I want it to go FAST

[Dancy]

I am trying to coaless everything I have been reading but not having much luck. I bought a Katana from Chief Aircraft and I am trying to figure out the best engine prop combination to make this plane go fast. Our little club has a speed contest each year and I would like to take a stab at it. The current record is 127mph. I have been playing around with the Thrusthp program (http://freespace.virgin.net/barry.hobson/) and now I am confused! Although that’s close to my normal state I am hoping for some help from those more well versed it this subject than I. So just how much power (in hp) will it take to get this plane moving? I am hoping to see 130 to 150 mph. Maybe that is too fast for this plane. I don’t know. It seems to be very well constructed and is very light. The specifications are:
· Span – 72”
· Fuselage – 8” wide 66.5 long
· Area 992 sq/in
· Weight 7.8 lbs with all gear except engine, mount, spinner, prop, fuel
· Wing root – 15.5x2 inches
· Wing tip – 8.5x1.125 inches
· Airfoil NACA 0014
It is balsa/plywood conventional construction. The wing has an I beam type balsa spar and is sheeted 100% forward of the spar and 30% aft. Covered in monocoat type covering.
I considered the OS engine 91 VR-DF, a ducted fan engine. I was thinking high rpm (18000), small diameter prop, nearly 5hp in a small, light engine. But according to the ThrustHP program I would end up with an 11x8 prop. That only extends past the fuselage by 1.5 inches on each side. Does not seem like a workable situation to me, plus it looks silly! ThrustHP tells me what the static thrust would be for a given combination would be (13.42lbs) but I don’t understand the connection between static thrust and max speed. The program does list a speed of 136 mph but I don’t trust it because there are just too many things it does not take into account like the width of the fuselage. DRAG in other words.
I have seen some discussion on static thrust to weight ratio and I can make sense out of it as it applies to hovering your plane but how would it be applied to max speed? It does not take drag into account. Everybody knows slippery airplanes go faster. I have heard of this inverse square law as it applies to hp vs. speed now I need specifics.
Back to the original question, what engine/prop combination will produce the results I am looking for, a speed of 130 to 150 mph. I want to know HP, RPM, Diameter and pitch.
:p

[canadagoose]

I'd say you need an engine meant to turn a high pitch prop. YS 110 or 140 four stroke comes to mind. The ducted fan engine might be rated at 5 hp but that is at an rpm that you could never achieve with a prop of any size worth condideration. Consider this, an engine turning a 14 in pitch prop at 10,000 rpm could theoretically achieve 132 mph, that's not accounting for any slipage though. You're deffinely going to need alot of power. You shouldn't go with a prop that's too small because you'll just get more slippage so the engine will have to turn faster to make up for it. Then you would have to go with some sort of shrouded prop to try and reduce the efficiency losses.

[Ollie]

Model gliders have hit over 200MPH by drag reduction. That is proof that drag reduction can pay off big time. Andy Lennon's book, R/C Model Airplane Design shows how to reduce drag in many ways. You can design the model to go fast based on an engine prop combination by sizing and configuring the model to minimize drag. Pylon racers take this approach to speed. Copy their configurations, airfoils and other drag reduction techniques. Get rid of all external control linkages by using the rotary aileron driver (RAD) hardware. Cowl the engine for low drag and adequate cooling. Lennon's book shows how to design a proper low drag cowl that cools efficiently. Loose the NACA 0014 airfoil and pick one the pylon racers use. The smaller the plane for the engine prop conbination, the faster it will go. Use flaps to reduce landing and takeoff speed to managable levels with the minimized airframe size. Use retractable landing gear or streamline the fixed gear as shown in Lennon's book.

[BMatthews]

If you really want it to go fast at the expense of all else then I would suggest that your design numbers describe a model that is much too "fat". What I'm seeing in those numbers is more a pattern model with a fatter than normal fuselage.

As Ollie said go for a better pylon racing type airfoil and slim down the fusleage numbers.

In fact you could do a lot worse than to consider one of the FAI pylon racing models enlarged by about 50 %. Or if simpler is your desire then a Quickie 500 design enlarged by the same 40%. Or split the difference and enlarge a Formula 1 by 50% and loose the canopy and cheek cowls for more streamlining yet. Or a european Club 20 class model blown up about 80% and equipped with retract gear. Or one of the many Speed 400 electric designs blown up by 100%. All these will provide you with a sleek, fast model that does large high speed jet style maneuvers very well.

[Dancy]

Thanks guys! I am still looking for more detail. I would like to be able to set up a spread sheet where I could enter values and see the estimated speed. I would need a way to calculate the drag of the model, then to take that value and apply it to values like horse power, prop diameter, pitch, slippage ect. Some sort of simulation to predict performace. Any ideas?

[Ollie]

The maximum speed is simply the speed at which thrust equals drag.

PC Soar is freeware that can be downloaded from:
http://my.athenet.net/~atkron95/pcsoar.htm
It will accurately calculate the drag of a clean airframe. However it has no provision for including the drag of landing gear, uncowled engines, exposed control linkages, etc. If you are prepared to clean up your model it will solve half your problem.

A stumbling block to calculating the thrust is that the pitch number stamped on commercial propellers isn't very accurate. Often it is in error by 20 to 30%. If that isn't bad enough, there is the uncertainty of the airfoils and airfoil characterstics of various propellers. The RPM at which an engine puts out its peak power is often reported in the modeling press. The engine RPM on the ground can be easily determined by a tachometer but estimating how much the engine speeds up in the air with a particular prop can only be roughly estimated. The actual power an engine produces also varies with atmospheric pressure and the fuel mix used. The sum of all these uncertanties, variations and errors is large indeed.

A practical solution is to put a powerful engine on a small, clean airframe and try various props until you find the one that gives you the most speed. The wing has to be big enough to produce enough lift at a slow enough speed to allow practical landing and takeoff. What is practical for landing and takeoff depends on the length and smoothness of the runway that the plane will fly from.

[vintage1]

The key parameters are as Ollie says. Lots of horsepower delivered at the correct RPM, and ultra low drag airplane (and that usually menas a poor stall/tipstall character) and teh riht prop to match it all.

Drag goes up as the cube of velocity (I think) so gettimng it slippery is the way to go. he oroginal spitfire was all flush rivetted (expensive) so the story goes they simply tried out as many different combinations of flush and doned to see where it actually made the real difference, knocking pounds of the prduction cost and only a few MPH off the top speed.

IIRC the optimal RPM for 100mph pls is probabl;y in teh mid teens of thousand RPM - i.e. 13k-17k, whereas a DF engine is rated up to 30k.

You will need a pitch in the 10-20" range to achieve those sorts of speeds, and an engine capable of driving it at those RPM.

Light weight helps low speed handling and reduces airframe stress. Long thin wings are lower drag, but hard to build strong. something in teh middel with some taper is good. Fuselages should be as small in cross section as possible - just enough to smooth the airflow over e.g. the engine and tuned pipe exahust system. Undercarriages are pure drag - even retractable ones - so a hand launched model or dolly undercart is a good idea.

[Dick Huang]

Dancy,
The table below is my analysis of an 11x8 prop turning at 18000 RPM from 0 (static) to 140 mph.

Analysis of 11 x 8 at 18000 RPM

Airspeed Torque Power Needed Thrust Efficiency Inflow
(mph/kph) (oz.in/ N.m) (HP/watts) (Lbf/ N) (%)
--------- ------------ ---------- -------- ------ -------
0/ 0 91/10.3 4.1/3084 18.2/ 81 0 52/ 83**
10/ 16 92/10.4 4.2/3130 17.5/ 78 11 46/ 74
20/ 32 93/10.5 4.2/3164 16.8/ 74 21 41/ 65
30/ 48 94/10.6 4.3/3183 16.0/ 71 30 36/ 57
40/ 64 94/10.6 4.3/3186 15.2/ 68 38 31/ 50
50/ 80 93/10.6 4.3/3172 14.4/ 64 45 27/ 44
60/ 96 92/10.4 4.2/3137 13.5/ 60 51 24/ 38
70/112 91/10.3 4.1/3080 12.6/ 56 57 21/ 33
80/128 88/10.0 4.0/3000 11.6/ 52 62 18/ 28
90/144 85/ 9.6 3.9/2893 10.6/ 47 66 15/ 24
100/160 81/ 9.2 3.7/2759 9.6/ 43 69 13/ 20
110/176 77/ 8.6 3.5/2596 8.5/ 38 71 10/ 17
120/192 71/ 8.0 3.2/2403 7.3/ 33 73 8/ 13
130/208 64/ 7.3 2.9/2177 6.1/ 27 73 7/ 11
140/224 57/ 6.4 2.6/1922 4.9/ 22 71 5/ 8

** indicates blades probably stalled at this speed

If you do have an engine that is putting out 4.1hp static it will do what you want. To go 130 mph the total airplane drag must equal to 2.9 lbsor less; to go 140 mph the total drag should be 2.6 lbs or less.
Hope this helps.

Dick Huang






















:) :)

[Dick Huang]

Dancy,
The table did not come out very clearly so here is a second try.
Dick Huang:)

[JMP_blackfoot]

Hello,

Maybe you will be interested in our experience of setting a French straight line speed record of 328 km/h (204 mph) back in 1992?
The engine we chose was the OS61 Marine with tuned pipe, which was converted to aircooled.
The best power is around 23,000 rpm, which can be attained in flight with a 200 x 300 mm (8 x 12) prop of my design. Ground rpm is about 18,000.
The model is 1290 mm. span (51 in.) and weighs 1.9 kg. (4.2 pounds) The airfoils used are NACA 63A110 (root) and 63A108 (tip).
This gives the model very good accelaration capabilities when the nose is put down, yet it is exceptionally stable in flight, requiring minimal control from the pilot. It is also a very good glider (glide speed about 100 km/h - 60 mph-) with very gentle behaviour during landing approach.
The important features of the model are :
Lighness
High L/D ratio at low lift
Low Drag
Stiffness (beware of control surface flutter!)
KISS !!!
The FAI rules for straight line records specify that the model must fly between 10 and 40 meter altitude, for 100 meters before entering the 200 meter speed trap.
If allowed to dive into the speed measurement zone, much higher speeds can be attained. Our model can fly over 410 km/h (255 mph) in such conditions
Good luck.

Jean-Marie Piednoir

[Dancy]

Thank you all again;
Dick, how did you perform the analysis on the prop? I also do not see the relationship between the values in the table and the speed of the aircraft. Your table lists 4.9 lbs of trust at 140 mph but you predict that the drag must not exceed 2.6 lbs. If as Ollie says that top speed is achieved when trust equals drag then the plane should still be accelerating at that point. Also what is the significant of the value "inflow"? Seems like it would be the relative speed the air is being drawn into the propeller there for the inflow plus the air speed should be greater than the air speed by the current efficiency the propeller is working at. I have missed something here.

As to calculating drag. Seems like you could calculate some sort of reasonable value based on the frontal area of an object but experience tells me that a one inch round ball will produce less drag than a cube with the same frontal area as the two objects move through the air. There must be some way to predict this mathematically.

Jean;
Fascinating! Are there any online pictures and reports of your attempt? I can only imagine what it must have been like to fly a plane at that speed. I have "flown" that fast only using the RealFlight simulator and it is very difficult to control the craft as things happen so quickly.

[Ollie]

Dancy,
It takes a net force to cause acceleration (F=ma). When thrust equals drag, in horizontal flight, the net force is zero and there is no further acceleration, hence the top speed.

[Dick Huang]

Dancy,
"Dick, how did you perform the analysis on the prop? "
Basically using the relationship:
In flight thrust,T = [550*Eta*BHP]/Vfps, T in lbs and Eta=prop eff.
I am sending you the program in zip format.
will follow with two more files on drag calculation.
BTW,if the drag is only 2.6 lbs and the thrust is 4.9 lbs your plane will go faster than 140 mph. You need to plot thrust and drag as a function of speed to find Vmax.
Dick Huang:)

[Dick Huang]

Dancy,
More on Drag.
Dick

[Dick Huang]

Dancy,
More more on Drag.
Dick

[Dick Huang]

Dancy,
I just had another thought. If you have Elec-Calc the thrust/drag graphics looks just like the thrust required (drag) curve I sent you.
You may be able to docter this program to provide you the drag info your looking for.
Dick Huang:)
PS the inflow came with the program; I don't use it for any thing!

[Bill Mixon]

I'm confused.

Why a Katana for a speed competition?
You could build or buy lots of other planes that are suited to speed vs. the Katana which is an aerobatic design.

Bill

[BMatthews]

Bill, this is exactly what I was thinking in my post about this being too "fat" for fast flying. And now that I think of it the large and light surfaces of such an aerobatic model are going to be that much more prone to flutter at the sort of speeds that are being considered.

Dancy, you are just barking up the wrong tree with trying to make this model fly at the sort of speeds you want to see.

[Dancy]

Bill & Bmatthews;
Of course you are correct. This is not the ideal speed plane, far from it. It will spend most of its life with closer to normal aerobatic configuration. This is more of an experiment for the advancement of my knowledge. I have been around control line speed planes quite allot in my shady past! I know in practice what must be done to attain speed through experimentation. I am attempting to "predict" the speed of a known configuration then test the results to confirm the endeavor. Hence my approach. At what point would this plane be grossly over powered? What speed would cause structural failure? (If I can plot drag against speed I could see the pound of force applied to the airframe). Etc. Does that make more cense? You see I am just an old dog learning new tricks!

[Ollie]

The only way to make an aerobatic model go fast is to over power it. Aerobatic models must be draggy to prevent excessive acceleration on the down line. They have draggy engine cowls, exposed control linkages, fat fuselages, draggy airfoils and fat wheels. Any streamlining is for stylish looks not for functional drag reduction. The aerobatic performance would be better without the wheel pants and cockpit canopy.

[vintage1]

As far as structural falure goes, if it doesn't get a bad case of flutter, its really a question of how tight radius turns you pull.

Smeone somewhere recently ent down teh route of calculating at what point a model would in fact stall at flat out top speed due to simply pulling so many G's that the wing could no generate the lift. That would seem to be the maxiumum G the wing would ever and could ever see - basically pulling out from a high speed dive. If its stressed fopr that, it won't break. Needless to say that is a LOT of G on the average model. Probably in excess of 20.

[Dick Huang]

vintage1and Dancy,

The formula that I use for maximum load factor in general is:
(Vmax/Vstall)^2 in g's
using 130 mph and 20 mph the max g = 42.25!

Dancy what do you think of the drag calculation method that I posted? Lots of work huh!

Dick Huang:D

[Dancy]

Yes Dick it is! but that is just what I was looking for. Something that may be accurate is going to entail some serious detail work. 42g's huh....lets see ten pound plane, balsa wing, 400 some odd pounds of force........... SNAP! Ah hah! Lets not go for the high speed stall!