Hi

I have used the online motor/batt/prop calculator at
http://brantuas.com/ezcalc/dma1.asp

A 17/10 prop has more static thrust than in flight thrust. ok
But a 18/6 prop has more in flight thrust than static thrust ?

Motor/gear/batteries are the same except for the prop size.
Is this possible? Are there times when the inflight thrust
exceed the static thrust?

Thanks in advance.

Mark S

Absolutely. Many props "unload" a lot in the air. They're more efficient and/or turn a higher RPM in a moving airflow than a static one.

I got an O.S. .32 that turns a 10-4 at 14.1K on the ground. I also have a Webra Speed .32 that turns the same. On identical planes, the Webra is MUCH faster in the air than the O.S.

Dr.1

There are so many variables that making judgement about what works best for a specific application is usually done by some experimentation. calculators do not take into consideration enough variables to make absolute predictions about performance but would give you a place to start. but if its any help, an 18/6 prop will unload alot quiker in the air than a 17/10, but you would use those props in entirely different applications so the comparison is meaningless.

You sure the prop numbers are the right way 'round? I'd expect the higher pitch to provide more inflight thrust and the low pitch to work best in static testing.

I've done a lot of static testing of my own and found that the pitches higher than about 8 tend to run in a stalled condition on the ground as shown by very high current readings and an odd sound from the prop compared to the lower pitch props. I've never run motocalc but I wonder if it can deal with a stalled blade static run situation.

I have used motocalc, and yes, it does calculate for a stalled prop. Usually there is still more thrust static than flying, but as the prop begins to bite more it draws more amps. (I was calculating with a Himax 4100 6.6 to 1 gearing with a 9x7 prop)

That's a great illustration of the importance of engine performance characteristics in prop selection. Mathematical thrust calculators do not deal with those parameters at all well.

They don't
because thay can't
Just as some believe that wide blades make more thrust than narrow blades --
the blade and it's speed (rpm) and the load it is trying to fly -all interact
FWIW on my high performance 3D (flip flop jump up etc., models the small narrow blades running fast rpms work best.
The best prop - allows the IC engine to make it's best power -- FIRST-
Unless that happens all the rest of the exercize is just jer-- off.
On electrics - the game changes --but you still have to operate the motor in a range where it does not go up in smoke.

The only real application for wide blades is as a compromise solution to avoiding another blade-set. If physical constraints limit prop diameter & if the power transmission capability of an existing prop has been reached, a somewhat less costly & arguably less inefficient solution to adding extra blades, is to broaden the blades in the existing configuration. This is seldom the case with models, but it does occasionally occur.

It is/was more common in full scale aircraft, where another blade significantly adds to weight & cost, & somewhat to loss of efficiency.

The Mustang & Thunderbolt are nice examples of that compromise solution.

With the Spitfire, Supermarine took the other path -- add more high-aspect ratio blades, as required. Who was right -- who knows?
The Spit was an outstanding performer at all altitudes with the Mk XI, XIV & XVIII. The Jug was certainly not at its best on the deck, but the 'Stang was like the Spit -- pretty good everywhere.

60 year old tech-
but was then state of the art - imagine what would have happened if they had engines capable of turning far more revs reliably
It is all a compromise.
On the models we can turn all the revs we want -really
I run 50 cc ga emgines intoth e8000rpm range --smaller engines far faster
and in racing glo engines - over 30,000 rpm
It is all a compromise which must include the task and the aircraft

During WWII, propeller design was well understood. Most propellers from that era were operating at pretty high efficiencies.

If you had engines capable of turning the existing props at higher revs, you would have had to increase the width of the blades or increase the number of blades to slow them down. The reason is that for a given prop diameter (which was already limited by ground clearance), your maximum revs are limited so you can keep the tip speed subsonic. Once the tips go past the drag divergence Mach number, the efficiency of the propeller decreases dramatically. Of course, you could decrease the diameter of the prop and then spin it at higher revs, but smaller propellers tend to have lower efficiencies than bigger diameter propellers (assuming both are sized correctly in terms of pitch, blade width distribution, etc).

If you had an engine that gave peak power at a higher rpm than your ideal propeller rpm, then you would have had to gear it down. In fact, many fighter aircraft engines and also many modern aero-engines are geared so the prop would not have to spin at such a high rpm before the engine reaches its peak power.

What happened during the evolution of WWII fighters was that (a) they increased prop diameter as much as they could within ground clearance limits. (b) RPM was limited by keeping the tips subsonic. (c) As bigger and more powerful engines were installed, the only two ways to absorb the additional power without increasing prop diameter or rpm were to increase the blade width or to increase the number of blades.

Theoretically not unlimited as the tips would also eventually hit transonic Mach numbers, but with our current engines and typical prop sizes you are quite correct. The props on our models are so small that you would have to spin them extremely fast before you start experiencing shock-related losses. They also gain a bit of efficiency when they turn faster since the Reynolds numbers that the blades experience increase with rpm, which generally go along with a decrease in drag on the blades and therefore an increase in overall prop efficiency. In addition, most model airplane engines produce peak power at very high rpm, so spinning a smaller prop faster often work better.

I have read through several of the "prop" threads, and I would like to make a few further comments:

Dick, you explained several times that you do static thrust tests with your motors and choose your props based on that. Just remember that you generally use your models for aerobatics and in particular, 3D aerobatics (at least from what I gather from your posts). During these types of maneuvers, you are generally very slow or even static, and therefore your static thrust tests are quite valid for the way you operate your models. Many of the modelers requesting information on these forums fly different types of models than you. For instance, in the case of a pylon racer the static thrust tests will certainly lead you in the wrong direction. You would need to test the model with the motor running in a wind tunnel with the wind tunnel operating at or near your maximum expected flight speed. You then take thrust measurements at that freestream speed and choose your propeller based on those results. Of course, very few if any modelers have access to that type of equipment, so the other alternative is to simply test different props in flight and to select the propeller that gives you the highest maximum speed. You will find that it will not be the same propeller that would give the maximum thrust on a static thrust test stand and it will also not be the same propeller that gives you the highest rpm when static.

The second thing I would like to comment on is your criticisms in general about the use of theory. I have worked on several UAV projects, ranging from mini-UAVs to much bigger ones, and also on manned aircraft. The propellers selected (on the prop-driven projects) were selected based on theoretical analysis which narrowed it to a very small number of candidates, followed by wind tunnel testing that lead to the final selection. In all cases the propeller/engine combination performed very close to that predicted. Note that the criterion here was not as simple as "yes, it can do aerobatics" - the criterion was very specific target endurance or range numbers, climb performance, etc.

When designing a new aircraft, theoretical analysis and simulation is the cheapest way to try out ideas. Next follows wind tunnel tests and finally comes flight tests. In one project in which I was involved, we ran more than 700 candidate designs through our analysis software to narrow things down to a few promising candidates. Imagine building 700 models to find out which ones are better, and doing so while trying to meet tight deadlines? Wind tunnel tests are generally much more expensive, especially when you need transonic tests. So, by the time you go into a tunnel, you want to know that you have a very good design and that changes will be kept to a minimum from there onwards. If you need enough accurate data for example to design a fly-by-wire control system, you are probably going to need 1000s of polars to capture enough possible points: Mach ranges, angle of attack ranges, side-slip ranges, different control surface positions, power simulations at different throttle settings, dynamic tests, etc. So, in general you don't want to repeat a series like that because you got your theoretical analysis wrong. Then comes flight test, and here you are very limited because of both cost and risk. So, often flight tests simply serve to validate that everything tie up with your original theoretical analysis and wind tunnel tests. When they don't, you stop the flight tests and start to study your calculations and simulations very carefully and go spend a bit more time in the tunnel - you try to prevent this from happening often...

What I am saying is that theory is capable of giving you very accurate results. Of course, to get those very accurate results needed in the aerospace industry, you need fairly expensive software and people with the knowhow to use them correctly. However, there are some cheap aerodynamic software tools and simple equations that can be a great help in pointing a modeler in the right direction, or to help you understand what factors influence your model's behavior. This is a "model aerodynamics" forum - but there are many geeks here who actually like to make a few calculations to help get them some ballpark numbers. When I was a kid in highschool, I was certainly one of those who were always looking for a formula to help me design my models - just to give me a bit more confidence in my design. In the case of propeller selection (the theme of the current thread), at least the theory or a tool such as motocalc can help you narrow down the candidates so you have to buy only a few propellers to test on your model.

One final comment before I go back to "lurk mode" - I have seen you make comments such as "life is not a windtunnel"... Aerodynamicists are not idiots (well, at least that was the case for most of the ones that I have known ) and the limitations of wind tunnels are well understood and acknowledged. However, there is very little that cannot be accurately tested in a wind tunnel these days - certainly power tests with a scaled-down motor/prop or a jet engine simulator are quite common just to give you one example. The effects of tunnel wall interference, tunnel blockage, mounting system tares, etc., are all well understood and can be corrected for to leave you with data that correspond very well with in-flight test data. So, I wouldn't be so quick to dismiss wind tunnels as a source of accurate data. The data that Prof Michael Selig has published in his "Low Reynolds Number Airfoil Data" books is one example of wind tunnel tests very useful to the moddeler. Of course, properly interpreting the data and using it correctly is another matter.

Alright, I have said what I wanted to say - I'll go back into lurk mode now...

thanks - I understand -and I appreciate your abstaining from adding formula or using technical jargon.
I have a background in reconstructing industrial accidents - which requires that information be kept in plain speak . I really appreciate when it is used for any explanation.
Theory is always the first step on the path to a proven result.
That is a given
For a model furum - I still like to think that most of the information solicited - by modelers -- is for use -on a model .
So - the simple , direct, hands on approach -if it supplies the needed info -I see as the best aproach.
For specific purpose vehicle ,such as you apparantly do - The new hi tech stuf is the way to go . It costs a lot but saves a lot.
Modeling is a hands on thing -At least it is for me . Most of my comments are directed toward low speed aerobatics and the tiny electric model setups -currently (no pun) the rage.
These models present an opportunity to do hands on experimentation never before available to anyone - anywhere -for any amount of money.
The results I have seen and the pure pleasure from trying the experiments can't be overstated .
Life isn't a wind tunnel -That is not meant as a slur- I meant to say that other paths and methods can produce the looked for results.
If no one gets anything more than an interest to do their own investigations of "facts"- -then I will feel I have contributed something to this forum.

Thanks, Dick, I appreciate what you say. I also fully agree with you on your comments about both the pleasure that can be derived from experimenting in our hobby, and the usefulness of such experimentation. Some of your posts just sometimes come across as you having a certain disdain for theoretical analises and in particular towards wind tunnel testing - which is of course also a type of experimental evaluation. I think that you probably don't intend to come across that way, but that you rather want to highlite the possibilities evailable to us these days - especially with the small, cheap foamies. I apologise if my post came through as a bit attacking, but I get the impression you have a thick skin anyway. I also wanted to use the opportunity to explain that with the necessary equipment and know-how, very accurate results are possible with both analises and wind tunnel testing - modern aircraft (and UAV and mini-UAV and micro-UAV) design depends on that accuracy. No reason for modelers not to make use of the simpler formulas and aerodynamic tools if they enjoy that approach and as long they understand the limitations. Often it is possible to quickly run an equation through your calculator to save you the time of testing something that is not going to work. My example earlier of narrowing the number of props you need to test for an application is one such example.

For what its worth, I have great respect for your experience in model aircraft design. Unfortunately I currently live on another continent - otherwise I would have loved to exchange notes. I think you would also enjoy seeing what is possible on the non-experimental and the wind-tunnel testing side these days...

The relationship of static or ground thrust being LESS than flying thrust is simple. The propellor is unable to draw in air to the blade fast enough.---It is starting to operate in a very weak intake vacumm-- It is begining to cavitate.

That is why it can develop better thrust in the air at a flying speed.

When you can size props like that, you have really reached the top of prop selecting. [sm=thumbup.gif][sm=thumbup.gif][sm=thumbup.gif]

Rule of thumb and TLAR engineering works well in our toy airplanes mostly because few of us, particularly sport flyers, know or care what our model's top speed, cruising range, or usable payload is. In models, it is so easy to just bolt in a bigger engine. Horsepower overcomes mediocre design. It's mostly aerobatic performance we seek, i.e. unlimited verticle and no yaw to roll or pitch coupling to make knife edges easier etc.
Now if your goal is to design a model airplane that can be flown across the Atlantic ocean without refueling, you had better do some computer modeling and wind tunnel testing.

getting the right prop for speed -is black magic-
The interaction with the airframe confounds the whole exercize
On old speed control line models - the setups would gurgle around -slowly gaining speed -till the craft finally unloaded th prop enough --to allow the engine to reach into it's best operating range .
This was sometimes a real crap shoot as the engine is simply an air pump --which has to not "cavitate" internally. Tuned systems on two strokes show incredible power gains when the resonant point is developed and the engine simply flows huge gulps of fuel mix. That is another aerodynamic ( OK ---fluid dynamic) exercize
-The prop -the airframe and the engine -all have to interact - change any one of em -all of em change in performance .
Back in the early '30s very fast, weird looking float planes went like gangbusters -on small (relatively ) odd shaped props . over 400 mph at sea level. (pun)
getting the right combo for a speed prop plane is a real case of cut n try.

What's really happening is that the propeller's airfoil is in a stall until the airspeed is high enough to reduce the blade's angle of attack below the stall point. It's mostly a problem with high pitch speed props. True cavitation happens to hydrofoils used in marine propellers. When the suction on the low pressure side takes the pressure below the water's vapor pressure, the water boils and this is what is called cavitation. A hydrofoil can be in a stall without cavitating. Aeriation of a hydrofoil is also often confused with cavitation. Aeriation happens on rudders or fins when the suction side of the hydrofoil sucks air down the side of the hydrofoil allowing the water flow to separate, resulting in a sudden loss of lift and a windsurfer's "spinout".

Close, but not quite -- they went like hell on huge 2-blade fixed pitch props. They used all of the diameter that could safely clear the floats, combined with as much pitch as the engine could pull. This was to transmit the gigantic output of the racing engines, but avoid going to 3-blade props, if at all possible, in order to maximize efficiency.

yeh--BUT they were not using the huge paddles as used on WW2 fighters
those planes had to fly waay up high to keep from getting shotup by the opposition.
Props are again --compromise devices . Those guys needed huge paddles to get a grip on air at over 20000ft.
I see lots of info --extrapolated from WW2 craft -to use on models -at sea level - and a lot of it does not cross over - not bad info-- just not all that applicable .

I disagree -- the paddle-blades were a dodge to avoid extra blades. The Spit outperformed all of them at high altitude, with skinny high-aspect ratio blades.

I agree with you for sure about the non-applicability of full-scale prop comparisons to models. As soon as variable pitch, constant speed & high-strength metal blades enter the picture, it doesn't apply to models at all. In reality, the fixed pitch props from the early full scale birds are the ones with any relevance to models.

Not all the Spits used those blades -- earlyones -first one- had two blade - then they progressed up to 6 -I think on last Mk

You are correct -- the Mk I had a fixed-pitch 2-blade. The Spit didn't use paddle-blades at all, regardless of the number of blades -- (five max).

Post war, the Spit 24's were used as reconnaisance fighters well into the 1950's, including in Korea. They were nearly imune to interception by the first generation jet fighters of the time, being capable of 460 mph @ 28,000 ft & 400+ at altitudes in excess of 47,000 ft. They were nearly as fast as any jets capable of reaching that height & could easily out-turn them -- continuing to evade until the jet was forced to break off the attack for lack of fuel. NATO forces practiced intercepts against the high-flying Spits by using pairs of fighters, so that the Spit would be forced to turn toward one of the attackers, while evading the other -- this was still no guarantee of a "kill", but it cut down the odds in favour of the Spit quite considerably.

The MK XI reconnaisance Spit was the world's fastest man-carrying vehicle until the advent of swept-wing jets, most notably the XP-86 Sabre. In 1944, a Mk XI Spit hit 0.9 Mach in a dive, while being formally compared to the Mustang & Thunderbolt. The Mustang hit 0.8 Mach & the Jug hit 0.78 Mach. Regular fighter version Spits were limited to "only" 0.86 Mach.

Skinny blades rule!!

now you went n dun it!
The p51 is known to be the best , fastest ,highest flying fighter ever made .
List exceptions here:

Just an interesting (to me) note: A few of the Navajos I used to fly had narrow four bladed props with Q-Tips as opposed to the three bladed props. Their sound (from inside) was higher pitched and annoying and they were difficult to sync and keep synchronized. I don't know if they were actually louder but they seemed like it. Nobody liked flying them because of the sound. They might have climbed slightly better than the regular three bladed prop versions but they didn't seem significantly faster in cruise. The three bladed props were easier to sync, but not as easy as the two-bladed props on some of the Barons I flew. I could quickly and easily sync those without a syncrophaser and they tended to stay synchronized for the entire leg.

OK:

Spit IVX

Spit XVII

Spit 21

Spit 22

Spit 24

Seafire XV

Seafire XVII

Seafire 45

Seafire 46

Seafire 47

Seafang

Spitefull

Fury

Seafury

Martin Baker MB.5

Dornier Do 335

Focke Wulf TA 152H

Kyushu J7W1

you realize you have just wrecked history for a thousand boys?