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Dec 11, 2017, 10:35 PM
Ken's CAD Models
dz1sfb's Avatar
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Oshkosh Special 30 in MPF


This thread has been quiet for some time. Though at the request of a flyer from Pittsburgh PA, I have modified the Oshkosh Special 30 for manufacture out of MPF. That customer is sending the electronics so I hope to have it completed this week. Looking for this to be available as a kit.

Ken
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Dec 12, 2017, 07:48 AM
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Great, let us know when we can start ordering.
Dec 12, 2017, 01:05 PM
Registered User
Will the wings in the kit have the airfoil already baked in?
Dec 12, 2017, 03:23 PM
Ken's CAD Models
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Quote:
Originally Posted by Lostcity
Great, let us know when we can start ordering.
Will test fly and trim noting any tweeks needed.

Quote:
Originally Posted by Michael763
Will the wings in the kit have the airfoil already baked in?
Yes, the airfoil will be baked in as part of the kit.

Ken
Jul 20, 2018, 12:48 PM
Registered User
After a while, I managed to finish my 30'' model, but it didn't fly much and it crashed . I think it was too heavy.

Now I want to start building a 40'' model and I was wondering what motor should I use. I haven't been able to find any of the suggested motors on hobbyking, though I have been able to find some similar ones (I guess?). Also, I was thinking of buying a bigger battery, so that I can get a bit more flight time out of it. Can someone please suggest me a good motor-battery pair (and even ESC, if either the battery or motor have special needs)?

And as a second question, why is it that the recommended motor for a 30'' is a park250 with 2200kv, while a 40'' model flies well with just 1200kv? Seems a bit counter-intuitive that a bigger plane needs fewer kvs.

Thanks!
Blue
Jul 20, 2018, 03:03 PM
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justj's Avatar
I use a 2812 1535kv from headsuphobby.com in mine with a 12 amp esc, 9x6 SF prop and a 2s800. I get 12 to 14 minutes depending on throttle use.
My 40" Oshkosh weighs in at 11.6 oz with the battery installed. Light is good, there isn't enough mas to hurt anything in a crash!
Very relaxing flying!

Jerry
Jul 20, 2018, 05:15 PM
Registered User
Quote:
Originally Posted by justj
I use a 2812 1535kv from headsuphobby.com in mine with a 12 amp esc, 9x6 SF prop and a 2s800. I get 12 to 14 minutes depending on throttle use.
My 40" Oshkosh weighs in at 11.6 oz with the battery installed. Light is good, there isn't enough mas to hurt anything in a crash!
Very relaxing flying!

Jerry
Thanks for your reply!

I checked out the motor and seems like a good choice combined with the 2s800 battery. I'm considering buying the same motor as well!
Jul 20, 2018, 07:03 PM
Ken's CAD Models
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Quote:
Originally Posted by blue-one
After a while, I managed to finish my 30'' model, but it didn't fly much and it crashed . I think it was too heavy.

Now I want to start building a 40'' model and I was wondering what motor should I use. I haven't been able to find any of the suggested motors on hobbyking, though I have been able to find some similar ones (I guess?). Also, I was thinking of buying a bigger battery, so that I can get a bit more flight time out of it. Can someone please suggest me a good motor-battery pair (and even ESC, if either the battery or motor have special needs)?

And as a second question, why is it that the recommended motor for a 30'' is a park250 with 2200kv, while a 40'' model flies well with just 1200kv? Seems a bit counter-intuitive that a bigger plane needs fewer kvs.

Thanks!
Blue
Hi Blue,

Probably it is a typo, but for the 40" model a 2208 1800 Kv motor is recommended and works great on 2s with a 9x4.7 SF propeller. Easy flying will get you 25 minutes duration on a 900 mAh 2s battery if your build is not too heavy.

The bigger plane / lower Kv motor is more a factor of air density and propeller size. Smaller propellers need higher RPMs due to the space between air molecules are relatively large compared to the propeller blade.

Ken
Jul 21, 2018, 04:26 AM
Registered User
Quote:
Originally Posted by dz1sfb
Hi Blue,

Probably it is a typo, but for the 40" model a 2208 1800 Kv motor is recommended and works great on 2s with a 9x4.7 SF propeller. Easy flying will get you 25 minutes duration on a 900 mAh 2s battery if your build is not too heavy.

The bigger plane / lower Kv motor is more a factor of air density and propeller size. Smaller propellers need higher RPMs due to the space between air molecules are relatively large compared to the propeller blade.

Ken
You are right, of course. Sorry for the typo, I should have said '1800' instead of '1200' when talking about the KV.

Regarding the fewer KVs for bigger props, it makes sense. Thanks for the clarification!

As a side question: do the specs above also apply to the Oshkosh Sport?

Thanks!
Blue
Jul 21, 2018, 08:55 PM
Ken's CAD Models
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Quote:
Originally Posted by blue-one
...

As a side question: do the specs above also apply to the Oshkosh Sport?

Thanks!
Blue
Yes they do.

Ken
Jul 31, 2018, 10:09 PM
flyin' fool
goldguy's Avatar
Quote:
Originally Posted by dz1sfb
Smaller propellers need higher RPMs due to the space between air molecules are relatively large compared to the propeller blade.Ken
????

Just to be clear on the subject, there is no such thing as air molecules. It's all about moving air, which in it contains different molecules, and generating thrust. Small props need to spin fast and big slow, to do the job. The amount is determined by how many amps the motor/ESC can handle and how many the battery will deliver.
Aug 01, 2018, 06:15 AM
Ken's CAD Models
dz1sfb's Avatar
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Quote:
Originally Posted by goldguy
????

Just to be clear on the subject, there is no such thing as air molecules. It's all about moving air, which in it contains different molecules, and generating thrust. Small props need to spin fast and big slow, to do the job. The amount is determined by how many amps the motor/ESC can handle and how many the battery will deliver.
Yes, I am in agreement. Air molecules is not correct. Maybe I should use "atmospheric molecules" which would be inclusive of all the gaseous molecules that constitute the air we breathe.

Ken
Aug 01, 2018, 12:31 PM
flyin' fool
goldguy's Avatar
My wife would agree that some of gases we breath are very toxic.
Aug 01, 2018, 12:36 PM
flyin' fool
goldguy's Avatar
Nice to see these proven designs are still getting attention.
Aug 01, 2018, 06:09 PM
AKA Don
Quote:
Originally Posted by goldguy
????

Just to be clear on the subject, there is no such thing as air molecules. It's all about moving air, which in it contains different molecules, and generating thrust. Small props need to spin fast and big slow, to do the job. The amount is determined by how many amps the motor/ESC can handle and how many the battery will deliver.
REALLY? Air is indeed made up of molecules. The molecules are not all the same because air is a mixture of elements, but they all have molecules. So, "air molecules" is perfectly OK with me.

Smaller props are normally turned faster than large props but they don't have to be. Props work by moving air and the more air they move the work they can deliver. Increasing the diameter or pitch will increase the work done. So, to fly a given plane a given speed a small prop must be turned faster.


The max. useful RPM of any prop is limited by two things, the stress on blade from centripetal force trying to pull the blades off the hub, and the speed of the prop blade tips compared to the speed of sound. Strange things begin to happen as the tips go sonic. For a given rpm the smaller blades will cause less hub stress, and the tip speed will be lower. Those things allow small props to be used at speeds that would cause larger props to fly apart, or operate with poor efficiency.


Note, no mention of the space between air molecules....


BTW I am always amazed at the slow speed of the power generating wind mills. And, the blade tip blade speed is so high at that huge diameter that to turn faster would not be as efficient. And, I am told if the automatic controls fail to limit the speed those huge blades will indeed fly off the hub in spectacular fashion.


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