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Author Topic: Thrust  (Read 876 times)
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Mike
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« on: August 05, 2011, 06:16:22 AM »

My son is fast becoming an edf speed freak. Unfortunately, being of the silent flight fraternity I now nothing about powered flight.
As my son wants to source a kick ass powerful edf unit for his latest project (a Hawker Siddily Hawk)I assume llbs thrust is what we need. How does thrust relate to model weight? Sorry if thats a stupid question.

Cheers Mike H  Roll Eyes
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Zeiss Ikon
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« Reply #1 on: August 05, 2011, 08:52:39 PM »

If you can build a model that weighs less than the thrust from the installed fan, it'll perform like an Air Force or Navy fighter -- vertical climbs won't decay into tailslides, unless you want that, for instance, and you'll never have trouble getting off the ground.  Historically, the Me-262 that startled P-51 pilots in the last few months of WWII had thrust of about a quarter its weight, and took a lot of runway (it was fast in flight as much because of the low drag design, as because it was jet powered); the F-86 Saber Jet had thrust close to half its weight, and could still (barely) break the sound barrier in a shallow dive; the F-104 Starfighter had sea level thrust slightly higher than its weight (though that decayed a bit by the time you hit 50,000 feet), and the F-14 Tomcat had thrust significantly higher than its weight (and could outclimb virtually anything else flying as a result).
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ScaleAero
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« Reply #2 on: December 05, 2011, 10:21:50 AM »

Typically the thrust to weight ratio for successful "scale" recip or electric ducted fan use is each pound of thrust (#) is capable of flying a model weighing 1.62 times or 162% of that thrust value. Note, flight is defined as sustained flight utilizing the lifting and contol surface... not a 1:1 thrust to weight ratio which is not realistic.
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Hepcat
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« Reply #3 on: December 06, 2011, 12:28:09 PM »

It sounds to me as if you know the answers already so I don’t know if the following will add anything.

For level flight, thrust needs to equal drag so level flight thrust equals weight divided by the L/D ratio.  Greater thrust than this will cause the aeroplane to climb.  If the level flight thrust is called To and the available thrust is called T then a book on aerodynamics will give you the following equation:

T/To = cosA + (L/D)sinA  where A is the angle of climb.

Example:
Using your figure of W=1.62T then T=W/1.62 and assuming a L/D of 8 then To=W/(L/D)
Then  T/To=(L/D)/1.62=4.94
Putting this value of T/T0 into the above equation and solving for A gives a climb angle of just over 30 degrees.  It seems to me that the suggestion of 1.62 times the weight is realistic.  As a matter of interest a better (higher) L/D ratio will give a smaller angle of climb.

John
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ScaleAero
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« Reply #4 on: December 06, 2011, 12:47:39 PM »

Unfortunately, with a hunger for speed as the premis for jet model flight Vs. scale maneuver and performance with realistic angles of attack...you end up the day with a full trash can at your flying field... ;^) That sound you hear right now is the hiss of the vaccuum on your checking account when speed is the goal. Jet modeling is experience based....typically the candle lit at both ends will extinguish itself half way through its anticipated existance... Grasshopper... ;^)
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« Reply #5 on: December 06, 2011, 01:26:48 PM »

Hi Mike,  When I started RC I was just enthralled with the prospect of flying jets.  But it does take quite a skilled pilot to fly anything that is going to be FAST, like you're talking about.  One tiny over control and you have a nice pile of junk. 
There are all kinds of power systems out there for just about anything you need.  One source is http://www.ductedfans.com/
  They carry what I consider the top of the line, Schubler, which is a composite fan that come dynamically balanced.  And they usually can handle a bit more RPMs than the run of the mill fan unit.  Another good fan is Wemotec.
  Probably the best place to ask about a power system setup for a particular model is to go to RCGroups or RCUniverse.  There are people there who are doing what your son wants to do, push the speed envelope.  Just describe the model he has, what kind of performance he wants, and I'm sure someone will recommend a power package that will get the airframe screaming.  Caley
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ScaleAero
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« Reply #6 on: December 06, 2011, 01:41:06 PM »

Actually dynamic thrust is going to give you less speed with realistic scale maneuverability. This would be the result of a higher pitch loading of the edf/rdf with a blade pitch that results in greater initial thrust at the bottom end of the rpm spectrum Vs. that of a less critical pitch blade as found in the Schubeler and Viojet.

The Dynamax provides an excellent value for a scale edf project with the capability of delivering in "excess" of 25# of "usable" thrust when it is packaged with an appropriate electric motor. Appropriate being the operative word here... ;^) The XPS packaging of Tommy's Dynamax has the greatest return on investment (ROI) for any larger scale project with the "headroom" to provide excellent speed across an exceptional range of rpm/aka airspeed...the XPS-Dynamax is currently the King of Speed. Which should make your son happy...yet it can drive a what-ever with equal performance.

The latest battery technology will yield in excess of an 11 to 12 minute scale contest flight round sustainable power curve to enable one to manage their throttle to deal with all those unanticipated and uncontrollable incidents which occur at such an event. While Stu Maxwell's Stumax provides close parity to the Dynamax, it is alas manufactured in Australia or roughly 180 degrees from you... :^)

The price point Vs thrust is no issue with respect to the Dynamax's scale flight sustainability. Our 157" w/s B-47's will bifurcate a pair of XPS-Dynamax to sustain flight at our AUW with batteries of 31# with appropriate angles of attack through the required 9 maneuvers.  
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ScaleAero
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« Reply #7 on: December 06, 2011, 01:49:01 PM »

While too often "foamies" catch the eye of a relatively new modeler with some sense that a light wieght foam airframe will in turn provide the greatest performance with a hybrid motor fan combination....the reality of this activity is a waste of money on a parent's part because this is a DYI element of this hobby...or as in the world of a boat owners' point of view....a vortex into which $ disappears at an alarming rate.

EDF and RDF or even turbine flight can be an enjoyable endevor yet not require a third mortgage to participate in. Turbine was the only way to replicate a jet but it requires you to attain a level of mechanical and piloting skills that typically are beyond that of the typical young aeromodeler in "today's world". RDF will tend to consume larger amounts of time, energy, & patience, as well as provide less performance and satisfaction than the better alternative of EDF.

EDF, as I have explained in previous post, provides sustainable reliability without the pilot/builder having to invest an inordinate % of their modeling experience to qualifying for and maintaining the issues related to kero, Jet-A, or Jet-A1...in your country. I recommend comparison of flight styles your son desires to attain...using YouTube to prove the value of any given airframe, power management (ESC), EDF (BRANDED Indentity as a sold as configuration...not a DYI hack of an EDF), and explicit batteries. By explicit I mean brand, voltage, amp capacity, cell count, serial/parallel or combination(s) of same that result in the video performance you witness in the YouTube video.  

Brand loyalty has become an issue with respect to understanding differences in the EDF units unless you have a huge population of multiple brands or dive deep within the EDF community and find a mentor who has been there and done that.... That will occur, be patient

I have placed images of the Dynamax and the StuMax in a similar nacelle application so you can see they are relatively the same size at 5"~5.125" diameter.
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