Why the Speedboy Patriot went 170 mph
 

OK, I have too much time on my hands but someone had to figure out this pitch x rpm stuff.

Here is how this Patriot made it to 170 mph in a dive with a 10x8 and why I predict 201 mph in a dive with a 9x10 prop. The theoretical speed considering pitch and rpm must also consider the propeller airfoil and airframe drag to be accurate. Since the propeller airfoil and the airframe drag is usually ignored and counteract each other, the straight pitch x rpm is often not to far off. As we try to go faster we use sleeker airframes to over come the ever increasing drag with speed. This often keeps the simple pitch x rpm some what accurate in the speed range our models fly. Put the same prop and engine on a 60 trainer and we see the formula is not even close.

To calculate speeds I used information from http://www.apcprop.com/ and http://www.grc.nasa.gov/WWW/K-12/airplane/foil2.html . I think I can trust APC props and NASA. I would like to start with a quote from the APC site that says

“The dominant basis for the primary airfoil shape used in most APC propellers is similar to the NACA 4412 and Clark-Y airfoils, except the leading edge is somewhat lower. Also, the aft region is somewhat thicker. This alters the zero-lift angle by approximately one degree and provides greater lift without having to twist the blade even more.”

The NACA 4412 airfoil has a camber of 4% and if we put that in the foil simulator on the NASA site we can see that it has a lift factor equal to an AOA of about 4 degrees. We then add 1 degree because of the altered zero-lift angle as stated on the APC site. We then have an airfoil lift factor equivalent to 5 degrees pitch. A 10 x 8 prop is twisted 15 degrees at the tip to give it an 8” pitch. Therefore, because of the airfoil shape, we have a pitch equivalent to 1.33 times 8 (15 degrees for pitch and 5 degrees for airfoil) or a 10 x 10.64 prop.

The Rossi 53 is not timed for high rpm and gets a static 15,500 rpm with a 10 x 8. A typical unloading of about 1500 rpm would take it to 17,000 rpm. After that the engine timing and the pipe would act as a governor and not allow much more, even in a dive. At this point the prop would act as a huge break if the plane tried to go faster. Calculating a theoretical speed using 17,000 rpm, using our equivalent prop size of 10 x 10.64 and not losing anything to drag because of the steep dive you come up with, you guessed it, 171 mph. Now where have we heard that # before?

Do the same thing with a 9x10 prop and we come up with an equivalent pitch of 12.5. The same rpm is used because of engine and pipe timing and no loss to drag because of the dive. The speed will be 201 mph if flutter does not take it out. It is interesting to note that the actual speed of 133 mph in level flight would be an actual prop efficiency of 78%.

No arbitrary views but any other thoughts are welcome if you made it this far

RE: Why the Speedboy Patriot went 170 mph
 

Thanks Mike, that was clear and made a lot of sense. The braking effect of a spinning prop can be powerful. I've notice on some planes that if the prop is not turning, that the plane actually lands hotter vs. a running engine at low idle. I wonder if the prop not turning would have less braking/drag (air stalled) in a dive than a spinning prop?
Just try to stop an engine from wind-milling at those speed. :)

RE: Why the Speedboy Patriot went 170 mph
 

Then we are in the electric hot liners region, climb to altitude fold the prop and zoom down.

RE: Why the Speedboy Patriot went 170 mph
 

With a few thousand hours in single and multi engine propeller aircraft I did not see a stopped propeller in flight unless it was feathered. I did have two props (one twin and one single) quit turning during the landing roll out. however, someone else has answered you question.
Either the stopped prop or the windmilling prop could have more drag. It depends on pitch, prop length and speed. For more info and graphs visit.

http://www.goshen.edu/physics/Propel...pellerDrag.htm

RE: Why the Speedboy Patriot went 170 mph
 

More then most think. During multi engine training there was a huge difference when you went from windmill to feather

RE: Why the Speedboy Patriot went 170 mph
 

Good information on the APC airfoils and zero thrust angles on the APC props Mike. However, I’m afraid that just a change in prop to 9x10 will not achieve 200 mph even if the airplane dove straight down from altitude. Just not enough power available from the engine and conversion of the aircraft’s potential energy. However if another 2.5 lbs. of weight is added to the aircraft (estimated now at 6 lbs.), you might be able to reach 200 mph briefly. The pullout from a vertical dive had better start before you reach 150 feet AGL if you want to keep the g loading below 20 g’s.

You need to do a little more research on the concept of prop efficiency. I’ve seen it referred to on RCU quite often, and almost all have it wrong.

RE: Why the Speedboy Patriot went 170 mph
 

My first post was a combination of facts and theory so maybe I should have put in a disclaimer. You must remember that I used the word "predict 201 mph" and not will go 201 mph so that should have given it away. There is no way to know all the exact facts but I still predict the engine power and gravity will allow up to but not much in excess of 200 mph with a 9x10 prop. Time will tell.

I was a pilot and not an aeronautical engineer so I do not have all the answers. I do try to back up what I say by explaining my reasoning and point to reputable web sites that support me. I was hoping to learn something in this thread but those with more knowledge then me could elaborate on why I am wrong and not just tell me to go do more research. At least point me to a web site that contradicts me. There is usually a difference between theory and reality so the real test will be the radar and Doppler #'s.

RE: Why the Speedboy Patriot went 170 mph
 

I think that gliders have proven that you do not need a strong engine or weight to reach 200 mph.

RE: Why the Speedboy Patriot went 170 mph
 

A pretty good explaination of prop efficiency was made in this thread by Oryx.

http://www.rcuniverse.com/forum/m_43...tm.htm#4345826

For Speedboy's Patriot to reach the vaunted 200 mph, it will require around 6 1/2 hp. Since the engine is not going to provide that amount, gravity has to provide the rest. Since you know the terminal velocity goal, it is a simple matter to determine the weight required.

And gliders do use ballast to gain speed.

RE: Why the Speedboy Patriot went 170 mph
 

One of the first model aviation guide books I ever cracked open claimed that some C/L speed plane combos acheived 100% prop efficiency, if not better.

No doubt that weight = power in a dive. A heavy enough plane can push its' engine well beyond any rpm that it could reach on its' own. Reminds me of the time I landed [a very tail heavy] plane after a nice power dive, with just a short stub of connecting rod hanging out of a ripped open crankcase.

It was a G-21 SuperTigre .35

RE: Why the Speedboy Patriot went 170 mph
 

I guess you need to define efficiency here. A 0 pitch prop could fly a plane because of the airfoil shape. Is that exceeding 100% efficiency? I think a dive would be cheating. Other then that we would have perpetual motion and I would like a piece of that pie.
Now let me think about HighPlains and terminal velocity.

RE: Why the Speedboy Patriot went 170 mph
 

Your thread link was interesting but much of it is a little over my head with out some serious effort. What I got out of it is that there is a lot of theory in propeller designing and the "proof is in the pudding".

help me out with this terminal velocity of a Patriot. Maybe it is me in the land of Oz.
Here are my #'s and I think they are very conservative in favor of slow speed.
6 lbs - aircraft weight
.75 sq ft - cross section - my delta is .25 so I tripled that
.08 drag Coefficient - With retracts it is fairly clean but I still almost doubled a typical wing drag coefficient.
1740' altitude - Monterrey Mexico
Terminal velocity is 297 ft sec or 202 mph
Where am I going wrong?

RE: Why the Speedboy Patriot went 170 mph
 

I find it ironic that my prop efficiency #'s based on theoretical pitch slippage was so close to your (claimed and widely agreed upon) typical prop efficiency of 80% considering other information.

Edit - added bold words

RE: Why the Speedboy Patriot went 170 mph
 

This is true. A terrific amount of study was done on the subject, mostly in the 30’s and 40’s but a limited amount still goes on even today. Yet the result of all of it is efficiency in the range of 80-85%. The prop designer has a great number of tradeoffs to make. Pitch and diameter most are aware of, but toss into the mix the pitch curve (actual pitch of the prop from the hub to the tip), airfoil selection, and blade width. Other requirements have to do with the distribution of mass in the blades relative to the plane of rotation, modes of vibration and stiffness, and material strength.

Usually the airfoil is picked more for strength towards the hub, while the airfoils on the outer portions of the blade does most of the work. With most props washed out at the hub (lowered pitch), sport and racing props differ at the tips. Many sport props are also washed out at the tips, while racing props typically have the highest pitch at the tips. For a sport prop, this washout at the tip gives better acceleration on takeoffs and more rpm in the air.

So if we assume that any well chosen, well made prop has roughly the same efficiency, say 82%, then selecting a prop comes down to matching it to both airframe and power available from the engine. This means finding the optimum size to allow the engine to unload to a point where it makes the maximum power. Fortunately, this peak power rpm is usually fairly flat over a narrow rpm range. Generally, once you get very close, then changes of a couple tenth’s of inch in pitch or diameter is about all you can make, and atmospheric conditions start to dominate which prop over a very limited range will work best.

Knowing the airspeed that a certain power output achieves with a well matched prop then allows calculation of how much power is required to achieve another speed. Simply stated, power required is a simple cubic function, so doubling the power only increases speed by 26%. So when a 6 lb. airplane flys level at 135 mph while hitting 170 at a 45 degree dive, we can calculate how much power is available from both gravity and the engine. It turns out in this single case, that engine power (82%) and gravity power are roughly equal. With the airplane diving it moving at 250 feet/sec, and the downward component of this is (sine 45) x 250 ft/sec, or 177 feet of altitude per second. Multiply this by 6 lbs. and you have 1060 ft^2-lbs/sec. Divide that by 550 ft^2-lb/sec and you have 1.9 hp added by the dive. Since this is roughly equal to what the engine is putting out, we figure the engine power by 1.9/0.82 (prop efficiency) and end up with an engine output power of 2.3 hp.

Now to see if 200 mph is possible when in the 6 lb configuration, we need to increase the available power by 63% over what was available at 170. This works out to 6.2 hp (my bad earlier, I used 2 hp from the engine in a rough estimate). Subtracting the 1.9 from 6.2 means that gravity has to provide 4.3 hp. This is 2365 ft^2-lbs/sec, and 200 mph is 293.3 ft/sec. So dividing out gives 8.06 lbs., thus at 6 lbs, the airplane will not be able to reach 200 mph regardless what propeller is used.

RE: Why the Speedboy Patriot went 170 mph
 

There are a few model airplane references out there with BS like that between the covers.

100% efficiency? I think not! What is missing is the definition of that pitch number and how it relates to the airfoil used.

It seems most of the hoopla over the airspeed versus rpm and pitch has to do with the oversimplified definition of pitch used by (most or all?) model prop manufacturers. Pitch as listed is typically not measured from the airfoil's zero lift angle but from the chord line or a datum line parallel to that. Cambered airfoils generally have a zero lift angle less than 0 degrees AOA. Therefore a "zero pitch" prop with a cambered airfoil (versus non-cambered or "symmetrical") will still produce forward thrust when rotating, since it's true pitch is still positive. Therefore, one manufacturer's 6" pitch prop may have a different true pitch than a 6" pitch prop from another manufacturer. Pitch numbers are certainly important, but if the manufacturers use different airfoils, it is possible to be mislead by comparing the numbers directly.

RE: Why the Speedboy Patriot went 170 mph
 

I think it is a mistake using 170 mph at a 45 degree dive to calculate the power from gravity. At that point my calculations show the prop is acting as a break and the real speed from gravity is unknown. I also think it is a mistake trying to use HP to calculate speed in this case.

The conservative #'s in my earlier post show a terminal velocity of of just over 200 mph. Forget starting the engine and take the prop off and drop it from a high altitude. I am not sure why you want to complicate terminal velocity with engine power.

We know the engine is turning the 10x8 prop at 15,500 rpm static and can make an educated guess that the unload rpm is around 17,000. Trying to incorporate HP ratings into these facts only complicates things and could hurt accuracy. Engine and pipe timing should keep it from going much faster even though the 9x10 prop has less load using widely accepted methods to calculate prop load. Using the adjusted pitch #'s, 12.5" because of the props airfoil shape, and 17,000 rpm we come up with just over 200 mph. Considering the terminal velocity speed and the gravity assisted dive there is no reason to not achieve 0 slippage with the prop.

My prediction stays at 201 mph. Now if Speedboy would just do his thing to prove me right or wrong. :D

RE: Why the Speedboy Patriot went 170 mph
 

I just had a thought about timing/pipe "holding back" the r.p.m.
Just wondering...
If an angine looses it's prop, and you have a shaft run, can't the engine's r.p.m. go waaaayyyy beyond it's "normal" range?

I'm thinking if you put a fairly clean airplane like the Patriot into a WFO screaming full throttle dive, the engine isn't really gonna hold things back much :D and the r.p.m. could go well beyond what could be considered the "normal" operating range. (no matter what the timing #'s are or the pipe "says";))

I'm not talking astronomical r.p.m. figures now...but maybe 20K or thereabouts?

Whadoyathink?

RE: Why the Speedboy Patriot went 170 mph
 

you bring up an interesting point. What is the difference in a shaft run rpm and the rpm of an airplane capable of a high terminal velocity pointed straight down? I have seen evidence that there is a difference but can not explain why a zero load opens up another can of worms. Maybe one of the engine gurus can help us out. As Combatpigg stated earlier, enough weight in a dive will push an engine past it's normal timing limits.

RE: Why the Speedboy Patriot went 170 mph
 

FWIW, CP's got an interesting thread going in the 1/2A forum...he's checking the timing figures of several engines.


I'm just wondering now if maybe it's the carb and/or the size of the venturi that would limit the r.p.m. in a dive? (pumping losses or words to that effect? )...the engine couldn't suck in enough air to allow it to spin any faster? That in conjunction with the timing.

I think that's what keeps some engines from blowing when they have a shaft run...the venturi size is such that it gets to a certain r.p.m. then chokes. (I know there are other considerations such as cross sectional area of the port runners, runner saturation, crank's I. passage, the pipe, etc. etc. )

Just out of curiosity, does anyone have the timing #'s for the Rossi .53? Being a Rossi, I don't think it's outside the realm of possability that it could turn to 20K (give or take a little ) in a prolonged WOT dive and not fly apart?

Maybe Speedboy could do another video, and do some dive tests...gradually increasing the angle of the dive, and you could do the analysis of the r.p.m. (right now, I'm picturing Clark Gable in "Test Pilot" or maybe Fred McMurray in "Dive Bomber" :D )

RE: Why the Speedboy Patriot went 170 mph
 

From another post.

The Rossi 53 has very mild exhaust timing. I also used a degree wheel to document the timings (by the way, TDC is 0, BDC is 180 on a 360 degree wheel). Here are the results.

Exhaust timing 160 (open at 100 closes at 260) changed this to 170 (open at 95 closes at 265) I opened up the top center middle third of exhaust port - rounded.
Intake timing 134 (open at 113 closes at 247) left this alone
Crank intake 208 (open at 212 closes at 60)
Blowdown 13 degrees, after exhaust timing change it increased to 18 degrees


Since the exhaust mod alters the compression ratio, I decided to document this as well. I used the timings to position the piston in the cylinder and measured the before and after compression strokes. I also used a buret to measure the volume of the combustion chamber. Here are the specifics on that:

Button volume (combustion chamber) .5 CC
Bore size .876
stock compression stroke .585
new compression stroke .545
shim .005
quench area (area between the piston and the button at TDC) .018

I put all of this data into a compression calculator and this is what I got:

Stock compression 8.94:1
New compression with shim installed 8.4:1
New compression with no shim installed 8.94:1

I opted for the no shim installed configuration since it put the compression about where it was before the exhaust timing mod. I also changed the glow plug to an OS #8 and kept the 5% nitro 18% oil Power Master fuel. I decided to see just how fast the motor would spin after the mods, so I installed a small (sorry no markings) prop and tached it at 22,500 RPM

I then installed the 10x7 and tached it at 16,300 RPM . Much better! ThrustHP calculates HP at 2.165, not 2.3 but I can live with that number for now. I wonder how much a full pipe will help?

Rick

< Message edited by 5.0Rick -- 12/8/2007 12:39:30 AM >

RE: Why the Speedboy Patriot went 170 mph
 

Hi Mike......

Great job starting this thread, and with the initial research.

Makes my brain hurt thinking about it. :) Reminds me of many years ago when I studied some of this.

The important thing that should be learned here by others (despite debate on the full details and physics) is that the prop does work, and that it is not just a flat piece of wood/plastic/other that pushes with the back side of the blade. And more importantly, the engine/prop/airframe work together as a system to create the performance of the "aircraft".

There are litterally thousdands of pages of actual prop test data in the old NACA and NASA archives. Most of it is boring, and good examples of what "not" to do. But at least someone tried it once :) Yet a good deal of it was application specific.... and of good value for others seeking to proppel similar applications/speeds/loads.

As for the pipe and a shaft run..... the engine will accelerate until it finds another "node" it is comfortable with. Just like throttling up or down... you can see where the harmonics are sometimes.

The ultimate fun thing is an in-flight adjustable pipe (been tried.... its not easy) ..... or taking time to design one that will multi-stage (the QM engines sorta do this) jumping from one harmonic to a higher frequency harmonic when the engine unloads.

RE: Why the Speedboy Patriot went 170 mph
 

I remember a few post from Bob recommending a low load prop on a particular Jett engine and magic will happen. What I remember is the low load let the engine jump to the next harmonic tuning level and add a couple thousand rpm. This may be what is happening during a shaft run and the engine skips through the harmonic levels until something gives. If this is the case then it would only make sense that the same thing would happen in a full power dive IF the terminal velocity was high enough.

It would be interesting to put a 9x6 on a normally timed Rossi 53 and see what the rpm's would be. If that light load would let it go to the next level up in rpm then a high speed dive should take it past my prediction of 17,000 rpm. I doubt the engine would explode until well past 20,000 rpm.

RE: Why the Speedboy Patriot went 170 mph
 

Well , unfortunately I do not have near the experience of you speaking to modified engines or making equation matts to obtain a result approximated to the reality [>:] but......... what if I can do is to put an 9x10 in my engine , go to the field , fly it , take video and show it you :D :D:D

Mike :
your are making a exelente work, to explain the reason by as arrive at that speed model, for my he has been very util the data that you have contributed, and those of dems also, I am going follow this treads very close and trying to learn each one of tips that you mention here (all include ), thanks for take part of your time in this post.


Mike :
less than I can do for you is to prove 9x10 and say that it says the radar and Doppler of course, consider a fact the new video ;)

I dont know how much can increase the speed with 9x10 , but we for a moment suppose that arrives at 201 mph , can I still have control surfaces at that speed ??? I mean which is the hi-speed stall of a patriot ??? I dont want lose my "prieto" on one dive [:o]

By the way the weight of my patriot is 6.83 lb. (3.1kg) with pipe .

RE: Why the Speedboy Patriot went 170 mph
 

Generally racing 40’s don’t explode until around 40K. Then the bottom end of the rod feeds the bushing material though the rest of the engine and out the exhaust. This can cause the engine to seize and the rotational energy to transfer to the case in a rapid and extremely violent manner.

My prediction for top speed in a terminal dive is 185 mph. This based on the data provided: speed in level flight, speed at a 45 degree dive, and 6 pound flying weight.

RE: Why the Speedboy Patriot went 170 mph
 

Speedboy, don't forget to bring 4 pounds of lead to the field for the "big run", then figure out a way to dump the weight before landing.