Aerodynamics Discuss the physics of flight revolving around the aerodynamics and design of aircraft.

What happens if I raise engine on air frame?

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Old 01-04-2018, 10:59 AM
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ziggy138
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Question What happens if I raise engine on air frame?

I am starting on a pylon mounted engine sea plane. Plans are for 12 inch prop clearance and the engine I will be using takes a 13 to 15 inch prop. I would like to raise pylon about 1 1/4 to 1 1/2 inch higher than plans call for. This sea plane has 0-0-0 incidence angles. By raising engine higher than design call for give me any problems in the way plane flies? If so, what can I due to help stop problems? This will be my first try at flying a sea plane and I hope to have a good flyer. Building is no problem, but Aerodynamics are well out of my skill set. PLEASE help me out, I need ALL the help I can get.
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Old 01-04-2018, 12:59 PM
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JohnBuckner
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If the airplane uses a tractor pylon (engine facing forward) there will be nose down pitching moment every time power is applied and raising the thrust line higher (for prop clearance ) will make this even worse. The norm for single hull type seaplanes using a tractor pylon is to use up thrust and if it were me I would build in about five degrees of up thrust.

Now most folks have a far easier time transitioning to twin float ships rather than the single hull types and that is what I would tell anyone to start with first.

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Old 01-04-2018, 03:44 PM
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ziggy138
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Default Engine thrust.

John thanks for quick answer. Yes engine faces forward. I am starting a Sea Dancer , 72 inch wing span for a 60 two stroke. I am thing of using a RCGF 15 cc gas engine with a 14 inch 3 blade prop. I picked this plane because of good to great write ups in reviews. I am a fair pilot and thought I would be able to do some basic aerobatics better than with two big floats hanging down under fuse. Sea Dancer is a take off of Cloud Dancer pattern plane. I just thought it would be a better plane than a Cub. I have not even seen a seaplane fly yet. Now you know why I am asking for HELP.
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Old 01-04-2018, 06:07 PM
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JohnBuckner
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Yes both are fine airplanes. I had the opportunity to meet Fred Reese the designer of both a short time before he passed away at one of his clubs night flys on Jean dry lake near Las Vegas. At those events I usually took along in addition to the RC night flyers, a .35 sized controlline stunt ship to fly while waiting for total darkness.

Well Fred was quite a successful competitor, designer in controlline back in L.A. long ago. And he took a great interest when I was flying my ship so I just had to pressure him into flying my airplane, I think he said his last flight in the circle had been 30 or more years! Well he just could not stand it and relented. He got to make one flight and it was a good one even presenting a reasonable stunt pattern. Man was he tickled the rest of that nite. I think it was about a year later and heard he had passed.

Sorry for the digression. I would go with the same thrust line Fred had used (can,t remember if he had used much up thrust) . Now just moving the thrust line up while retaining the same thrust line angle will technically increase the down pitching moments with power. However every thing is relative and small changes are probably not a problem. So that begs the answer how much do you need to raise the pylon??


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Old 01-05-2018, 09:14 AM
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ziggy138
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John you are a lucky guy. I would have loved to have meet him in person. I have 3 Cloud Dancers, 40 - 60 -120 sized. They are my go planes for just pure fun flying. I am still waiting for plans to get here. Mailed to me on 29 Dec and still not here today. Talk about SLOW, U S Postal Service must still be on vacation. I may just go with two stroke an fly it as Freed designed plane. I can always build a bigger one and power it with a DLE 20 just like my 120 sizer Cloud Dancer.
As per higher thrust line, what do you think would happen if I mix throttle with elevator? I need to raise pylon 1 1/4 to 1 1/2 inch higher than plans call for. Freed said a 12 inch prop caused water spray problem and he used a 11 inch to fly with. I plan to use a 14 inch three blade prop so I will need about 1 1/2 inch more clearance. Could use a 13 inch prop but I think that would give to much RPM and less thrust for 15 cc gas engine. 14 inch is middle of road prop and 3 blade should help keep RPM down and give a little more thrust if my thinking is right.
You said that I should start float flying with twin float plane over a single hull. WHY ? Thanks for help. Ken
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Old 01-06-2018, 10:37 AM
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For a pylon mounted sea plane I'm a little surprised it does not use some up thrust on the engine. A little upthrust aids with combating the nose down pitching tendency with power that high above both the main source of drag while on the water and also the main source of drag when flying. Namely the hull for the first and the wing for the second.

Raising the thrust line will increase that moment so I'd suggest you at least make some accommodation for angling the engine upwards a couple of degrees. If you don't need to then fine. But if you find the model trying to dig the nose into the waves then it might fix the issue.
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Old 01-07-2018, 07:39 AM
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Default Engine thrust line.

So far all replies say I should give at least 2 deg. up thrust, or more. If I do raise engine higher above hull and wing I will add the 2 deg. of up thrust to engine firewall and 2 deg. right thrust built in. NO ONE has come up with any ideas about mixing throttle and elevator to help with controlling pitch???
Having NO float plane experience I must trust help from more experienced fliers. SO please help me.
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Old 01-08-2018, 11:17 AM
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John's suggestion of 5 degrees is a good starting point. You might be able to look at seaplane designs of the past on outerzone, see McGovern, Willard for two designers that did a number of designs. Not really a fan of mixing functions until the aerodynamics are right.
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Old 01-13-2018, 01:39 PM
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da Rock
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It was designed for 12" prop clearance? There is another option that could work, and certainly has in the past in full scale and on my models. Use a 12" prop. A 3-blade. They have only two drawbacks.

Finding them can be an effort, and a slight decrease in efficiency (that won't be noticeable to most modelers). Almost my whole squadron uses them. I've got a hangar full of 60 and 90 size aerobatic and warbird models. The aerobatic ones are propped with them because the dia/pitch 3's that are available give me better thrust. The vertical performance is stronger with less loss of speed. Those aerobatic models don't need to be propped for speed anyway. The warbirds on the other hand can be pitched for speed. My bigger Corsair has been clocked at 118 on radar. It was a bit of a surprise since the prop wasn't chosen for flat out speed. But it got it. It's also partly from running it rich and the power from the OS95.

I've got a couple of electrics (warbirds) with 3s on them. I actually picked the motor to match the prop/model combination for one of them. It's got very good acceleration. It's a hoss.

Efficiency ever give a problem? Never noticed. In fact, I've been waiting for a very long time to meet someone who has actually measured it's effect in models.
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Old 02-22-2018, 07:12 AM
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Default Sea plane with raised engine pylon.

I did raise engine pylon 1 inch so I can use a 13 or 14 inch prop. I am going to try first flight with a 13 x 6 three blade, but also have a 14 three blade prop just in case. As I over built plane and it weights close to 10 lbs. I changed engines and put in a Supper tiger 90. I am sure this is more than enough engine and should be able to fly at 1/2 speed. My Sea Dancer is all ready to go, just need some warm weather up here. to try her out. Thanks to everyone for help. K C
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