Extreme Speed Prop Planes Discuss the need for speed with fast prop planes (Screamin Demon, Diamond Dust, Shrikes or any REAL sound breakin'''' plane)

prop diametre ?

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Old 06-07-2007, 02:30 PM
  #1  
max plank
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Default prop diametre ?

if you took 2 identical speed planes (delta or conventional) and a lets say 50 something engine and one running a 10 x 7 prop and the other a 11 x 7 which one will be faster ? you will properly say the one with 10 x 7 ... but the question is why... will it not be the resistense off the plane that will diktate the speed...
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Old 06-07-2007, 04:56 PM
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Default RE: prop diametre ?

Not enough info in your post. You would need to know the specifc plane and engine for starters.
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Old 06-07-2007, 07:13 PM
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Default RE: prop diametre ?

The theoretical speed would be higher for the 10x7 because it would turn more rpm.
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Old 06-07-2007, 07:34 PM
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Default RE: prop diametre ?

10x7 also has less disk area = less drag..."theoreticaly" smaller prop is faster....Unless it's on a P51
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Old 06-07-2007, 08:17 PM
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Default RE: prop diametre ?

Yeh, ok, put a 5X10 prop on it. A 3X10 and watch it turn it's guts out at over 30k rpm. How fast do you think it will go then?

You have to consider how much drag the air frame presents. Remember in unaccelerated flight, thrust = drag and lift = mass.

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They say the first thing to go is your memory and I can't remember what the second one was.
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Old 06-07-2007, 09:10 PM
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Default RE: prop diametre ?

ORIGINAL: Kaos1

Yeh, ok, put a 5X10 prop on it. A 3X10 and watch it turn it's guts out at over 30k rpm. How fast do you think it will go then?

You have to consider how much drag the air frame presents. Remember in unaccelerated flight, thrust = drag and lift = mass.

Kaos1
....Unless it's on a P51
That's what this statement was for..

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Old 06-08-2007, 05:16 AM
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Default RE: prop diametre ?

the reason fore the question was that when somebody ask for what prop to run on a plane it semes that someone always rekomend cut the prop down to get more static rpm.. and i mean that if the plane not have mutch resistense the rpm will go up as soon as you get airborne ewen whit the larger prop diametre.. so is the pitch not more importent ?
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Old 06-08-2007, 06:53 AM
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Default RE: prop diametre ?

Well, the 10x7 prop size will only provide better top speed (because of higher revs thus higher slip stream) IF the airframe does allow that.

So it is crucial to minimise the drag, both induced and parasitic.

If using a “perfectly“ shaped airframe such like some of the speed contenders at the SpeedCup in Germany this weekend, very high pitched props (oversquared) can be used successfully.

Even props like the mentioned 5x10 will work fine then (only for a typical speed flight pattern, not for pylon racing). With such oversquared props the only critical situation can be the start procedure, due to a quite low static thrust. Once in the air the (stage two) pipe resonance starts (during the first dive at the latest).

It does require some routine to find the “sweet spot“ where the engine will work at the optimum (=rev and torque peak). Meaning that airframe and all components should harmonise with each other:

- airframe design (drag components)
- engine timings and compression setup
- tuned pipe length
- prop size (the prop diameter has to be adjusted to the target engine rpm to avoid transsonic prop tip speeds, the pitch on the other hand does determine the potential speed plane target top speed)
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Old 06-08-2007, 06:55 AM
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Default RE: prop diametre ?

So here are some examples how to gain speed or to lose speed depending on airframe design:

This black speed plane is set up with a .40 Nelson FAI F3D engine.

Because this airframe is optimized for straight line speed (minimised sections, fully cowled engine, completely hidden linkages, special airfoils), the use of overquared props is highly recommended in this case.

It is not surprising that - if mounting an oversqured prop - a higher top end speed of this plane can be reached versus using a regular .40 F3D Pylon propeller size (e.g. 7.2 x7.2) with this plane.
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Old 06-08-2007, 06:57 AM
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Default RE: prop diametre ?

Next example:

This time it is explained why a .60 powered speed plane can be considerably faster than a .90 powered speed plane.

As we can see the below pictured plane has no engine/pipe cowl. The fusealage section is quite fat as well as the airfoil thickness’ of wing, stab and fin. This plane comes with the strong and reliable OS 91 VR-DF ducted fan engine plus BVM tuned pipe (about 4.7 HP of ouput). The fastest runs of this combo do reach about 195 mph using a 9x10 prop made of carbon fiber (only carbon provides the needed strength). If using more pitch (and less diameter to unload the engine better) top speeds will be slower than 195 mph because of the inability of this airframe to transform the increased propeller slip stream into higher top speed.
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Old 06-08-2007, 06:58 AM
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Default RE: prop diametre ?

Final example

Now we can see a speed plane powered by a .60 sized OPS VAE engine and OPS non-muffled pipe (about 4 HP of ouput). Like the above shown black speed plane this airframe has been purposely designed around a certain speed engine/pipe combo to minimise any kinds of drag – as slippery as possible.

In this case it would be a waste of engine energy to use the above mentioned 9x10 prop size. Now a smaller and higher pitched carbon prop can be used effectively, leading to top speeds exceeding 220 mph. So the recipe for better speed results is NOT simply using the next bigger engine!
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Old 06-08-2007, 06:59 AM
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Default RE: prop diametre ?

After all a picture of a propeller mold plus hand made CF speed prop. Using such hand laid carbon speed props is most important for safety reasons. Never use regular plastic props for an ambitious speed project! Even wooden speed props cannot handle the power of modern .40, .60 or .90 high performance engines.
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Old 06-08-2007, 09:19 AM
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Default RE: prop diametre ?

Excellent thread. Thanks to all for contributing and explaining the question.

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Old 06-08-2007, 11:46 AM
  #14  
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Default RE: prop diametre ?

thank you fore all input ... i am a litle weiser now..
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Old 06-08-2007, 01:00 PM
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Default RE: prop diametre ?

After watching the video from the last German Speed Cup, I wonder why so many guys are building low wing airplanes when a few of them almost crashed from bad hand launches. The black mid-wing airframe looks like a winner but having handlaunched pylon ships for years it's one of the worst ideas to have a low-wing, hand launched airplane. If it was faster, more guys would do it despite the difficulty. Is it just easier to start and tune an upright engine and have the tuned pipe go along the top of the fuselage over the wing (easier to make a smooth, aerodynamic cowl) or is it just because everybody else builds low wings so other people copy the style? A high wing is also generally more stable and a lot of those speed passes looked a little wild (obviuosly more trim and pilot issues than just high wing vs low wing). Just curious...?
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Old 06-08-2007, 03:02 PM
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Default RE: prop diametre ?

Hello, the main reason for seeing more low wing designs is the improved (upright) engine/cowl/tank access. That helps much during pre-flight fine tuning. In addition the dirty air of the wing does not affect the required clean airstream at the horizontal stab (being located clearly above the wing level). Last not least the electronics (servos, receiver and battery) won't be impacted by these high thermal pipe emission when applying a low wing with an upright mounted engine...

In fact there are only little (almost no) bad issues with low wing speed planes during take-off.

There are absolutely no disadvantages of low wing speed planes in terms of less stable flight performance compared to high wings. It is just quite tricky to find the “door” meaning the sensor barrier of the electronic speed trap after a 1000 feet dive.

The helper should apply one of the belown shown launch techniques. I don’t know the reason for the mentioned last years take-off crash, however it was a mid-wing 10 cc speed plane with upside down engine + pipe configuration.
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Old 06-08-2007, 03:04 PM
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Default RE: prop diametre ?

Of course the low wing design does demand a very accurate horizontal landing approach. Wing leading edges should also be very sturdy on a low wing speed plane.

In general the set up, be it high, mid or low wing, does not affect the top speed. Below attached a picture of a former national speed champion of Germany. His mid-wing 10 cc speed plane with upside down engine /pipe should preferably take-off using a launch ramp because of too high tuned pipe temperatures (for hand launching). BTW only hand launch is allowed if trying to get a new FAI world speed record.
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Old 06-08-2007, 03:07 PM
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Default RE: prop diametre ?

Lastly some pics of different wing-level designs and take-offs:
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Old 06-08-2007, 05:28 PM
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Default RE: prop diametre ?

Interesting... I'll ask my other question over in the 'Speed Cup Thread'
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