still4given

I get asked all the time how fast my Warbird racers are flying. Yesterday I had my son film just short clips of two of them. I am hoping someone here who knows how to use the Dopler software can tell me how fast they are going in the clips.

Thanks, Terry
[link=http://smg.photobucket.com/albums/v357/still4given/?action=view&current=BHSpitfire.flv]BH Model Spitfire w/ YS45[/link]
[link=http://smg.photobucket.com/albums/v357/still4given/?action=view&current=YS110Strega.flv]PM Strega w/YS1.10[/link]

Mike Connor

My Feedback: (5)

I was unable to get good Doppler lines for a speed check. Part of the problem may be the camera positioned where the aircraft started it's turn as it went by. A low and close pass without turns works best. Maybe someone else extract better info from the file.

still4given

Hi Mike.

I will try and get better vids next weekend. I don't think my son realized what I needed. That, plus the fact that I had a bunch of guys flying 3D to my right causing me to need to turn just past me. I didn't think of that until I got home to DL the video from my camera. If you are willing to run them for me I will try and get a good video for each of my Racers.

Thanks, Terry

wjvail

My Feedback: (3)

What prop and rpm are you running?

Bill

still4given

I'm running an APC 13x10 on the YS 1.10. Getting around 12k on the ground. The YS .45 turns an APC 10x6 at about 16,5k on the ground.

wjvail

My Feedback: (3)

A 13x10 at 12,000 may unload to around 13,000 in the air. That is about 123 mph. You can add about 10% if she is going down hill and the plane is not super draggy (60 deg dive) for about 135 mph. You can add or subtract for a head/tail wind so a 5 mph tailwind would be about 140. Finally, you can add about 2% for each 1000 feet you are above sea level. So if you're at, say 2000', you can add another 5 mph for about 145 mph.

A 10x6 at 16,500 on the ground may unload to 18,000 in the air. (The muffler/pipe plays a big part here). 18,000 with a 6" pitch prop is about 102 mph. Adding 10% again for a dive, gets you to around 112 mph. Add or subtract for the wind and altitude as you see fit.

These are just a wag and not a substitute for a radar gun but over the years I've found them to be pretty close.

Bill

still4given

That's pretty cool, It will be interesting how close those numbers are. I will try to get better wav files next time.

Blessings, Terry

airraptor

My Feedback: (66)

you cant go off pitch speed for your plane. I have one plane with a pitch speed of 150 or so and it does an easy 185 straight and level.

Plus allot of engine dont unload in the air in fact some bog down even more once in the air which is good sometimes. that 13x10 is stalled on the ground and gets working in the air. put a telemetry on the engine and run in the air to see REALLY how much the engine is unloading. most i have seen on sport engine turning sport props like 10x6 and so on only unload about 20-500 rpm. the higher the pitch the less unloading.

some engine will "unload" in the air because they are not "on" the pipe during take off. this has been where i seen the most unloading in the air.

Mike Connor

My Feedback: (5)

And the real answer is....
Well, maybe we will know next week end. If you can get a good recording Terry, we will know the aircraft speed and the unload rpm of each engine. All of the above make good arguments but do not always hold true depending of the variables.

wjvail

My Feedback: (3)

That's funny... Out of curiosity, what engine, prop and plane do you have that does an easy 185... 35 mph faster than the prop?



Errr....???...??? What? I don't even know where to start with this...

vicman

My Feedback: (10)

Sometimes it's best to leave the chemically abused ones alone

I've never seen anything go 180+ with ease. They all take a good bit of effort to hit 150.

wjvail

My Feedback: (3)

My dad always said, "Never get in a augment with an idiot. Someone watching may not know who is the idiot." I try and live by his advice but sometimes things come up that I just can't lay off. I have to comment.

airraptor

My Feedback: (66)

so you calling me an idiot?
the plane i have that does over 185 is an LR1 with a BSE 35 with full pipe and no gear thats 3.2 lbs dry.

And before you guys start comenting on stuff you seem to know very little about please do some research. It is possiable for an aircraft to fly faster than pitch speed. Props do have airfoils correct? Do airfoils contribute to lift????????? hint hint Also on this aircraft the engine might be unloading since its just on the pipe on the ground turning 21,500 on an 8x7 apc.

Have you every had an engine run perfect on the ground then richen up in the air? if so it could be the prop was stalled on the ground and then with the fwd airspeed it started "working" loaded up and lost some rpm.

Some aircraft engines will unload in the air and some will stay the same and some will load up even more.

look up some telemetry results on the net I am sure some one some where has posted some info.

Mike Connor

My Feedback: (5)

I started a thread sometime back exploring why an aircraft CAN outrun it's simple pitch speed.
Anyone interested can see it here.
http://www.rcuniverse.com/forum/m_68..._1/key_/tm.htm

The basic concept as stated in the thread is:

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.

Exceeding the pitch speed is common in slower aircraft but because of drag it is more difficult as speed increases.

As far as a prop being stalled during a static run up, it seems I remember that being a problem with the high pitch over squared props used by the German speed guys.

wjvail

My Feedback: (3)

Shipmate... I've been flying models for over 40 years and done more research than you can imagine. I've got a college degree in mathematic, flew airplanes for the Navy for 12 years and commercial aviation for another 12 years. I've got more time working on props and holding a radar gun than most have flying.

Still, I really don't think were are completely disagreeing. Not only do I agree that an plane can go faster than the prop, I factored it in! The calculation I gave to answer still4give's question was "calculated prop speed (accounting for the prop unloading in flight) x 110% + 2% for each 1000 feet of altitude +/- wind."

Using this calculation and your numbers of 21,500 static rpm with a 7 inch pitch prop we could calculate your planes speed. Being generous, say your engine comes on the pipe in flight to around 23,500. That is about 157 mph. Adding 10% is 173. If Fairfield is at 1,000 feet msl you can add 3 mph for 176. Now add whatever wind you'd like. Say 9 mph tail wind for 185 mph.

I very often radar aircraft going faster than their static pitch x rpm calculation would suggest. VERY rarely 10% more. As I originally stated, calculations will not replace a radar gun but they will prevent folks running a 10x6 on an OS .46 FX claiming they are going 200 mph.

Mike... I've read a considerable bit on this and a lot of what your referring to can be chalked up to how prop pitch is measured. This becomes increasingly obvious when using a flat bottomed airfoil with considerable modification to the leading edge... such as the Clark-Y.... as used in an APC.

In this case most prop measurements are taken from the flat part of the (flat-bottomed) airfoil but the mean camber line for the airfoil is actually at a considerably greater pitch. I've attached several (poor) photos of an APC 13x8 on a Prather pitch gage. To APC's credit, the prop measure VERY close to 8" pitch when measured on the flat bottom. I've also attached a few drawings of both a Clark-Y and NACA 4412 (the Clark-y is the top and the 4412 is the bottom). Notice that the chord line is NOT the flat bottom. The chord line is around 3.5 to 4 deg steeper than the bottom of the airfoil.

Using the chord line of an APC 13x8 I measure the pitch closer to 9 inches. That is why airplane can go faster than the pitch speed would suggest. A 13x8 is really closer to a 13x9. I've simplified this by adding 10% to the calculated top speed. Another way to do this would be to measure every props pitch using the chord line to get a more accurate idea of it's true pitch. Using this new pitch you could then calculate a new pitch speed and you would find it is almost impossible to go faster than the prop.

Measuring prop pitch was common before APC when wood was king. It was not uncommon for a Rev-Up or Top Flight to be off a ton (i.e. a 10x6 may measure as much as 10x7.5 or as little as 10x5) Planes often went fast than their rpm x pitch would suggest but it was usually due to the prop being misrepresented.

Bill

airraptor

My Feedback: (66)

my numbers were wrong lol. The engine was only turning 20,700 on the ground and the plane did 174 up wind and 191 downwind on the radar gun.

WJVAIL you keep saying the higher you are the faster the plane is this true? You would think with the air being thinner that the engine would make less power and the prop wouldnt "move" as much air either. It would be easier to pull the plane thru the air though.

wjvail

My Feedback: (3)

Mike... Just to continue.... Attached are a few better pictures of a Clark-y and NACA 4412 I generated. What is of interest is the difference between the chord line ( the straight line from the TE to the LE) and the bottom of the airfoil.

The above quote if from Wikpedia defining Angle of Attack. (I know, it's not exactly a textbook but it was the first reference I could get my hands on that defined AOA.) Notice AOA is defined as the angle of the chord line and relative wind. http://en.wikipedia.org/wiki/Angle_of_attack

The above is also from Wiki for the Clark-Y. The important point to note is the quote"... is flat on the lower surface from 30 percent of the chord back" suggesting the first 1/3 of the leading edge is NOT flat. Also notice the statement "the flat bottom simplifies angle measurements on propellers....".

It is the difference when measuring and marking propellers with reference to the bottom of the airfoil vise the chord line that leads to faster than calculated speeds.



One last thing, and you've already suggested this in other posts, but we are really only talking faster planes. Obviously planes go faster than their prop speed would suggest every day. The best example would be to take an Alpha trainer up to 1000' and kill the engine for a dead stick landing. In this case you will glide back to earth at 45 mph with a calculated prop speed of... ZERO! Obviously in this case the power source is gravity and altitude.

Basic aerodynamics says thrust MUST EQUAL drag. Thrust is the sum of engine provided force plus gravity provided force (zero in level flight). The thrust of the engine is the mass of the air through the propeller times the amount it is accelerated. As a plane accelerates to close to the speed of the air behind the prop (calculated prop speed), the acceleration of the air goes to zero and prop thrust goes to zero. Put another way, if the speed of the air into the prop equals the speed of the air behind the prop, the prop is providing no thrust. If we calculate a prop speed of 150 mph, using the prop true pitch, it will be VERY difficult for that plane/engine/prop to exceed that speed. Most gains will come from the engine unloading (the engine isn't accelerating air so it unloads to a higher rpm) and adding some vector of gravity force (i.e. point the plane straight down![X(] ) At higher plane speeds, the pointing a 4 lb plane straight down will provide 4 lb of thrust to the propulsion of the plane. Add that to the thrust the engine is providing (quickly going to zero as the plane accelerates to it's prop speed) and you will see why a very slippery airplane that does 150 in level flight doesn't accelerate as much as you might think when pointed down hill and why it is so difficult to go fast than the prop.

Bill

airraptor

My Feedback: (66)

then explain why my plane does around 182 average with such a low pitch speed?????

Mike Connor

My Feedback: (5)

Using the NASA foil sim (link above) we can show that the Clark-y or NACA 4412 can move an aircraft forward, even with a negative angle of attack. If you could find a 0 pitch prop (cord line or bottom of the airfoil), it would probably fly a trainer type airplane without a problem.

As altitude increases, true airspeed will increase if indicated airspeed is constant. However, unless you have a turbo charged engine it is hard to take advantage of thin air. Engine HP decreases with altitude and HP = speed. Maybe I did not understand what you were trying to say.

HighPlains

My Feedback: (1)

Not in all cases.

What happen to thrust equals drag?

wjvail

My Feedback: (3)

The point of all this is that in the case of piston engine propeller driven planes, strange as it may sound, at some point, the frontal area of the engine and prop become a serious liability to faster speed. Unless you can increase the prop pitch or the engine rpm, you are NOT going to go faster. For the first 2 decades I modeled, the model speed record was held by a glider in the Alps. By maximizing L/D for the airframe to include eliminating the prop and engine, they were able to go fast than any driven airplane. To achieve the speed suggested in the video below, you would have to turn 12" of pitch in the 30,000 range. Adding a motor and prop to this slope glider would immediately limit its top speed to rpm x pitch + maybe 10%.

Mike.. to make your point I've attached a scan of the lift plot for the NACA 4412 previously discussed in this thread. I think the point you are making is that even when the AOA is zero, the coefficient of lift is positive for a non-symmetrical airfoil. The point I am making is that this zero AOA is NOT defined from the flat bottom of the airfoil. Unfortunately, the flat bottom is exactly where APC measures pitch from. If you look at the drawing in the upper left portion of the right hand page you will see where zero AOA is referenced and it is not the bottom of the airfoil. I accounted for these factors by adding 10% to the calculated speed to come up with a ball park figure to answer the question presented in subject line of this thread. That was met with the comment "you cant go off pitch speed for your plane." Again, the calculation is not a substitute for a radar readings, but you CAN go off pitch speed to calculate a good ballpark number for how fast your plane will go.


I can't answer this... partly because I have serious doubts it is true. Sorry. Your use of an 8x7 prop on this setup raises a flag to me. It is not a good choice if going fast is your goal. I have a Q500 with a Nelson turning a 8.5" pitch prop (as marked by APC) at 19 ish on the ground. It will do 150 ish straight and level and 168 in a long 60 deg dive and a little tail wind. A typical Q40 prop would be something in the 7.5x8 range and that is for close course racing.

To average 180 with 8" pitch prop (that is your 8x7 corrected for the above discussion) you will have to be turning around 24,000. Possible I suppose but you would have to be running a pretty sporty pipe for it to unload that much. Now if you took the prop and engine off, removed the cheek cowel and canopy, added weight and pointed the thing down hill you might see well over 200.



http://www.youtube.com/watch?v=Vi0hrjqU15I

Bill

Mike Connor

My Feedback: (5)

Considering the airfoil shape and pitch of the prop along with unloading of the engine I calculate a best case top speed in level flight of 175 mph. This would require a very low drag airframe but the LR1 without gear would be fairly clean. I suspect you got a little help from a decent as you approached the radar gun.

wjvail

My Feedback: (3)

Yup in all cases... in level flight and if velocity is static. If thrust does not equal drag the plane is ether accelerating or decelerating. It's simple statics. The sum of the forces must equal zero if the object is static (i.e. not accelerating.) If you'd like to talk about a situation where thrust doesn't equal drag, that is the area of dynamics and we see that on every takeoff roll. That is not what we are really talking about here. (It gets a little tangled in the usage of terms if you are talking about, say, a plane in a steady torque roll. In this case you have to say lift = weight. In this case we would be using engine thrust plus wing generated lift (0) to equal aircraft weight.)


Thrust, in this case, is the sum of prop provided thrust plus the component force of gravity and MUST equal the drag of the airframe. Airplane drag is the drag coefficient of the airplane times velocity squared.

An example may help. Take the example of a 7 lb Alpha .60 trainer with a Evolution .60 turning a 12x6 at 12,000 rpm. If you remove the engine and prop and replace them with ballast, I think we can all agree the component of gravity contributing to thrust in level flight is zero and thrust provide from the (missing) engine is zero and the total trust is... zero! Now take this same arrangement and point it straight down. Engine provided thrust will still be zero but gravity will be providing 7 lbs of thrust (force). The plane WILL continue to accelerate until airframe drag equals 7 lbs.... and NO faster. I would suggest this speed for an .60 trainer is around 90 mph.

Now 1/2 way into this dive reinstall the engine. A 12x6 may have an actual pitch of around 7" and may unload to 13,000. That computes to a prop speed of around 86 mph. At this point the newly installed engine is now not pulling the airplane. It is increasingly becoming DRAG. At about 90 mph engine trust will be close to zero and gravity will remain... 7 lbs. 0 prop thrust + 7 lbs gravity = 7 lbs thrust = 7 lbs drag.

Bill

Mike Connor

My Feedback: (5)

Yep, and as I pointed out in an earlier thread you make a good argument for your case. It would be nice to get theory down well enough so we could actually calculate reality.

airraptor

My Feedback: (66)

your not making any sense with your trainer. if the prop pitch speed is = to the aircraft speed and the prop isnt making the air accelerate anymore then whats keeping the prop from spinning faster. this is like saying if the aircraft was on the ground tied to a fence and then the wind was blowing at the pitch speed the only thing keeping the prop from spinning faster is the drag on the prop at 13,000 rpm which a 60 spinning a 12" prop with no pitch would spin way faster than 13,000.


as to my LR its a very clean plane with low weight and a Jett BSE 35lx with short pipe.

As to your quickie take the gear off add some wing fillets, and cowl the engine and I bet your could reach 180 pretty easy.

Dont put all your faith in Pitch Speed ok. Specially those calulators