Lake Flyer

My Feedback: (13)

Being that I am between float planes at the moment (there all broke), I picked up a Sig Sealane kit just to get back in the air quickly. A search on the Sealane here at RCU revealed that it changes pitch at different throttle settings. I had a previous flying boat that did this and it was a royal pain. So I decided to check the wing and engine incidences. The airfoil is your basic Sig flat bottom they use on most of their kits. The engine is set up so it is parallel to the bottom of the wing. However when you draw a line through the wing chord the engine now has negitive incidence. I have never seen a set up like this on a flying boat before. Past experience tells me that I would be a lot happier if the engine were 0-0 to the wing chord. Any of you experts want to add your $.02 on what I should do with the incidence?

JimCasey

My Feedback: (1)

SInce the engine is high-mounted, it has leverage to push the nose DOWN when you add power. So the engine should have UP incidence, so that when you add power, the engine lifts, too, and the nose of the plane is not forced down. SInce the engine is now meeting the air at a siginificant angle, a liberal dose of right thrust is also in order. Most flying boats also have a high-mounted horiz stab that lives well inside the propblast. The downwash over the stab from the inclined thrustline also helps keep the nose up when power is added.

The recipe I used in the last Flying Boat I built: Not a sealane but still had the engine on a pod: Wing incidence zero, thrustline up 3 degrees and right 4 degrees.

Completely opposite to this: On a high-wing trainer, the thrust line is significantly LOWER than the main source of drag-the wing, so adding power will make the plane balloon. Adding downthrust in this case cancels the tendency for the nose to point skyward when power is added. ALso, the stab on many trainers has positive incidence to help keep the nose down when power is added.

jrf

My Feedback: (551)

Does the Sealane climb or dive with increased speed? If it climbs, as most flat bottoms do, adding upthrust will make the problem worse.

Stab incidence is an important factor in this kind of behavior as well. What is the incidence of the stab in relation to the wing?

Jim

Lake Flyer

My Feedback: (13)

Here's the incidence lines on the plan.
jrf, The stab incidence is the same as the bottom of the wing. All the flying boats I have had,even the flat bottom wing ones, upthrust in the engine.
I am thinking along the same lines as Jim Casey, my experience has been that all pod mounted engines, even those with upthrust tend to push the nose down. All the kit reviews I have read on the Sealane say it turns into a submarine on takeoff. I can see why.
I think I am going to leave the stab where it is, call it 0, the wing chord will be + to the stab, and I will make the engine +1 to the wing. That is pretty much what we used on the Piranha's we built.
This means I will have to move the front of the engine up, or the back of the pod down, or split the difference. If I move the front up even a little bit I will be able to swing an 11" prop which gives me a greater range of engines to chose from

JimCasey

My Feedback: (1)

jrf: Speed is different than thrust. If he aborts a landing, I want him to be able to punch throttle without making a big splash. With a fixed-pitch prop, thrust drops off as speed increases.

Theoretically a (true) 6" pitch prop turning at 12000 rpm makes zero thrust at 68.181818 mph. But the airplane is going fast enough by then so it would be a good idea to unload the wing or the plane will balloon up because of the speed, not the thrust.

jrf

My Feedback: (551)

Jim Casey: No disagreement here. He will definitely need upthrust to counter the configuration down-force under acceleration.

However; I've flown the Sealane and my recollection is that it climbs at higher speeds and dives at lower speeds. Enough that re-trimming is required for level flight. Nothing unusual about that in a flat-bottom wing. But adding upthrust will make that characteristic worse.

Since the stab is negative to the wing, it's not surprising that the airplane changes trim with airspeed. The fact that the high mounted engine is set at 0 to the stab means that the Sealane is, in effect, using the configuration down-force on the nose to balance the down-force of that negative stab at the tail.

So what is my point? It is this; if you are going to add upthrust to make getting off the water easier and go arounds safer, it would be a good idea to reduce the negative incidence of the stab in order to better balance the flight forces. I'd try setting the stab at 0 to the wing chord and the engine at 2 degrees positive to the wing chord as a starting point.

On the other hand, if you just want to avoid "submarining" on the take off run, add spray rails.

Jim

Lake Flyer

My Feedback: (13)

jrf, I'd planned to add spray rails anyway. It's the retrimming every time you change throttle setting I wanted to try and cure. The one flying boat I had that did this was no fun to fly at all and I don't want another one like it. I will give some consideration to changing the stab incidence.
I guess what has me confused is the one pain in the ***** boat I had had +3 on the engine and 0 on the wing. I don't remember the stab setting but this thing went down at high throttle and up at low throttle, and the trim changes were extreme. The original designer was the only one who could consistantly get his design off the water. I was told recently that he did it by going across the water at high speed then pulling the throttle back so the nose would rotate. The way that thing flew I believe the last part of the story.

Lake Flyer

My Feedback: (13)

Ok I thought about this overnight, forgetting about my boat from hell. jrf, said the plane climbs with power and dives when you throttle back. Since the engine and stab are negative to the wing chord then the wing chord must have to much positive incidence in it. So shimming up the trailing edge should cure the climb under power, I think????

jrf

My Feedback: (551)

LakeFlyer: These relationships are complicated. I said it climbs at high speed. It will nose down when you apply power from low speed. It dives at low speed, but it will climb when you chop the throttle from high speed. That is the nature of high lift (flat bottom) airfoils.

You are correct, changing the wing incidence would normally be part of the solution, but doing that will change the incidence between wing and the fuselage bottom. That would not ordinarily be a problem on a land plane, but on a flying boat you can't change the angle of attack on the water very much with elevator. In other words, it may not take off. That is why I would recommend changing the engine and stab incidence and leaving the wing alone.

Adding upthrust will minimize the pitch down when you throttle up from low speeds, while the reduction in negative incidence in the stab will reduce the tendancy to climb at high speed. Obviously these changes have opposite effects, so you need to balance them to obtain the performance you want.

Jim

manix

A thing that really impressed me about the Seamaster (Satio.72) is how pitch stable it is when making large power changes at just about any speed.

alexei111

What are spray rails?

JimCasey

My Feedback: (1)

Spray rails are deflectors mounted to the corners of the hull. A lot of the water displaced by the hull goes sideways, and the spray rails turn that down so there is not a curtain of water getting into the prop ( More likely with twin floats and a front-mounted motor). When the spray rails deflect the spray DOWN, it creates a reaction force the lifts the hull, so the plane pops up more quickly onto the step. Pretty easy to make from thin aluminum or plastic, or from triangle stock.

PeterC

I just have to jump in on this one. As has been noted the Sealane has a large pitch tendency and the Seamaster does not. I would think that the Seamaster has some but just not as pronounced as the Sealane; so whats the difference? The most obvious is the airfoil. As has been mentioned speed and the application of power create undesireable results. A lifting airfoil will generate lift at a slower speen than a symetrical one so when power is applied to gain speed there is a greater tendency to generate lift than a symetrical airfoil. The lifting airfoil is essentially a one speed airfoil and if you try to make it fly faster it is going to generate lots more lift and thus pitch up. If you look at where the forces are located on a seaplane (lift, drag, thrust and weight) it seems obvious that ALL designs are going to have a pitch change when power is applied except some will be more pronounced than others. The closer the forces can be grouped the better. Look at aerobatic planes for examples of a design that have all the forces passing through a common point. I think the Seamaster does the best job of grouping these forces than any other seaplane design out there so it probably is the best flier of the entire selection. Personally I like the nuances of a seaplane. IMHO
Peter

0.k.lakeflyer

I have a 40 size pond hopper, which had a pusher setup now converted to a tractor setup 46ax 12x4 apc while trying it out on the bench under aceleration seems that the nose is being forced down tail wants to lift, do i need upthrust on motor, if so how much, how do i measure it. I am confused as to what do you measure the upthrust, top of wing or what?

PeterC

You must remember that the Pondhopper was originally designed for a 40 engine so you are probably using a lot more power than was originally envisioned. I would suggest,
1. going easy on the throttle when accelerating; 2. increasing your up elevator throw to counter the nose down pitching, 3. using lots of exponential (-80%) to soften the elevator when airborne. This should be a good start to good flying.
I don't think upthrust will help much for the problem you are describing but, all incidence and thrust measurements should be measured from a common datum line which is usually parallel to the stab. Since seaplanes have wierd thrust arrangements because of the high thrust line I would make my measurements relative to some easilly found datum like the top of the fuselage side. If you consider the datum line to be 0 degrees then record the other angles relative to that; ie: wing +2, stab 0, engine +4 or whatever.
This should help sort it out.
Peter