New tandem tractor design and build

[ILikeJetsToo]

I know this write-up is a little long, but please read the following and comment with constructive critique and technical commentary with the intent of constructive design improvement.

I’ve debated for a while about posting project info on forums. In the past, I have completed large projects (aircraft and other) completely in the privacy of my personal shop, and I have also shared all the details of other large projects openly on forums. I have had mixed feelings about the process both ways. One of the main reasons for deciding to post here now is for everyone to learn and discuss for the betterment of the experimental aircraft community.

Professionally, I’m an aircraft design engineer. I have worked in the engineering field one way or another for the last 18 years. I’ve worked on many design projects over the years, and I think I’d like to share some information here about my current build for your review. I respect the talent and experience in this community, and I'm interested in constructive feedback.

:: PROJECT BACKGROUND ::
I’ve been working on this project off and on for about four years. I started the project by listing out my aircraft desirements and sub-selecting configurations that met those desirements. I then went out and got training in several specific gyros to better understand the fine differences in handling/performance from classic pusher gyros to several new European gyros. I also got additional training in trikes and flew a number of different fixed wing aircraft to double check that a gyro is what I wanted to go with. In the end, I decided on a two-place tandem, tractor gyro.

:: DESIGN INTENT ::
The intent of this project is to build a one-off prototype, stable/docile, open, two place, gyro with good visibility for fun local flying and short cross country flights. I started my configuration study by benchmarking my basic layout against 15 other “technically relevant” (mostly older tractors) gyros for comparison.

:: DESIGN STYLE ::
I have always gravitated to the classic tractor designs, so I really wanted to design a new gyro that was a “modernized” gyro roughly based on a classic Cierva. Because this gyro was to be a modernized classic tractor, I had been calling it the “Variant”, but after the recent unveiling of the similar themed Bulldog gyro, I might rethink that name as it was never meant to be a “Variant” of the Bulldog.

As I got into this design, I wanted to try for a more aesthetically pleasing gyro layout than some of the gyros I have trained in. I also didn’t want to just put a fairing on a metal frame like many of the Euro-tub gyros do. Not that the “standard” metallic frame gyros aren’t very practical, but I find that they leave a little something to be desired from a modernized structural design perspective. I have often worked with structural composite design, so I decided to go with a completely composite structure and incorporate a classic theme to the stressed-skin monocoque layout. During the final conceptual design phase, I also built a plywood cockpit mock-up from CAD templates to check ergonomics and get a better feel for the sight angles and basic visibility.

:: CURRENT PROJECT STATUS ::
Right now, I have built or purchased 81% of the gyro by mass. I still have quite a few parts to make and lots of assembly work to do. Monetarily, I am 83% through paying for the parts and material (including tooling).

:: BASIC SPECS (more in the following text)::
-- 627 lb empty weight
-- 1,200 lb max design weight
-- 280 mile cruise range (with 35 min reserve)
-- Total length 16 ft (rotor removed)
-- Width 7 ft 5 in
-- Height 9 ft 3 in
-- Solo from AFT seat only
-- Rudder and brakes AFT seat only
-- Throttles and sticks both seats

:: STRUCTURE ::
Fully composite monolithic structure. The structure is primarily carbon fiber-282/epoxy-PTM&W/Divinycell-H45 with localized fiberglass-7725 and Kevlar-49 buildups. There is also localized carbon fiber-IM7 unidirectional tow reinforcements. The horizontal stabilizer is Epoxy-PTM&W /fiberlass-7725/extruded-polystyrene with unidirectional pultruded-Graphlite carbon fiber spar caps. The fuselage tooling method is a seamless composite shell molded over a sacrificial/removable foam male tool. The fuselage tool was CNC milled from the CAD outer mold line loft. The composite structures were sized using hand calculations and shell finite element analysis (FEA) to a minimum safety factor of 2.0 on stress and 3.0 on buckling.

:: WEIGHT AND CG ::
I checked 82 different weight and balance scenarios (example: heavy pilot, no passenger, full fuel) and iteratively refined the configuration to establish as little CG movement as reasonably possible. From a realistic low flight weight of 851 to a maximum takeoff weight of 1,200, the CG swings FWD_AFT +/- 1.8 inches and moves vertically +/- 0.6 inches. The vertical CG position is within 2.0 inches the crank centerline for all weight and balance scenarios. Depending on the part, I am also carrying a 5-20% weight growth margin on all still to be built components. The expected average takeoff weight is 1,055 lb.

:: TAIL GEOMETRY ::
The horizontal tail volume is 17.3% of rotor volume and compares well against other benchmarked tractors. The H-stab is a tapered NACA 0012 airfoil. The V-stab is a modified NACA 0020. The vertical tail volume is 67 ft^3. The vertical and rudder are positioned in the cleanest air available on the underside of the fuselage. The intent is to maximize yaw stability with the nose raised at high power and minimize spiraling-slipstream turn/roll tendencies.

:: INSTRUMENTS ::
Despite the temptation to go overboard with avionics, I want to keep the flying experience super simple and focused in flying, so there will only be very basic steam-gauge cockpit instruments and a Flightline FL-760 radio.

:: POWERPLANT ::
I compared 44 different engines for this application in the 85-150 hp range. Not surprisingly, the 912 and 914 were good options, but I decided to go with the Rotec Radial R2800 swinging a 76x57 Culver wood prop. I love the style and sound of the engine, and I pretty much based the gyro design around it. I have the engine now. It’s a little heavy at 224 lb, it takes auto or aviation fuel and is quite a work of art. The fuel tanks are structurally free-floating tanks that are mounted in the fuselage cheeks on either side of the front cockpit. Total fuel load is a little over 19 gallons.

:: ROTOR ::
Unmodified RFD 28 ft Cruze rotor with modified RFD double-bearing rotor head. The rotor is mounted to a relatively flexible “limber” composite mast. I have not selected a pre-rotor yet, but I am leaning toward a brushless “outrunner” motor.

:: FLIGHT CONTROLS ::
The rotor flight controls are very standard pushrod with rodends and mixed at the base of the rotor mast. Rudder and tail wheel steering is via aircraft cable. Flight controls are per ASTM F2352-05.

:: LANDING GEAR ::
The main landing gear are modified Cessna 140 landing legs (canted out for a wider stance and similar “root” bending moment to the heavier C140). The tailwheel is a Maule castering wheel assembly with spring steering links from the rudder. The gear is 3g at 10ft/sec per configuration applicable FAR23 load cases. The rolling stock is FAA/TSO Parker Hannifin 500-5.00 with single puck differential hydraulic brakes. All TSO and custom landing gear parts are structurally sized to a minimum safety factor of 2.0 using solid element finite element analysis.

:: DESIGN CHANGE OPTIONS I AM CONSIDERING ::
-Remove wheel pants
-Larger tires on 5.00 rims (380x150x5)
-Larger wheels and tires (maybe 600-6.0)
-Add H-Stab vertical tip plates for increased yaw stability
-Remove engine Townend ring (“NACA ring”)
-Change to underslung “torpedo” fuel tank

:: SENSITIVITIES I HAVE BEEN CONSIDERING ::
-Nose-over/prop-strike tendencies
-Tail-dragger ground handling sensitivities
-Drag penalties from a tractor configuration with open cockpit
-Windy flight experience
-Vertical tail/rudder effectiveness (x-wind , etc.)
-Limited FWD visibility
-Limited tractor training options
-Safety of fuel stored in fuselage

 

[Boots]

Michael,
Looks like you have done your homework . That is a Very nice gyro your building .
I will be watching Your progress on this one thanks for posting this cool project !

 

[Leptronjohn]

I am impressed. I would never have been able to go that far with out blabbing.
I will be listening in with much interest.
Thanks for sharing that. I look forward to all info as you go..
John oakley

 

[Abid]

I wonder why you selected a tractor.
I also wonder why you selected tandem for this configuration because the mast hinders the forward view of the back pilot tremendously as the mast is substantial. You obviously knew that as you have noted it. That would be my biggest negative and suggestion to remedy.

I am not sure what your structure is where the tailwheel is but that is an interesting idea if the structure could take the drop tests there and survive without damage.

The fuel storage also as you know should be able to stand emergency loads for it which are substantial. I am guessing you have put the fuel right behind the engine in front of the front seat pilot? or ??

 

[Matt_S]

Nice tractor! I know you have a bit of concern over the fuel tanks in the front seat cheek plates. Instead of looking at a belly tank, why not look at simple NACA23012 12% stub wing airfoil tanks set at -0.5deg & set close to the COG? This will outboard your fuel like done with a lot of fixed wings, plus provide enough lift to add some additional fuel for a longer cruise range. According to an old report (NACA #523) you might even be able to push a safe top speed up to 140mph without unloading the rotor too much using this method. Just a thought.
I'm curious, did you shop out your CNC work, or do it yourself? Looks like nice work!
- Matt

 

[DennisFetters]

Nice looking gyro. I always liked the look of the old classic autogiro's with a modern twist.

I would like to make two friendly observations;

1. Redesign your Center of Gravity where the rotorhead will be positioned over the passenger set. That way you will not need to rebalance with or without passenger.

2. When you find out you don't have enough vertical stabilizer, and you will find out that, just redesign the horizontal with two more verticals on the ends with moving rudders, and that will solve your upcoming problem.

Very nice build.

 

[Kevin_Richey]

What an exciting project...

What an exciting project...

Great thread on your build. Kudos for sharing it with all of us.

 

[ILikeJetsToo]

I wonder why you selected a tractor.
I also wonder why you selected tandem for this configuration because the mast hinders the forward view of the back pilot tremendously as the mast is substantial. You obviously knew that as you have noted it. That would be my biggest negative and suggestion to remedy.

I am not sure what your structure is where the tailwheel is but that is an interesting idea if the structure could take the drop tests there and survive without damage.

The fuel storage also as you know should be able to stand emergency loads for it which are substantial. I am guessing you have put the fuel right behind the engine in front of the front seat pilot? or ??

Thanks for the encouragement Rodney and John.

Fara,
Thanks for the questions/comments.

When I was starting this process, I modeled up quite a few different configurations (tandem and side-by-side). One of my favorite gyros is the Magni M16, so I actually started with a tub pusher configuration. However, I really have always liked the classic Cierva and Pitcairn gyros. In the end, I just decided that I could probably make a nice stable classic tractor gyro that I would be happy with.

When I decided on a tractor, I let myself play around with a lot of strange configurations for a while. Some were really ugly. One that I kind of liked for a while was identical to the Bulldog with the mast coming up from the back. I worked with it for a short time, but the geometry was limited by rotor plane and pilot head clearance, so the structural load path was pretty bad and the structural deflection was pretty high. I liked the open visibility, though.

I did make a nice cockpit mockup and the visibility seems surprisingly acceptable with the current mast. Not that it’s a real justification, but I think the visibility isn’t really much different than a Pitcairn PA-18. I did try a mast configuration where there is better forward visibility (see attached), but I was a little concerned about compromising the flight control simplicity, and making the mast trussed and very ridged without much significant visibility improvement. It isn’t too late to change the mast though! I can go with a slider head with a trussed mast.

I’m actually pretty happy with the unconventional tailwheel. There were some structural additions to get it to work in FEA. The local laminate is pretty thick right where the tail spring connects in, but most of the added structure was actually the bulkhead at the narrow part of the fuselage “waist” to help buckling stabilize the tail from side load in the tailwheel. We will see how it works.

Having the fuel on either side of the forward fuselage works great on this layout from a CG perspective, but I’ve never been super happy with having the fuel in the fuselage from safety perspective. I was planning on bladder lined Kevlar composite tanks, so it’s probably better than an MTO-Sport or seat tank, but I’d rather move it outside the fuselage if I can. I plan to look at a belly tank again, but for now, it’s on the sides of the FWD fuselage.

Thanks for the input,
Michael

 

[ILikeJetsToo]

Nice tractor! I know you have a bit of concern over the fuel tanks in the front seat cheek plates. Instead of looking at a belly tank, why not look at simple NACA23012 12% stub wing airfoil tanks set at -0.5deg & set close to the COG? This will outboard your fuel like done with a lot of fixed wings, plus provide enough lift to add some additional fuel for a longer cruise range. According to an old report (NACA #523) you might even be able to push a safe top speed up to 140mph without unloading the rotor too much using this method. Just a thought.
I'm curious, did you shop out your CNC work, or do it yourself? Looks like nice work!
- Matt

Thanks for the fuel tank recommendation Matt. I did try out a fuel filled stub wing very early on when I was doing concept work, and it was a little odd looking. I think I’ll try it again when I look at a belly tank. Maybe my aspect ratio just made it look odd. It’s not too late to change the tank position.

Edit Note:
Matt I forgot to answer your questions about the CNC. I had it cut by a friend that has a foam molds business. He's a great guy. If you ever need a foam tool milled, let me know, and I'll set you up with him.

Thanks,
Michael

 

[ILikeJetsToo]

Nice looking gyro. I always liked the look of the old classic autogiro's with a modern twist.

I would like to make two friendly observations;

1. Redesign your Center of Gravity where the rotorhead will be positioned over the passenger set. That way you will not need to rebalance with or without passenger.

2. When you find out you don't have enough vertical stabilizer, and you will find out that, just redesign the horizontal with two more verticals on the ends with moving rudders, and that will solve your upcoming problem.

Very nice build.

Thanks for the recommendations Dennis.

1)
I think it’s a little hard to tell from the semi-isomeric graphics I posted, but I actually did just what you are recommending on the CG. The passenger CG is very very near the overall vehicle CG, so there is very little global CG shift if the passenger is not there. I'm actually pretty happy with it. Good recommendation for sure,

2)
Adding tip plates to the H-Stab is one of the changes I have been considering. I have actually been having trouble finding a good technical reference for vertical tail volume and aspect ratio sensitivity. I was trying to keep the vertical stab in clean air, but I still wouldn’t want it to stall at high engine power and cause the gyro to spin (P-Factor and such). That would be a very bad thing close to the ground. I’m pretty happy with the position of the vertical, and I think it being below the CG will be a benefit for spiraling slipstream yaw/roll sensitivities, but I do want to make sure it is enough to counteract that torquey engine spinning that big prop. I have already made the horizontal stab, but I have not finished the tips yet. Do you by any chance have a technical reference for vertical tail sizing?

Thanks,
Michael

 

[ILikeJetsToo]

Great thread on your build. Kudos for sharing it with all of us.

Thanks for the encouragement Kevin. Much appreciated!

Michael

 

[DennisFetters]

Thanks for the recommendations Dennis.

1)
I think it’s a little hard to tell from the semi-isomeric graphics I posted, but I actually did just what you are recommending on the CG. The passenger CG is very very near the overall vehicle CG, so there is very little global CG shift if the passenger is not there. I'm actually pretty happy with it. Good recommendation for sure,

2)
Adding tip plates to the H-Stab is one of the changes I have been considering. I have actually been having trouble finding a good technical reference for vertical tail volume and aspect ratio sensitivity. I was trying to keep the vertical stab in clean air, but I still wouldn’t want it to stall at high engine power and cause the gyro to spin (P-Factor and such). That would be a very bad thing close to the ground. I’m pretty happy with the position of the vertical, and I think it being below the CG will be a benefit for spiraling slipstream yaw/roll sensitivities, but I do want to make sure it is enough to counteract that torquey engine spinning that big prop. I have already made the horizontal stab, but I have not finished the tips yet. Do you by any chance have a technical reference for vertical tail sizing?

Thanks,
Michael

Well, you are already more ahead of the game than others by understanding the problem of P-factor that the old autogyro's suffered from until it was understood.

No, I have no data, only knowledge of the situation, but when I need to learn about such things, I'll dig out the pictures or prints of the old autogyro's and measure the ratio of what they did. It's not an accurate science, but gets you close enough. Learn from what others have already found out saves a lot of experimenting.

Your idea to keep the vertical stab in clean air is what I feel you should do the opposite. You should concentrate in keeping it in the prop-wash at all times. That is the only thing that will keep you from spinning out of control at slow nose high attitudes. That is why you need the other two vertical stabilizers on the tips of the horizontal stab. They also need to have the same area as the center vertical stab, because when your prop-wash veers off the central vertical stab, the outer vertical stabs still need to do the same amount of work, or more. The trick is the distance how far out it would be best to place the outer vertical stabs, and again that would be a good idea to find a ratio of what the old-school did way back when.

That, or limit your slow-speed flight to where your foreword speed would have enough force on your single vertical stab where you would not spin... but that would be a horrible waste of autugyro at its best abilities... slow, low flight.

 

[TNVD]

looks very simple and well done (well designed, well constructed)

dont bother with CG right now too much (you will move head if necessary after or will move battery as necessary, etc)

construction is frameless - means you will need to reinforce body significuntly....

- what is the method of attachment of must to body (how do you plan to attach it) ?
- what is the method of attachment of landing gear to the body (how do you plan to attach it) ?

 

[bryancobb]

If I were to design a gyro FOR MYSELF, it would be the one you designed here. I love it.

 

[ILikeJetsToo]

Well, you are already more ahead of the game than others by understanding the problem of P-factor that the old autogyro's suffered from until it was understood.

No, I have no data, only knowledge of the situation, but when I need to learn about such things, I'll dig out the pictures or prints of the old autogyro's and measure the ratio of what they did. It's not an accurate science, but gets you close enough. Learn from what others have already found out saves a lot of experimenting.

Your idea to keep the vertical stab in clean air is what I feel you should do the opposite. You should concentrate in keeping it in the prop-wash at all times. That is the only thing that will keep you from spinning out of control at slow nose high attitudes. That is why you need the other two vertical stabilizers on the tips of the horizontal stab. They also need to have the same area as the center vertical stab, because when your prop-wash veers off the central vertical stab, the outer vertical stabs still need to do the same amount of work, or more. The trick is the distance how far out it would be best to place the outer vertical stabs, and again that would be a good idea to find a ratio of what the old-school did way back when.

That, or limit your slow-speed flight to where your foreword speed would have enough force on your single vertical stab where you would not spin... but that would be a horrible waste of autugyro at its best abilities... slow, low flight.

Thanks for the comments Dennis. All good points.

I guess what I meant by clean air over the vertical stab is air that has not been messed up by flowing over the cockpits. I agree with you though that with the tail on the underside, when the gyro is very nose-high, the vertical could protrude outside of the prop slipstream and the vertical effectiveness would quickly degrade. I will think more about putting verticals on the tips of the horizontal.

Thanks for your input,
Michael

 

[ILikeJetsToo]

If I were to design a gyro FOR MYSELF, it would be the one you designed here. I love it.

Thanks Bryan. Much appreciated!

Michael

 

[Vance]

A Very Nice design!

A Very Nice design!

That is a very thoughtful design Michael and very nicely fabricated.

I feel fences/vertical stabilizers on the ends of the horizontal stabilizer would be a nice stability enhancement.

Aesthetically larger diameter but narrow wheels and wheel pants would appeal to me. I would find something off a scooter or small motorcycle alluring.

I like the speed ring.

I like the landing gear.

Regards, Vance

 

[Smack]

the outrunner

the outrunner

Michael,
Wow, I like your design's look.
So, about this outrunner for pre-rotating... that parallels my thinking on this subject, but you are way ahead of me!

Please provide updates.
Brian

 

[neutrax]

That is going to be one nice sightseeing machine.

Gilbert

 

[DennisFetters]

Thanks for the comments Dennis. All good points.

I guess what I meant by clean air over the vertical stab is air that has not been messed up by flowing over the cockpits. I agree with you though that with the tail on the underside, when the gyro is very nose-high, the vertical could protrude outside of the prop slipstream and the vertical effectiveness would quickly degrade. I will think more about putting verticals on the tips of the horizontal.

Thanks for your input,
Michael

Michael, I'm really happy that you are open minded to this input. It is the most important feature on a tractor gyro. After nearly 30 years of designing and building rotorcraft, I still find a way to learn something new every day.

Back on 10/21/2012 I offered the same advice to Mr. Paul Komender (PTKAY) to give to his buddies.

https://www.rotaryforum.com/forum/showthread.php?t=34125&highlight=new+polish+tractor

He said he would "mark my words" if they would have trouble. He has yet to do so after they sadly but expectantly crashed. Of course beforehand, a heated argument about my advise pursued, and the inevitable tragic crash in that configuration is a sad redemption.

Sure, they had all the computer simulations showing the tails stability.... if it were and airplane, but a gyroplane is a different beast. Do all the computer simulations and calculations you want, they are a pointer in the right direction, but in the end, just look at what the old-timers..... "maybe about time I include myself in that group :-( " ... have done in the past to finally make it work around all the compromises needed to cooperate with mother nature.

I wish you the best of luck. It's a hard road you have taken, and I greatly respect you for the fortitude to design and build your own.