Kweasel

My Feedback: (29)

I remember running a 1st generation YS120 on a small .60 size pattern plane nearly 20 years ago. It certainly wasn't slow.

RCER88

My Feedback: (25)

I don't know where those numbers come from. To me it is irrelevant. Because we do it all the time and have been doing it for years at the field. Radar gun verified.

I do not believe "computer programs" and their projections. Garbage in, garbage out comes to mind. I doubt that they have everything in there. Air density, barometric pressure, humidity, altitude., tempature.

I will take the plane out and fly it. THAT is how you know for sure.

I hate adding weight. A four stroke IMHO is added weight when a two stroke is lighter. Weight affects vertical perfomance and speed by creating additional drag.

Airframe design has just as much to with speed as power. a swept wing will fly faster than a constand chord with all other things the same.

MJD

My Feedback: (1)

Naturally - a given airframe requries x amount of horsepower to achieve a given speed. WIthout that power available you won't make it. Both are vital factors.

Are you talking about swept wings, or tapered wings - this is apples and oranges. You can have swept constant chord wings, or tapered non-swept wings. A tapered wing is generally capable of flying faster since it works more efficiently due to better lift distribution and smaller tip losses. But swept wings are of little value for drag reduction in subsonic aircraft.

The top RC speed contenders in Europe generally use tapered non-swept wings, or pseudo-elliptical profiles (crescent shape, that sort of thing) since these offer the efficiencies that elliptical lift distribution offers.

A pitch speed calculation is very useful in targeting combinations for an application. The thing to take into consideration is that number on a prop is (a) rounded off, and (b) not always aerodynamic pitch, but often datum line pitch - meaning that the true pitch may be a few percent higher. So it is often observed, esp on some common APC props like the 10-7 for instance, that mdoels achieve a bit higher than basic math would indicate. Generally this is due to the pitch issue mentioned, along with engine unloading.

It is not a waste of time running some basic numbers up front to get an idea of what to do. The garbage in part of GIGO is entirely up to the user. If you are trying to achieve 150mph and have an airframe/powerplant combination with the combination of available power and streamlining to do it, then you are wasting time with the wrong prop combination - some thought up front to target possible combinations is useful. If I was trying to get one of my deltas to crack 150 with a Jett .50 (easy to do) that likes to run at 18-19k in the air, then why would I waste my time with 6" pitch props? It is pointless. A pitch speed calculator tells me so, and real-life experience verifies it. Instead, armed with the knowledge that I need a certain pitch value to even get close, I start with prop combinations that make sense. Then - yes indeed - that is where the proof is in the pudding. IOW, fly it and see.

To wholesale reject the notion of doing some basic math up front is the same as saying "I have no clue what to do so I'll fiddle around until it goes faster". That may eventually work, but there are better ways to approach the task. If I were to use that approach it would take about 17 flights on the aforementioned delta/engine example before I got to a point where I thought I was getting somewhere. Or I could tear around all day at 110-115 on a 9-6 and think I was doing great, and man look at that vertical performance and static thrust, and never realize I had another 40-50 mph to go.

Also at high speed, the lift coefficients are so low that although weight always plays a factor in induced drag (the drag generated by production of lift), the other drag factors achieve higher significance. It is generally agreed amongst the speed jockeys that weight should be minimized (as always) but not obsessively so, whereas one must pay very careful attention to drag reduction. Drag goes up with speed squared, as does lift, so you can see how the relative significance applies as speed increases.

In the end, use whatever method works best for you.

airraptor

My Feedback: (66)

pitch speed calcs are wrong. i have never used one but every time i post a plane with certain speed with rpm and pitch on the ground lol guys come on here and say it cant go that fast with that prop lol all i say is lets race. all my speeds are with GPS.

weight has a big factor in getting the most out of a plane. the lower the AOA the plane has the faster. Thats why i dont think that new Jackal will be very fast 8.5 lbs on 500 sq inch wing........ ever noticed how your plane is faster as fuel burns off???????????


dont get the saito 100 go ahead and get the 125 on the 60 revolver put a 14x10 prop on it with 35-45% nitro and should be in the 115 range. i dont know about that 60 mustang but i do know that minnow needs a four to go faster unless run a DF engine. you can run the 13x11 full blade apc to the 13x13N on the 125. the 13x11 spins just over 11,000 on 35% this is great on lower drag planes the good thing is that below 120 flight speed can set the engine near peak rpm because it doesnt unload much with this prop.


no matter the engine if it runs good on the ground and you take off and get going the engine richens up then to much pitch with the close or over sq props. these props will be stalled on the ground and the then unstall in the air and put more of a load on the engine.


for the smaller engine like the 63 and 70 you could get a APC11x14 prop take down to a 9" prop clean it up and run on some small warbirds and get in the 150 range.....as long as its very very clean

ton2di

My Feedback: (182)

Airraptor,

I have a Top Flite 60 size p-51 running on 14x12N Apc prop with an OS 120FSIII. It turns around 8700 on the ground with 15% nitro. Do you think a 13x13N would make it faster?

thanks,
Ton2di

MJD

My Feedback: (1)

I agree that there is often some disagreement. That's what I am getting at about pitch.. if you fly with say, an APC 10-7, you'll get one number, but it's been shown many times that this prop has speed potential beyond what the simple math would suggest. Again, likely due to "7" being a round number and likely datum line pitch, meaning that the zero lift angle of the prop blade is higher than the simple pitch number would suggest. IOW, maybe "8.2" is more accurate.

I am not suggesting pitch calcs are the ultimate in speed predictors. No way. But.. they are an excellent tool to get you in the ballpark, assuming you have some idea of the speed potential of an airplane and engine combination. You gotta start somwhere.

They are not so wrong that if the numbers say an 11-4 can only go 50mph at a given rpm, that it can magically go 110mph. They are a tool, and tools rely on the operator to make some form of judgement. So yes, GIGO is correct, but the GI part is still up to the guy using the calculator and interpreting the results. As an example, I tach an engine on a 9-7 and a quick calc tells me the pitch speed is 110mph. But, in the air the aircraft only reaches 70. I would interpret that to mean that the prop is overpitched for the engine/aircraft combo, and that by depitching and adding blade area, or bringing up rpm with the same blade area, I may get a more efficient system and top out higher. I guess I could figure that out by simply switching props and flying too.. but I'm not satisfied with that, I want to know more about what is going on.

airraptor

My Feedback: (66)

ton2di i think you would lose speed with that plane. its a large plane with a lot of drag. i am thinking of going the other direction of a 14x10 regular blade or a 15x10 cut down some. i dont know if apc makes one but I would see if they make a 15x9. try to prop for atleast 9,000 on the ground. alos if caan see about removing the muffler if you have it installed. The OS four strokes like some tank pressure so they way we do that is to take the vent line and install a large as will fit brass tubing into the vent line. Then have this mounted on the cowl near the prop or other high pressure area to provide just a bit of tank pressure.

sometimes going down in pitch and adding Dia will result in a faster plane. to go fast you need to more thrust than drag right so sometimes more pitch speed doesnt get that if low on HP and small props.


also for the little FS engines the OS 30 can turn up to 23,000 when change the valve springs and cam. you can also add a different carb. i modded mine with the springs,carb,intake, and valve spring from OS and it will tunr an APC 8x6 prop at 17,000 or so on 10% almost = to a old OS32 with muffler it is ten ounces so on those small 1/2A house of balsa kits might hit 120-130.... i am thinking of doing one by spring.

ton2di

My Feedback: (182)

Thanks for the advice Airraptor. I have my vent line just hanging somewhere on the cowl. I have tried the apc 14x10 already. I might try the 15x10 cut down a little as I have it already in hand. A friend's estimate is around 90mph with the current set up of 14x12N (8,700rpm on the ground) at level flight. SO maybe it's unloading a little more in the air to get to that speed.

TimT2000

My Feedback: (1)

<div style="margin: 0in 0in 0pt"><span style="font-size: 9pt">Pitch x RPM / 1056 = prop air speed

This will get you real close to your actual air speed.

The engine will unload in the air giving higher RPM's so this
will need to added.

The dopler method is very good for getting air speed. Get an
audio file of your plane on a fly by close to you. Then use a frequency
counter to get the approach freq. and the departure freq. and calculate.
Very accurate.

Gps's and radar's have there place if you know their limitations. Gps's are not
great when flying around in little circles. If they just remember the peak mph, that
number will often be wrong. A good radar lock can be very accurate, just make sure
it is repeatable and not a one time reading.

The numbers never lie,, math and science are the reason most of us are still alive.
I really get pissed at people that say "computers, technology, math, and science are the root
of all evil" Then the first time they get a chest pain, they run straight to the hospital to get a dose
of math and science.

Okay I am done, Cheers Tim</span></div>

jeffie8696

Tim can you tell me, Ihave been wondering how accurate the doppler method can be if the speed changes during the sampling period. Just curious.

TimT2000

My Feedback: (1)

Great point! yes it matters a lot. To make this work you do need a close level pass at a constant speed.
If the pass has varying speed you can not just take the high and low frequencies as this will give you a higher
airspeed than was achieved.

Thanks for the info,, I had not considered that problem.

cheers Tim

RCER88

My Feedback: (25)

All I was saying is that a Sig Kougar less than five pounds with a Rossi 45 with a mini pipe 10x6 prop will fly around 110mph religiously flat and level. Not done on a computer. Done at the field in the air with a radar gun from the ground with a good lock normally done on three seperate passes each time to insure the accuracy and get a good average and done into the wind.

We have been doing it that way for years.

Sure using a computer to get numbers is fine if that is what you want to do. I have no problem with that. I have just seen speed calculations wrong more than they are right.

I would rather take a tach and bunch of props and test them in "real life" at the field. Then there are no questions. You have real data done with YOUR equipment. I get to fly the airplane and see how different props act in the air.

But that is me. I would rather be at the field flying than sitting in front of the computer trying to get data that will probably be inaccurate anyway.

But as with this hobby to each his own.

But telling me my plane won't do what it does is nonsense based on a computer data or a formula is not right.

TimT2000

My Feedback: (1)

<div style="margin: 0in 0in 0pt"><span style="font-size: 9pt">Fair enough, I too fly a lot and that is what it is all about. Thanks for the conversation.

You have a great Rossi 45, I have had two of the original .60's and they are just perfect.

I know you don't like numbers but that 45 is doing 19,500 rpm on your 110 mph pass, very
respectable I might add. Do you think you could get a four stroke to go that fast in that plane?

Might be fun to try! As for the sound of the two, I can't decide, Love them both. If I might steal
a signature I saw on here,,"I have never met an engine I did not like".

Cheers Tim</span></div>

airraptor

My Feedback: (66)

rc88 i am with you. these dopp readings with just sound seem way off to me, also the pitch speed calcs are off aswell. I think gps is the best for giving ground speed. the speed meters with pitot tubes can be usefull but again they are flawed in that if dive straight down then pull into the wind could have a false reading. So many people will dive then pull out right at the radar gun and say i have the fastest what ever lol.

timT if you like messing with numbers and all thats fine too we are all different and thats what makes this hobby fun we are all trying to do our own thing and "tinker" in different ways.
we have three young guys at our field that are using a trainer and trying different power plants like adding electric DF under the wing, adding wings making it a low wing, putting gliders and all kinds of mods to it. most of this is pointless to me but they are having fun and its all over youtube.

blw

My Feedback: (3)

This is one area that can probably be ignored at first. The Reynold's number is unpredictable at the small scales our models use. It may be negligible for our purposes.

jeffie8696

Isnt it all about having fun? I love to mess around with powerplants , props, fuels etc but the next guy wants plug and play. As long as we are both having a good time what difference does it make? Neither one of us is going at his hobby wrong, just different.

Jezmo

I like jeffie's way of looking at it, if I'm not experimenting I not having much fun but for others at our field they just want to fly, clean up and go home and that's cool too. I'm sure most have heard this old saying but it sure seems to apply on our little monsters as well - "Speed costs, how fast do you want to go?"

freakingfast

My Feedback: (3)

Exactly!

blw

My Feedback: (3)

Eddy Hill was a member at one time on RCU. He was good about people asking about this dragster, that one, nitro, etc. I think his sig line was that you could never have enough nitro. He was into speed, of course.

mike early

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