New Flight: Pitts -Skelton Aerobatic model (page 9)

7 x 3 untouched should help for acro but speed would be quite slow.

You will have to eyeball it using the lettering on the prop as a guide.
Full 7" prop or 6.75" or 6.5"

Thanks Brad for your help, here I am with my Norvel playing a little! hahaha ..... she has given me a kiss of remembrance! ... are the tattoos that I really like ... I have many on my hands and on my arms, each one with its history!.
Well, I will be testing the MasterAirscrew 7x4 propeller "without touching" and after seeing the results, then maybe I will modify it ... it is strange to think about how I cannot take a propeller and modify it, but it happens that in my country there are so many restrictions that a propeller is an important piece and should not be wasted if it is not necessary to risk it ... maybe one day it will change ... but it is missing a lot!
So 12000 rpm and 415 grams of thrust, not bad!

Bernie has these new 7x4 props in stock / note they are narrow great for small IC engine use.
Ouch flip starting is fun lol

Fantastic data guys , Are you going to make the Pitts Dance for us in the future ? Good you have replaced the upper wing hold down with better wood !

1/2A Nut wrote:7 x 3 untouched should help for acro but speed would be quite slow.

You will have to eyeball it using the lettering on the prop as a guide.
Full 7" prop or 6.75" or 6.5"

Well friend, I decided and modified one of my 7 x 4 propellers, here are the results.
From the experience made, the 7 x 4 propeller goes better without modifying, what do you think?

7 x 4 original (unmodified) 420/430 grams of thrust at approx. 12000 rpm
6.75 x 4 (modified 7 x 4) 395 /400 grams of thrust at approx. 14340

According to this experience I have obtained the following data:
Original MasterAirscrew 7x4: 430 grams of thrust at 12,000 rpm
MasterAirscrew modified to 6.75 x 4: 395/400 grams of thrust at 14340 rpm.
Higher traction = 7 x 4
Best idle = 6.75 x 4
If a 7 x 4 propeller is used the idle setting will be higher because the carburettor needle works more open and therefore if you close the air valve pretending to lower the rpm to 4500 turns, the engine will stop by choking.

Good first steps!

Using thrust calc I get these results:  

After you fly the prop to prove if it has provided good thrust and speed
.. you can sculpt the trailing edge for a bit more performance.



The black prop was brought close to balance after rough forming the trailing edge.
Later it was fine sanded smooth like the grey prop.


If you need a bit more speed and don't want to give up too much thrust you can modify the prop further.

Ingenues to use the fish weighting device to measure grams of pull

1/2A Nut wrote:After you fly the prop to prove if it has provided good thrust and speed
.. you can sculpt the trailing edge for a bit more performance.



The black prop was brought close to balance after rough forming the trailing edge.
Later it was fine sanded smooth like the grey prop.


If you need a bit more speed and don't want to give up too much thrust you can modify the prop further.

How about Brad ..
.. the table that your samples do not match the data obtained in practice.
Well, according to your table the propeller that rotates at 14340 rpm, generates a thrust of 540 grams, however in the reality of my experience and video at 14340/14400 rpm, the propeller only generates 400 grams.
As the blade width is reduced, the engine gains revs, but loses thrust traction, causing the propeller to be discarded.
I am afraid of reducing the width of the blade more and that the traction or thrust will be reduced more.
This conclusion is related to your data, since the rpm coincide between your table and reality, but the grams of thrust in reality are very different, do you understand me, friend?

I see if you reduce the width of the prop you will get less thrust ?

getback wrote:I see if you reduce the width of the prop you will get less thrust ?

that's right friend, that gives me in practice as seen in the video.
Right now I am testing an original 6 x 3.5 MasterAirscrew (MA) propeller and it pushes 405 grams ... so when I get a 7 x 3 propeller (MA GF3) it would be nice to try and then see how much it pushes.
So far, the best result was
7x4 at 12,000 rpm in MasterAirscrew (MA GF3) with a thrust of 430 grams.
Then a propeller (MA) 6 x 3.5 15900 rpm with a thrust of 405 grams.
Then APC 6 x 3 at 17790 rpm with a thrust of 405 grams.

In the photos that I show now the propeller Master Airscrew 6 x 3.5

Yes the thrust calc does not know the width of the blade so can only calc based on stock props.

As I said:
After you fly the prop to prove if it has provided good thrust and speed.. you can sculpt the trailing edge for a bit more performance.
After flying the prop If you need a bit more speed and don't want to give up too much thrust you can modify the prop further.

In the air the 7x4N will provide the best thrust and speed around 65 mph.
All my modified props yield more thrust and speed when flying a good blend of both attributes required.  
A proper modified prop ends up being the best choice in flight when suited for the plane at hand.

A stock 7x4 will not unload enough to provide a good pace, the wide cord will load a too much in flight maybe 14.5k unloaded.
The 7x4N will allow good vertical climb and acro, (in flight it will grip the air better than in static state). The engine will
throttle well and not overheat.

7x4 at 12k turns slow at 45 mph static perhaps 53 mph unloaded. Best thing to do is fly the props and see which one
gets the job done. Sport biplanes require power if you can't provide that then the best choice is select a prop that gets
past that wide cowl and pulls 2 sets of wings / weight / drag of the craft at a solid pace.  

Friends! I have realized something really important! ... to see if I can make my comment understood.
As you know, I've been doing propeller tests on the Pitts in static, I removed and put the propeller over and over again to check the performance, well, at one point I noticed that the same propeller was working less, i.e. between 10% and 12 % less than in other tests ... I did not understand what was happening!?, then I looked at other previous videos and realized that I had done the static tests WITHOUT Cowl!, placing the Cowl and placing the same propeller, the propeller Due to the air shock expelled by the propeller, it yields 10% to12% less. It is understood?

From aerodynamic point of view  I would also expect lower motor performance with the cowl being placed on.. the obstructions too close downstream the prop may matter  much..

In my business I work with large axial fans of up to 36feet diameter and the fan and fan bridge ineraction can cause fan performance reductions

balogh wrote:From aerodynamic point of view  I would also expect lower motor performance with the cowl being placed on.. the obstructions too close downstream the prop may matter  much..

In my business O work with large axisl fans of up to 36feet diameter and the fan and fan bridge ineraction can cause fan performance reductions

Hi András, I think the engine is well ventilated, see in this photo, the carburetor takes perfectly air.
I think the great flow of air is swallowed by the cowl himself and instead of channeling it, he pockets it inside it.
Although the cowl itself has air vents, I think that makes the propeller perform less.

Yes Mauricio I learnt the cowl air outlet to inlet area should be 2....3 to minimize the cowl effect on aerodynamics and ensure proper cooling airflow

Thanks for this Mauricio. Yes, the compressed air from the propeller hit most of the front of the cowl, so performance lowers. There must be a propeller and cowl clearance, or a streamlined cowl is needed.

I wonder if supplementing here, and moving the helix away from the cowl at least 10 mm, will it improve ???

I think that air flow will improve.

Levent Suberk wrote:I think that air flow will improve.

Okay, friend, I will get down to work right now to make a supplement and we will see that it turns out ...

I agree itmay improve but it will magnify any vibration if the prop is unbalanced..

balogh wrote:I agree itmay improve but it will magnify any vibration if the prop is unbalanced..

yes it's true i will try to make a supplement accurate