpensive ($60) and quick to build.

(in reply to Cometlover)
       Post #: 11

Going fast - 3/28/2002 4:12:48 AM   
Ed_Moorman



Posts: 3701
Joined: 1/5/2002
From: Shalimar, FL, USA
Status: offline 		I agree, get a Diamond Dust and cowl your engine. I remember one like that which was clocked over 200 mph.

_____________________________

Ed Moorman, AMA 553, KD4QBM, Revver Bro #156
R/C Report Magazine, Fun Aerobatics Column

(in reply to Cometlover)
       Post #: 12

Re: All Moving Stab. - 3/28/2002 8:26:16 PM   
billf



Posts: 260
Joined: 2/1/2002
From: Hudson, WI, USA
Status: offline 		[QUOTE]Originally posted by Ollie
An all moving stab doesn't flutter if the pivot line runs through the aerodynamic center. [/QUOTE]

Ollie...
Somewhere I had heard that an hinged surface would not flutter if the pivot is behind the chordwise CG. Is this an incorrect impression? Or, must the pivot be right at the AC to avoid flutter? That is, neither forward or aft of the AC.

Maybe a better way to ask the question is: What should the relation between the AC and chordwise CG of an hinged surface?

Bill

(in reply to Cometlover)
       Post #: 13

Balancing the Control Surface - 3/28/2002 9:35:29 PM   
Ollie


 

Posts: 958
Joined: 12/8/2001
From: Punta Gorda, FL
Status: offline 		There are two kinds of control surface balance, aerodynamic and mass.

Aerodynamic balance is achieved when the pivot runs through the aerodynamic center of the movable surface. This kind of balance reduces the net torque on the surface to zero.

Mass balancing increases the period of oscillation to infinity which is the equivalent of reducing the frequency to zero.

If either type of balance is perfect, it is completely effective in supressing the associated mode of flutter.

In addition to the flutter mode of flapping around the pivot line there are several possible aeroelastic modes of flutter. There can be longitudinal flutter along the span and torsional flutter along the span as well as others.

_____________________________

Regards, Ollie

(in reply to Cometlover)
       Post #: 14

Going really fast - 3/28/2002 11:30:16 PM   
niccolai_m


 

Posts: 6
Joined: 3/26/2002
Status: offline 		Flutter occurs when the control surface modal response frequencies and the aerodynamic forcing function frequencies become equal (or close enough).

Works like this....

Surface moves, force on surface changes and pushes the surface back where is came from. If the push backwards is done at the resonant frequency of the surface, the cycle will get worse and you have flutter.

You can control this by balancing as mentioned previously but don't overlook the fact that the resonant frequency of the surface is affected by the stiffness and friction in the whole control system.

If you stiffen up the control system by reducing any slop in the connections from the servos to the surface you'll increase the resonant frequency and improve flutter resistance. The servo itself will be a contributor. You are looking to reduce backlash in the system. Gently rock your control surface with your fingers to look for play - in any mechanical system there will be some play - reduce it as best as you can.

Friction will also reduce the chances of flutter by giving the energy somewhere to get used up. Too much friction is obviously a bad thing though.

(in reply to Cometlover)
       Post #: 15

Going really fast - 4/7/2002 12:06:21 AM