amateur built floatplane question

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ruddersup?
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amateur built floatplane question

Post by ruddersup? »

I put floats on my amateur built floatplane and I'm wondering how do you determine what size ventral fin to install?
Is this determined by flying or calculation?

Hoping PilotDAR can help or?
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photofly
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Re: amateur built floatplane question

Post by photofly »

I can’t be the only person hoping this wasn’t plan A for determining this important piece of information, can I?
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ruddersup?
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Re: amateur built floatplane question

Post by ruddersup? »

I think that's a snide comment. Maybe I should have put "IF" at the start of my sentence but then you wouldn't have commented. Appreciate your input.
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AirFrame
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Re: amateur built floatplane question

Post by AirFrame »

The answer is "it depends."

How do you know you need a ventral fin at all?
What kind of plane and what kind of floats?
What analysis have you done to the airframe for the added structure to add the floats?
What are the current stability margins with and without floats attached?

You'll probably need to know all of this before you get Transport Canada to inspect it.
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ruddersup?
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Re: amateur built floatplane question

Post by ruddersup? »

Airframe - thanks for all that. I'll spill the beans - has been on floats for 20 years. It has the ventral fins on the horizontal stabilizer but I think they need to be larger. Question? How do you determine size?
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Re: amateur built floatplane question

Post by TrilliumFlt »

ruddersup? wrote: Wed Jul 14, 2021 8:14 am Airframe - thanks for all that. I'll spill the beans - has been on floats for 20 years. It has the ventral fins on the horizontal stabilizer but I think they need to be larger. Question? How do you determine size?
Your not giving us much to go on here to "help" answer your question.

What is the make and model of airplane ?

If the pervious set of fins where adequate why do you think they are now insufficient, i.e. engine change

Your asking a very specific question without providing any of the salient details so an "educated" guess is the best you can expect. :|
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ruddersup?
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Re: amateur built floatplane question

Post by ruddersup? »

It's amateur built and a one off so knowing what it is won't help.
I've flown it a lot and yes it has a recent more power upgrade of the engine. Just the way it flies, tracks, handles crosswind seems to me that a little more control from the tail might help. Larger rudder would really help also. Don't want to get into handling qualities though, it's fine, but I think it could be better. Should I measure the surface area of the floats, same with tail area and then longitudinal dimensions? Is there some sort of a formula (equation)? Something like positioning the float step a percent back from ????? Don't want to go down that road, lol.
I'm guessing but I think it really is trial and error to fine tune. Probably everything about an aircraft is really trial and error and that's why we have test flights, I guess. If you "really" know how to do it, please let me know. If you were to design a floatplane what would you do about the tail surface area to counter the float area? Does float shape enter into picture also.
I do appreciate everyone's input.
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Scuderia
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Re: amateur built floatplane question

Post by Scuderia »

If you haven't already tried, give a shout to your local EAA or RAA Chapter. I suspect someone there will be better able to point you in the right direction. Or someone from MDRA.
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Re: amateur built floatplane question

Post by PilotDAR »

There are many variables, but yes, in general, the addition of floats will require a noticeable addition in vertical tail area, or something to compensate in yaw control.

Too little (no added vertical fin) or too much (too big an additional fin) and crosswind handling will suffer. Too large a rudder, and rudder forces go up, too little, and you can run out of control (can't sideslip). Those are pretty obvious. Less obvious will be a loss of positive yaw stability, meaning that feet off the pedals, it won't tend to center the ball (dangerous to stall, as it is more inclined to spin). This is a characteristic of 180/185's, and the reason for the higher dorsal fins on 185's than older 180's. Some 185's do not have ventral fins, but rather a rudder centering spring kit to supplement pilot pedal centering forces. I once was given a 185 to test fly, in which this kit had been forgotten when the floats were installed - yeah, it sure needs that kit - horrible to fly without!

There may be formulae for this, that's beyond my skill set. I've done the assessments by flight testing. 'Enough control to fly crosswind approach with a 90 degree crosswind speed of at least 0.2Vso? But not so much that it weathercocks into the crosswind and is hard to maneuver? That's the ventral fin or finlets characteristic.

Test for enough rudder to oppose full aileron deflection. If you reach full rudder well before reaching full aileron in a stable sideslip, you may need more rudder size. Increasing rudder deflection is usually a less good idea. If the rudder size is changed, it must be mass balanced, and flutter tested, which is demanding and potentially dangerous testing - avoid.

Test for free return of the rudder when pedal pressure removed in slow cruise flight - rudder system friction. All the added pulleys, cables and water rudders can over friction the normal rudder return aerodynamic forces.

Then, test for "rudder lock" (the desired tendency for the ball to self center when a sideslip at high power is released). Power into slow flight (1.2Vs), and apply full rudder, while opposing to wings level with aileron, when you let go of the pedals, do they return to ball centered as you release the aileron? I have flown engine changed 180's (small dorsal fin) on floats, which will not return, they just hang until you apply opposite rudder. If it's really bad, it takes lots of pressure on the pedal, as though "locked". I've never had a 180 which "locked" but I've had a few where the rudder would not free return - again, a less safe plane to stall - 'could spin.

Yes, I have test flown floatplanes to determine the need for additional ventral fins or finlets. I suspect that their size was eyeballed, but the effects were as desired. This handling is kind of subjective, and if you're used to flying the plane as has been for a while, you're probably used to masking a deficiency with skill - like taildragger flying. If you want to read up on this, I suggest the book "Cessna, Wings for the World" by Thompson. He goes into lots of detail about original Cessna flight testing of the floatplanes, and the aerodynamic fixes.

For further reading, I recommend "Flight Testing of Fixed Wing Aircraft" by Kimberlin, it has excellent descriptions of these characteristics and test methods. Oh, and it has the formulae! What I have written here is a light "touch the cloth" of the whole topic, there' more to it...
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Re: amateur built floatplane question

Post by AirFrame »

ruddersup? wrote: Wed Jul 14, 2021 6:59 pmJust the way it flies, tracks, handles crosswind seems to me that a little more control from the tail might help. Larger rudder would really help also. Don't want to get into handling qualities though, it's fine, but I think it could be better. Should I measure the surface area of the floats, same with tail area and then longitudinal dimensions? Is there some sort of a formula (equation)? Something like positioning the float step a percent back from ????? Don't want to go down that road, lol.
The short answer is yes, there are *lots* of formulas to work through to answer what you're looking for analytically. I recommend finding a copy of Dick Hiscocks' book on light aircraft design, or perhaps "The Design of the Aeroplane" by Darrol Stinton. Both would get you started on the path.
I'm guessing but I think it really is trial and error to fine tune. Probably everything about an aircraft is really trial and error and that's why we have test flights, I guess. If you "really" know how to do it, please let me know. If you were to design a floatplane what would you do about the tail surface area to counter the float area? Does float shape enter into picture also.
I do appreciate everyone's input.
Yes. All aircraft get fine-tuned after they're built. That's why we have trim tabs.
No, most of it is done analytically and then flight tested.
I do, but honestly it's a lot more work than just one formula to get this answered for you.
I would design control surfaces that gave adequate stability on paper, and fine tune afterwards if necessary.
Float shape definitely factors in, both to yaw and pitch stability.

It sounds like you have a system that's pretty good to start with. It also sounds like you don't have a ventral fin (ie. on the fuselage ahead of the vertical stab), you have rudderlets on the horizontal stab/elevator (giving you a "triple tail"). You could probably make new rudderlets that are, say, 10% larger, and see if that helps. Don't double their size right off, make small changes, test, and move on. Make sure you recalculate your CofG each time.
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Re: amateur built floatplane question

Post by Bede »

Every float pilot thinks they need a bigger VS :) I have an Avid Magnum on floats. It has a ventral fin and it still isn't enough IMO.

You can check out the books by Daniel Raymer. Here's a link to information from Chris Heinz: http://www.zenithair.com/kit-data/ht.html see the one on the Horizontal Tail. It's the same principle for the VS though.

I have a book at home that has the information, but alas, I am not at home. I'll see what I can find when I get home if you need more info.

Here's what I would do: Figure out your VS volume* (VS area * wing area / distance from center of lift (wing) to center of lift (tail) * MAC) - see Heinz formula. Now try to figure out what this "volume" would be for your floats. Now design a ventral fin with the same tail volume as the "float tail volume". My guess is that this ventral fin will be way bigger than what you want so there will be a size/stability trade-off.

The other thing that you can do is look at existing floatplanes and see what increase of tail volume (in percent) that they use and design a ventral fin with that % increase in tail volume.

A lot of this is trial and error BTW, but this will get you started.

Good Luck!

*Don't get confused by "volume" as height x width x length. Tail volume is called that because when you work out the dimensions you end up with units^3 - same as a conventional volume.
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ruddersup?
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Re: amateur built floatplane question

Post by ruddersup? »

Wow - much more than I bargained for. Thank you everyone. I've got some reading to do. I new the forum had lots to give - never underestimate a forum, huh?
My thought right now is to extend the "rudderlets" and test fly. Changing rudders size is more challenging but I think it will have to be done down the road too.
PilotDAR - IMHO do not side slip a float or ski plane. I should not be here anymore because ...........
Bede - great help - plane is good but could be better - same as mine. I really only notice the decrease of directional control in turbulence and crosswind landings. It is an amphib btw so even the gear hanging adds to the crosswind component. I know the extra long wings contributes to yaw and demands more from the tail but no more to give back there. If you want an aircraft that is the ultimate stol aircraft I think there are trade offs.
Got some work to do.
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Re: amateur built floatplane question

Post by PilotDAR »

PilotDAR - IMHO do not side slip a float or ski plane.
I cannot speak for non certified planes. For any certified type, in any landing gear configuration, has to show compliance to full rudder sideslips. Stalling with full rudder applied is a poor idea, but down to 1.2Vs, a slip should not be overly concerning.
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ruddersup?
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Re: amateur built floatplane question

Post by ruddersup? »

IMHO - do not side slip a float plane or ski plane - maybe a test pilot can demonstrate but I'm not one. Floats and skis offer lots of drag - not necessary to side slip but maybe one wants to show off. Side slips are done close to ground - very unforgiving they say. There is a strong possibility that air is diverted away from the tail - maybe not this time but oops next time. Please listen to experience.
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Re: amateur built floatplane question

Post by PilotDAR »

maybe a test pilot can demonstrate but I'm not one.
Well, my courteously stated comment is that if you're flying an non certified plane, to some degree, you're a test pilot. The certified planes have been test pilot flown, non certified, not so much. I'm not saying that non certified are not good planes, they have just followed a less regulatory path to service.

I have flown a few non certified types which exhibited some non certifiable characteristics. Not necessarily unsafe, just a lesser margin of "does not require unusual pilot skill and attention". That said, the design requirements for certified planes are very well established, and a good starting point.
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ruddersup?
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Re: amateur built floatplane question

Post by ruddersup? »

IMHO - do not side slip a seaplane or ski plane. I did with a Cessna 170 on wheel skis and I was taught in a Champ on wheels so I'm sure everything is okay to play around in other planes. I say again - never...........
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Re: amateur built floatplane question

Post by TeePeeCreeper »

ruddersup? wrote: Thu Jul 15, 2021 3:35 pm IMHO - do not side slip a seaplane or ski plane. I did with a Cessna 170 on wheel skis and I was taught in a Champ on wheels so I'm sure everything is okay to play around in other planes. I say again - never...........
I’m genuinely curious as to your “never slip a ski or float plane” comment.

I’ve only flown my C180 (fixed pen skis) and she slipped just fine. Never thought twice to not slip it while in float config. Having flown several types on floats, (from flapless Citabria’s to amphib C208’s) they all slipped without any protest just fine.

What’s the story? What kind of odd ball “one off” type of home built are you operating? Why the need to be so secretive?

Always learning so I would really like to hear your “take” on it.

TPC
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Re: amateur built floatplane question

Post by ruddersup? »

from PilotDAR - cannot speak for non certified planes. For any certified type, in any landing gear configuration, has to show compliance to full rudder sideslips. Stalling with full rudder applied is a poor idea, but down to 1.2Vs, a slip should not be overly concerning

Can you really trust your airspeed doing a side slip? Apparently down to 1.2 Vs you can. Really? Are you even looking at the airspeed during side slip? I don't.

TeePee - If I've planted a little doubt in someones mind about side slipping a float plane or ski plane on short final I'm happy. Don't ask me about the theory of the whole thing all I know is the Cessna I was flying shook me up and I'll never do it again. Sure it's been demonstrated at certification.
The Beaver full flap setting is another - sure you can - but generally not recommended by experienced Beaver pilots- I've done it hundreds of times but in certain situations.

Just planting a seed - not sounding like I know anything - experience only.
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Re: amateur built floatplane question

Post by AirFrame »

ruddersup? wrote: Fri Jul 16, 2021 5:13 amJust planting a seed - not sounding like I know anything - experience only.
What you're planting is a seed of FUD (Fear, Uncertainty, and Doubt) of using slips--one of the more valuable tools a pilot has in their arsenal. And you're doing it based on a vaguely described, unsubstantiated event... "this one time" in a Cessna.

Without more details, your post comes off as an old wive's tale. Was the slip really the problem, or could it have been something else that happend to correlate with the time you were slipping?
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Re: amateur built floatplane question

Post by PilotDAR »

Can you really trust your airspeed doing a side slip?
Certainly not near the stall! A reference to the IAS to CAS tables is pretty important, and there is a likelihood of additional error from the yaw, which is not documented.

There is little operational need to sideslip at 1.2Vs. And, slowing toward the stall during a slip really is of little use, unless you're pulling into the flare in the slip (which I will do on wheels, but not floats). A sideslip has more benefit in creating drag at a faster speed, as drag increases as a square of the speed, so unlikely you're near the stall while slipping operationally.

For certification, it's required to demonstrate down to 1.2Vs to assure that the plane has no bad habits there. During Grand Caravan flight testing last winter, TC required me to demonstrate a stall to the break in all flap settings, 75% power, 30 degree bank, and one ball (which was full pedal) of slip. Yes, that is an intentional spin entry. Though as soon as the nose dropped, I recovered, so no spin developed. The plane handled it just fine in all cases with an easily recoverable wing drop with pitch down. Then, I had to fly as right seat PIC, so another pilot with no Caravan time could repeat it for formal certification.

Like some 172's, the 170B has the outboard end of the flap coincidentally located such that in a slip, a wake off the extended flap can burble over the horizontal tail, and momentarily reduce its effectiveness. It is a thing, and I have done it a number of times. That's why some 172's are placarded to avoid slips with flaps extended. During certification, it was deemed to be acceptable (so no "prohibited"), but I agree, it can catch a pilot unaware on a calm day.

Here's why I like slips, even on planes with effective flaps: You can plan a deliberately high approach, slip as required to set up your approach path, and once established with the runway made, extend flaps. The slip can be modulated, stopped, and restarted as required. Flaps, once extended, should be left out. So, if you misjudged, you're going to undershoot a power off landing. I will regularly practice a forced landing from deliberately high, slip most of the way in to set up the approach I like, and extend all of the flaps very short final. I did not use the flaps for glidepath control, so could unslip if I'd misjudged. But, I do want full flaps at touchdown. If this means touching the first mainwheel in a slip, it slides a bit, okay...

A slip is a tool, and a particularly useful one, if you must get into a tight spot. I like to think that pilots practice with all the tools appropriate to the type, but yes, you could fly a career without slipping - I think I have met pilots who have done it.
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