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

well friends, it really works !! .... here is a small video to quickly demonstrate.
I have managed to increase 30 grams of thrust, has recovered the thrust as if the cowl did not exist

Mauricio, you are a Cox engineer! Bravo to you!!

Levent Suberk wrote:Mauricio, you are a Cox engineer! Bravo to you!!

I have given the spacer a metallic blue color and now that I know it works, I am looking forward to making one in aluminum alloy!
I am enjoying this !!! ... thanks friends !! ...

made the spacer in aluminum alloy!

Very nice
The modified 7x4N should work really well in the air with the extension.

Surfer_kris wrote:I love the plane, it looks absolutely stunning, well done!

I think you need to let the engine rev a little higher, in my experience it needs to do more than about 18500rpm on the ground in order to well in the air. Preferably it should do around 20krpm and it will then unload nicely in the air on top of that.

There were lots of good discussion on the Norvel engines on the RCU forum in the past, and it is still all there. Here is one example by "Combatpig" with thrust measurements and different props: https://www.rcuniverse.com/forum/1570574-post27.html

As you can see the APC 5.7x3 is pretty hard to beat for these engines:

PROP RPM THRUST OZS
COX 5-3 21200 17.6
MAS 6-3 17900 20.0
MAS 5.5-4 22700 18.2
MAS 6-4 17600 19.2
ZINGALI 6-3 17900 14.8 OINK!
PUSHER 6-3 19600 reverse thrust
APC 5.7-3 21200 19.2
APC 5.5-2 24000 18.2
GRISH 5-3 24000 16.0
GRISH 5.5-4 17600 17.0
GRISH 4.5-4 21800 17.0

For your plane with the drag from two wings plus wing-struts and the large front, I think that the Norvel .074 would be better suited. It doesn't weight much more than the .061 and more importantly it can handle a 7x3 or 6.3x4 prop with ease.

Kris, thanks for all your info !
At 7.00 minutes I recommend that if someone wants to do it the same size, they can bet on .074 and they will have more possibilities of flying with adequate power, however I think that Norvel's .061 has a lot to give and that with some points To retouch as the propeller extension and displacement and find the indicated diameter and pitch, the Norvel .061 will surprise.
I like this challenge because many theories have been put to the test, regarding the aircraft itself, regarding the general technique of each sector and, of course, also the qualities of the engine. I am learning a lot with this project and I think it has a lot to give and enjoy.
I also believe that the plane for total flight safety, both at low speed and in acrobatics, the indicated would be a good electric motor, which I also plan to experience here on this same plane, (not what I like, the electric motor), but I must admit that, for this type of model, in this size and with low gliding qualities at the moment the engine is stopped in the air, an electric motor will help us at all times to maintain absolute tranquility in the flight.

Levent Suberk wrote:Mauricio, you are a Cox engineer! Bravo to you!!

Friend, you have encouraged me to try it, it is an achievement of yours too !!

1/2A Nut wrote:Very nice
The modified 7x4N should work really well in the air with the extension.

Thanks Brad, you always accompany me on the projects, I have a lot of joy thanks to you!
Yes friend !, now it is a matter of waiting for the right day (it rains here) and then go to the flight field and do the tests with the propellers, the modified one too. Thanks again!

Good news !! ... I have achieved 440 grams of thrust by placing a 15.5mm spacer and a MasterAirscrew GF3 7x3 at 12900 rpm.
According to the results, I see that it is the top of safe extension of operation.    

Nice experiment like always, Mauricio. I believe the engine thrust could be further increased by allowing the engine to run near the speed where its peak output is delivered that I assume will be somewhere near 22k or so. At 12.9k the engine delivers only a fraction of its maximum shaft power, because it is overpropped and cannot spin the prop faster. (For reference, a stronger TeeDee09 will spin the 7x3 prop at around 16...17k, near its sweet spot...so I believe your prop better suits a stronger engine)

As you know the effective engine shaft power is the combination of thrust and air velocity in the direction of the thrust:
P (W) = F(N) x v (m/s).

By maximizing  the maximum shaft power with the selection of the right prop (i.e., smaller in diameter and/or pitch), both the thrust and the airspeed could be increased. Just my humble opinion.

balogh wrote:Nice experiment like always, Mauricio. I believe the engine thrust could be further increased by allowing the engine to run near the speed where its peak output is delivered that I assume will be somewhere near 22k or so. At 12.9k the engine delivers only a fraction of its maximum shaft power, because it is overpropped and cannot spin the prop faster. (For reference, a stronger TeeDee09 will spin the 7x3 prop at around 16...17k, near its sweet spot...so I believe your prop better suits a stronger engine)

As you know the effective engine shaft power is the combination of thrust and air velocity in the direction of the thrust:
P (W) = F(N) x v (m/s).

By maximizing  the maximum shaft power with the selection of the right prop (i.e., smaller in diameter and/or pitch), both the thrust and the airspeed could be increased. Just my humble opinion.

Thanks Andras! It is always very good to have all the information possible and I appreciate your support. I am making these adjustments to check later on the flight, which really turned out better.
I am very pleased to have found this improvement from moving the propeller away from the Cowl. Right now I am watching my previous flight video and I am surprised to myself that the model finally took off and that I could then maneuver in the air, now that I see these improvements, I am "would say" almost certainly that the model is really great! than to check it in "clear" flight !, but with the above limitations, see how it behaved, I think the model is better than I imagined. Technically understanding the tuning of a model, can mean a resounding change in the concept and opinion of an airplane ... as I said earlier, I will be testing when the weather permits, here it rains and there is wind, so the Pitts & Norvel, you must wait, me too haha.
Here are the photos of today with wet surfaces due to the weather and there you can see how the extension of the motor shaft is compared to the plane as a whole. It is quite nice, I like it!



You can also see there the new shots of the upper wing in MIDWEST wood, much better than the previous material, definitely!

The whole plane and your experiments are top notch professional Mauricio. The engine shaft "extension" really makes it look technically very sound.

I wish you good weather!

balogh wrote:The whole plane and your experiments are top notch professional Mauricio. The engine shaft "extension" really makes it look technically very sound.

I wish you good weather!

Many thanks András for your words !!. I share here then, an important security detail.
I have drilled two holes in the original 1.5mm diameter motor plate and done the same on the extension. After this, I have unscrewed and pressed two steel bolts into the extension itself, leaving them fixed there.
This means that in the event of a counter-explosion of the motor itself, nothing slides and becomes loose.
Also during high engine rpm, and direction change flights, gyroscopic propeller motion, do not allow the center screw to loosen and some or all of the assembly to fly out. With this that I show here, the whole set is definitely safe and free of possible accidents.

Absolutely logical measure!

Because there is no key on the tapered shaft end which is also not splined in NV engines, I believe you must also span the prop screw well enough in order to prevent the prop plate from slipping on the shaft in case of a counter explision.

One can also glue a piece of fine emery cloth on one of the parts, that works well enough with spinners and other things that I have made (like a spring starter cam) that goes between the prop and the drive plate on Norvel engines.

friends wish me luck !!, if the wind allows me today I will make new attempts !!, now with the scroll of the propeller!

Best wishes Mauricio!!!!!!

Good luck friend..as long as you find the direction of headwind to (hand)launch your plane into, there should not be a problem..

My friends, thank you very much for your wishes, I must say that it was a SUCCESS, I hope you could see this model personally the spectacular and safe flight it has now, it is really great, the plane flies really spectacular, the Norvel .061 takes it in an excellent way No more is needed, just fun time!
The Pitts is one of the best models I have ever created. I feel happy!!

Congratulations Mauricio! Beautiful performance!

Mauricio

I'm very happy for you to see the results of all your tireless work! Very extensive effort put into this project and I'm glad you have shared it with us.