Going really fast
 

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 :D:D:D)

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

Going really fast
 

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)

Fast Aerobat
 

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.

Consider a delta
 

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).
http://www.usfamily.net/web/craighilgers/Rapier01.jpg

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 http://www.tufflight.com/videos.html

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.
http://www.rcuniverse.com/showthread...6540#post81320

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. :DD

Going really fast
 

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.

fast...
 

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....:D

Going really fast
 

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.

All Moving Stab.
 

An all moving stab doesn't flutter if the pivot line runs through the aerodynamic center.

Going really fast
 

Ahh, but the aerodynamic center moves with angle of attack, no?

AC movement
 

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.

Going really fast
 

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.

Going fast
 

I agree, get a Diamond Dust and cowl your engine. I remember one like that which was clocked over 200 mph.

Re: All Moving Stab.
 

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

Balancing the Control Surface
 

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.

Going really fast
 

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.

Going really fast
 

you want a Outlaw, ok its a delta, but i was blown away by performance, i only have a 36 on a mini pipe, and i can make a APC 10x4 sing, 10x6 and its a rocket, rolls with a touch of rudder are so fast its a blur, and yes lots of people say that but i mean it, it can also do inverted climbing flat spins, very cool to watch, and looping flips ( on video @ www.rcflyers.fsnet.co.uk )
stall speed, sorry there isnt one, even for an american type funfly.
I've got a Diamond Dust too with tuned MVVS 40 and the Outlaw can do the same high G direction changes, fly as slow, and i think its faster maybe than mine.
its simply all balsa and took me 2 days to build, it also survived a 150+ head on impact with another plane burying it wingtip in the foam wing of the other plane and comming in with no other dammage. ( pics on same site )
Rates off it's nuts, rates on, its damn quick and stable.

Going really fast
 

A .90 ducted fan engine will work just fine turning a suitable propeller, and will pull a similar size and shape model faster than it would with a ducted fan, since ducted fans are much less efficient than most props. Simply ripping the engine/fan out of a ducted fan model and hanging the same engine on the nose, turning a prop should generate more airspeed than the ducted fan could ever dream of. Weight will be much less than with a ducted fan, so you don't need as much wing area. Control surfaces should be on the smallish side, and mass balanced. against flutter. You will need something around an 11-8, or even a 10-8 prop, turning 18 to 20 grand static, for top speed in the 180 mph range, and 200 mph should be possible with a smallish, very clean model with the right prop. Such models are not expensive, provided you already have a half-decent fan engine kicking around, gathering dust, and they are great in windy weather. Just be sure to thoroughly flight test it, including lots of top speed flying before flying when many people are around, since it could become a rather lethal missile if something broke at the wrong time.

Don't even think about using a plastic prop. I used to carve my own high pitched wood props for such creations, since most stock wood props would be stressed a bit high. Reworking a larger prop, cutting the blade length,width, and thickness down, is one solution - gives you more cross section area in near the hub where props normally fail in flight. A good carbon prop should be OK, but I would check with its manufacturer to find its maximum safe RPM. Aside from the personal hazard presented by a deserting blade, I have models badly damaged, and one even crashed after chucking a blade, with all the servos shaken out of their mounts.

Going really fast
 

With an engine like that, a broken prop would most likely rip the whole nose off the plane. I looked at the apc prop web site and the only pylon props in a close size was a 13/13 for 120 warbirds.

Going really fast
 

A 13-13 would, of course, be far too much load for a fan .90. Somewhere between 8 and 10 inch pitch should do, at 20,000 RPM or so in the air, and 10 to 11 inch diameter, depending on how slippery the model is. If a Nelson .40 can pull a pylon model at close to 175 mph, a fan .90 should pull a model of the same drag at well over 200. I used a flat-topped fuselage, with a fiberglass or molded thin plywood half-circular section cover to enclose the pipe and clean things up, and let most of the engine cooling air flow over the pipe and header, then discharge near the rear.

Going really fast
 

Rotaryphile,

Obviously you seem to be sharing the same ideas as me on how the construction of such a model would look.
I was thinking in the same direction. Box type fuse for strength, but also fully sheeted in an aerodynamic manner. I was thinking about using NACA numbers to shape the fuse to minimize drag. And of course keeping it thin will help also. Just enough room for all the components to fit in and that's it.
I don't have a .90 df engine in the basement, but thats no problem. I would have to come up with a home made motor mount though. Or could the carb on the OS .91 VR-DF be turned in such a way that the needle valve doesn't interfere with a standard motor mount?
Finding a suitable prop is another issue of course. My guess is that a carbon prop of the right size would hold up, but that's only a guess. And guessing when you're messing with this kind of power can be very dangerous. I am convinced that if a prop blade decided to quit there will be nothing left of the plane. It wouldn't even matter if it happened in the air or on the ground. The plane would litterally be shaken apart. I've seen this happen once with a 1.20 four stroke engine on a sport model on the ground when tuning the engine. Fortunately no one was hurt, but there was nothing left of the model. Every glue joint was cracked!

Going really fast
 

What I've got in mind would of course be very close to what a pylon plane looks like. A little bigger probably, but not much.
It would probably turn out that the wing that has a smaller aspect ratio then common pylon planes. Just a question of keeping the strength up as this will be a full balsa and plywood constuction, laminated with some carbon and/or glass fibre in critical areas.

OS .91 carb
 

The OS.91 carbutetor can only be rotated 180 degrees. But you can remove the needle valve assembly and install a plug (provided with the new motor) and use a remote needle, BVM or JMP.

Going really fast
 

To me, it sounds like you are looking for a scaled up Quarter 40 or F3D racing plane. Similar to my Avatar picture. Heres a link with several different styles of Q40 racing planes that might give you some ideas.


http://www.pylonworld.com/rc_pylon/q...-40_planes.htm

here is a picture of a F3D racing plane. I think these look awesome!

Going really fast
 

Just go out and pick up a .19size Sig wonder and put in a .21 rossi buggy engine. I seen a video of some guy who did one like that and it was ridiculous..

The .19 wonder with a good .25-32 will be insane!!!! trust me!

Going really fast
 

You have got to look at the Whiplash from ACM.

http://www.geocities.com/roger_forgues/Whiplash.html

It's fast becoming the high speed trainer of choice for Jet Jockey's