Author Topic: Thundergazer design: Understanding its decalage (Read 11609 times)

Teodorico Terry · « **on:** January 27, 2022, 06:35:42 AM »

Hi -

I just bought a Thundergazer kit from RSM and I have been spending some time looking at the drawings trying to understand how to built it. By the way, the RSM kit is very nicely done with good wood and nice hardware.

So looking at the drawing and assuming that the wing is shown at 0 degree incidence this is what I noticed:

1) The design uses down thrust, 1/8" over the length of the engine bearers.
2) The tail plane has a slightly positive incidence, 1/64" up at the stab's leading edge.
3) The CG is rearward of what I have seen in other C/L designs, CG at 3 7/8" with a wing cord of 12" at the root, almost 33%. Considering the taper of the wing it would place the actual CG a little closer to 28-30% mark.

I also fly R/C, mostly the AMA pattern (Advanced) and they way in which the model is set-up would make me thing that it is in a tail heavy condition. The use of down thrust is pretty normal but in most cases the wing is set-up with a slight positive incidence (0.5 -1.0 degree) with the stab neutral. The Thundergazer in a sense is set-up with negative wind incidence if the stab is neutral.

Does anyone understand the reason why it might be set-up that way?

Thank you,

Teo

Mark wood · « **Reply #1 on:** January 27, 2022, 07:57:28 AM »

Yes, many here do understand the reason behind the trim elements in the airplane. First CG location is generally related to the Mean Aerodynamic Chord, MAC. When a wing is swept rearward the MAC moves aft so what you've estimated is likely off due to not having determined the MAC location. Doing that you'll find the relative CG isn't as far aft as 33% MAC and is likely further forward depending on the sweep and taper.

The thrust line and tail incidence are related to the propeller and trim bias. An RC pattern aircraft virtually never has to deal with propeller precession when compared to a CL model which has to deal with it continuously. The CL model is traveling in circle which means the propeller is pivoting about the Z axis continuously. The precession translates this through 90 degrees to the Y axis which results in a nose up or nose down moment depending on the rotation direction of the propeller. For a counterclockwise propeller this means a nose up moment is created. To compensate for this we can do one or both of two things, change the thrust line or change the tail plane incidence. Adding a little down thrust will help offset the precession as will a little tail incidence. Sometimes the thrust line is moved up or down for the same reason. Note that this trim correction is only good for one flight velocity and resulting rotational rate.

The tail plane incidence has been found to be beneficial in removing some flight trim bias. An airplane may have a difference in the trim from upright to inverted. For instance, the upright trim may be close to "hands off" while the inverted trim may have a "heavy" feel to it. Unlike the RC Pattern airplane placing the CG on or close to the neutral point is not very desirable as this makes the controls "twitchy" which is why we see the more forward CG being used for the CL models. By adding a little ail incidence the upright control feel becomes a little more pronounced and the inverted trim becomes a little less "heavy". So, by adding a little positive incidence the control position bias becomes more symmetric between upright and inverted. Having the balanced control moment upright and invented makes the airplane easier to fly the maneuvers.

Teodorico Terry · « **Reply #2 on:** January 27, 2022, 05:41:06 PM »

Motorman,

Looking at the kit it would be difficult to build it with the molded leading edge sheeting as designed. Even if you use the existing holes so that you can use a traditional JIG (i.e. rods/tubes) there is not way to support the 1/2 ribs. I am thinking that I might use the alternate process using two L-shaped extrusions to support the LE and trailing edge. To do this I will have to add a leading edge strip, probably 1/8x1/4. However, if I add a leading edge strip then I can probably use the traditional jig. I know that I will have to notch the ribs but since the C/L is marked it should not be a bid deal.

I think that in some respects the design is overly complicated with laminated spars which use CF for reinforcement. I will probably make some changes along the way. My intent is to build it as a one piece model in an effort to save weight as mine will be electric. We will see how it goes.

Teo

Teodorico Terry · « **Reply #3 on:** January 27, 2022, 05:42:25 PM »

Mark,

Thank you for your explanation.

Brett Buck · « **Reply #4 on:** January 27, 2022, 06:06:11 PM »

Quote from: Teodorico Terry on January 27, 2022, 06:35:42 AM

Hi -

I just bought a Thundergazer kit from RSM and I have been spending some time looking at the drawings trying to understand how to built it. By the way, the RSM kit is very nicely done with good wood and nice hardware.

So looking at the drawing and assuming that the wing is shown at 0 degree incidence this is what I noticed:

1) The design uses down thrust, 1/8" over the length of the engine bearers.
2) The tail plane has a slightly positive incidence, 1/64" up at the stab's leading edge.
3) The CG is rearward of what I have seen in other C/L designs, CG at 3 7/8" with a wing cord of 12" at the root, almost 33%. Considering the taper of the wing it would place the actual CG a little closer to 28-30% mark.

I also fly R/C, mostly the AMA pattern (Advanced) and they way in which the model is set-up would make me thing that it is in a tail heavy condition. The use of down thrust is pretty normal but in most cases the wing is set-up with a slight positive incidence (0.5 -1.0 degree) with the stab neutral. The Thundergazer in a sense is set-up with negative wind incidence if the stab is neutral.

Does anyone understand the reason why it might be set-up that way?

Yes, and the simple answer is that he experimented extensively with both thrust lines and stab alignment, and found that it worked best that way, after having the downthrust influenced by his 4-time National Champion mentor/hero Ted Fancher, and the positive incidence suggested to him by one of his dimwitted flying buddies.

A slightly more complex answer is that as long at the prop is spinning in the conventional direction, there is a fair bit of nose-up pitch torque from gyroscopic precession, and some benighted individuals think this explains why many airplanes end up rigged with lots of down elevator at neutral flap. This just builds it in, or in the case of the original airplane, was shimmed in using the removable tail hardware. The theory is that this compensates for this nose-up torque, making it neutral both for sensitivity and control loading.

Brett

Scientifiction . · « **Reply #5 on:** January 27, 2022, 06:44:41 PM »

The Thundergazer in a sense is set-up with negative wind incidence if the stab is neutral.

Someofus thinktheres DOWNWASH from the wing , & you need the tailplane upoish if its up , to neutralise it .

I think that in some respects the design is overly complicated with laminated spars which use CF for reinforcement.

You could leave half the bits out if you want it lighter !

RIDGIDITY & alignment under duress , 10 or 20 G's worth , the more carbon the better .

I think you might want to READ UP on CONTROL SYSTEM INTEGREDY .
It doesnt want one of these halfarsed poncey flexy remote control
that might do it bendy floppy cxontrol set ups .

The Control Action should be more like a rifle bolt , smooth , precise , dead accurate , no flex wotsoever twixt FLAPS & LEADOUTS .

Rear Ends theres a bit of conjecture , but likewise , deflection under load and itll steer like a 70s Jap Bike on a bumpy road . i.e. , it wont .

Brett Buck · « **Reply #6 on:** January 27, 2022, 10:52:41 PM »

Quote from: Air Ministry . on January 27, 2022, 06:44:41 PM

The Thundergazer in a sense is set-up with negative wind incidence if the stab is neutral.

Someofus thinktheres DOWNWASH from the wing , & you need the tailplane upoish if its up , to neutralise it .

I think that in some respects the design is overly complicated with laminated spars which use CF for reinforcement.

You could leave half the bits out if you want it lighter !

RIDGIDITY & alignment under duress , 10 or 20 G's worth , the more carbon the better .

I think you might want to READ UP on CONTROL SYSTEM INTEGREDY .
It doesnt want one of these halfarsed poncey flexy remote control
that might do it bendy floppy cxontrol set ups .

The Control Action should be more like a rifle bolt , smooth , precise , dead accurate , no flex wotsoever twixt FLAPS & LEADOUTS .

Rear Ends theres a bit of conjecture , but likewise , deflection under load and itll steer like a 70s Jap Bike on a bumpy road . i.e. , it wont .

I apologize to Matt. I wasn't able to decipher what he was saying, and he probably *was* reacting to the OP rather than offering criticism, as Pat suggests, so, I am sorry.

Brett

pmackenzie · « **Reply #7 on:** January 28, 2022, 04:47:56 AM »

Quote from: Brett Buck on January 27, 2022, 10:52:41 PM

Thanks for such a cogent analysis. I will for sure pass along your brilliant and carefully considered analysis to World Champion and Multi-Time US National Champion, not to mention my close personal friend, David Fitzgerald.

Brett

p.s. Here is a video including David winning the World Championship in 2008, with the airplane that Matt thinks is designed all wrong and "steer like a 70s Jap Bike on a bumpy road . i.e. , it wont ." :

https://youtu.be/wMhQ5lYY88I?t=364

It can be hard to decifer Airministry's posts

,
But I think he was actually quoting the OP's post about leaving out the carbon and some of the other overly complicated structures, and warning against doing that lest you end up with the "bendy floppy control set up"

Teodorico Terry · « **Reply #8 on:** January 28, 2022, 05:56:16 AM »

Brett,

Thank you for your answer and posting the video. All of my planes are electric and in my limited experience it seems that using a reverse rotation prop helps line tension. The intent is to build the TG as an electric model; if built to plan it seems that it would make more sense to use a normal rotation prop. Is that correct?

Thanks,

Teo

I was the one who made the comment about some aspects of the build appearing to be overly complicated. I get the quest for lightness and stiffness but looking at the photos which are part of the build instructions it appeared to me that the spar laminations were aligned vertically; for best results, they really should be aligned horizontally, parallel to the cord line. If I had access to spruce as we used to have many years I would have been tempted to replace the spars with spruce. At any rate part of the fun of building is making changes which make the build your own.

Ken Culbertson · « **Reply #9 on:** January 28, 2022, 08:00:05 AM »

Quote from: Teodorico Terry on January 28, 2022, 05:56:16 AM

All of my planes are electric and in my limited experience it seems that using a reverse rotation prop helps line tension.

All of mine are electric too. I was pleased at first with the perceived enhanced tension but after about a year decided that it was a near wash. The better tension places were offset going the other direction. In my case outside turns were enhanced at the expense of inside. Since my outsides are and always have been better than my insides, it was hurting my overall pattern. I switched back. What does not work is using the same trim for one direction as the other. I am far from the last word on this but I do know that many of the folks I fly with have switched back.

Ken

Brett Buck · « **Reply #10 on:** January 28, 2022, 08:24:11 AM »

Quote from: Teodorico Terry on January 28, 2022, 05:56:16 AM

Brett,

Thank you for your answer and posting the video. All of my planes are electric and in my limited experience it seems that using a reverse rotation prop helps line tension. The intent is to build the TG as an electric model; if built to plan it seems that it would make more sense to use a normal rotation prop. Is that correct?

Thanks,

Teo

I was the one who made the comment about some aspects of the build appearing to be overly complicated. I get the quest for lightness and stiffness but looking at the photos which are part of the build instructions it appeared to me that the spar laminations were aligned vertically; for best results, they really should be aligned horizontally, parallel to the cord line. If I had access to spruce as we used to have many years I would have been tempted to replace the spars with spruce. At any rate part of the fun of building is making changes which make the build your own.

If the airplane is too complex, then put it aside for a while, rather than "improving" it or trying to "lightening it up". Since David and I have been such close friends and flying buddies/collaborators over the years, I have watched the entire thing develop from the very first day. In reality, that design goes back 40+ years. While there might be a few things I would do a bit differently (and did, going more or less a different direction), you are seeing the results of very extensive development. There are no significant mistakes or glaring errors, at most minor improvements that do not involve anything you mention.

I *do* think it would fly just fine on a much lesser engine than a PA75 - but not a Brodak 40, a Stalker 61, Saito 56, etc. - a lesser *piped* engine, like a 40VF. In fact it is extremely similar in overall dimensions to David's first 40VF airplane, a full-fuselage Imitation, and substantially smaller than many 40VF airplanes. It would fly OK with a 4-2 break engine (David's first piped airplane was converted from and ST46, and was maybe the last gasp of the 4-2 break era, coming in 6th at maybe the 1991 Team Trials) but that's not what it is intended for.

Brett

Brett Buck · « **Reply #11 on:** January 28, 2022, 10:43:17 AM »

Quote from: Motorman on January 28, 2022, 09:51:14 AM

Please tell us more about the minor improvements to the Thundergazer.

No, thank you, and in any case, it is a moving target, which changes made every flying session.

Brett

Howard Rush · « **Reply #12 on:** January 29, 2022, 06:41:02 PM »

Quote from: Teodorico Terry on January 28, 2022, 05:56:16 AM

The intent is to build the TG as an electric model; if built to plan it seems that it would make more sense to use a normal rotation prop. Is that correct?

I think so. It's the devil you know. David made the TG for a normal rotation prop, so I'd stick with that prop. I built two electric Impacts. One worked with props turning either direction; the other only worked with normal rotation props.

Quote from: Teodorico Terry on January 28, 2022, 05:56:16 AM

At any rate part of the fun of building is making changes which make the build your own.

In my case, that really slows down the building. When I built my first Impact I had to continually remind myself, "This airplane was designed by a world champion who's a professional airplane structures engineer. Don't change anything. Keep building."

Mark wood · « **Reply #13 on:** January 29, 2022, 06:53:39 PM »

Quote from: Howard Rush on January 29, 2022, 06:41:02 PM

I think so. It's the devil you know. David made the TG for a normal rotation prop, so I'd stick with that prop. I built two electric Impacts. One worked with props turning either direction; the other only worked with normal rotation props.

In my case, that really slows down the building. When I built my first Impact I had to continually remind myself, "This airplane was designed by a world champion who's a professional airplane structures engineer. Don't change anything. Keep building."

Super good advice. My designs tend to work really well and take FOEREVER. When I want to get flying, I build other peoples airplanes the way they did so I can learn. I don't change those designs.

pmackenzie · « **Reply #14 on:** January 29, 2022, 07:48:11 PM »

FWIW, I have the same kit, and I would probably change a few things

- The take apart system is a variation on the Impact one, but seems more complex in the details. If I was going with a one piece wing I would be tempted to simplify it, and make it more like the impact. I have done a couple of ARC Vector 40s that way, so I know it works well.
- Having said that, I would want a two piece wing, so this involves changing a lot of things to accommodate it.
- Mine would be electric, so a bunch more changes to the front end.
- As was posted earlier, the geoditic wing implementation seems a little strange: No consistent angles and/or spacing. Perhaps it would all work out fine as is?
But a lot of work to find out there is a problem, so again I would be tempted to figure it out from scratch.

At this point, not a lot of the kit would be left

Facing this decision a few years ago, I opted to build a MaxBee instead. Which of course I did differently than Igor's

Built up wing and stab, Impact style removable tail, "my" type of 2 wing take-aparts, Orbit motor instead of AXI, etc)

I took a look at the kit again recently, still not sure if I will build it, or another MaxBee. Paralysis by analysis.

Brett Buck · « **Reply #15 on:** January 29, 2022, 08:12:47 PM »

Quote from: pmackenzie on January 29, 2022, 07:48:11 PM

FWIW, I have the same kit, and I would probably change a few things

- The take apart system is a variation on the Impact one, but seems more complex in the details. If I was going with a one piece wing I would be tempted to simplify it, and make it more like the impact. I have done a couple of ARC Vector 40s that way, so I know it works well.
- Having said that, I would want a two piece wing, so this involves changing a lot of things to accommodate it.

The second Thundergazer has a two-piece wing with the usual clip-type hardware, custom made by Dallas Hanna.

The original is as it is shown in the kit. I don't really like this method, although it is easily achievable with normal modeling tools, it still leaves you with a pretty big box. I particularly do not like the firewall attachment. When we were helping Paul Ferrell build his take-apart Trivial Pursuit, we replaced the conventional cap head screws with flat-head screws with nylon locking patches, and put a titanium plate on the front of the former, with countersinks to take the screw head loads. This has the advantage of always centering itself up the same way and proving more than just the clamping pressure to keep it from sliding side-to-side.

In any case, it is very minor secondary issue. Howard (even though he is certainly more than qualified to do his own engineering) has the right idea, and the most apt observation - we are (sometimes) pathologically original. The airplane was good enough to win the WC on the first time out, and the NATs multiple times, just as shown. It doesn't need to be fixed.

Brett

Teodorico Terry · « **Reply #16 on:** January 30, 2022, 07:02:43 AM »

Thank you guys for the replies, interesting to read. My intent is not to build a model that I can carry on a flight to a competition so I am looking at building it as a one piece model. Given that it is going to be electric removing the additional material that goes along with the take apart features saves weight which could be used to compensate for the additional weight of the electric power system. That is really what I had in mind when thinking about making changes.

Teo

pmackenzie · « **Reply #17 on:** January 30, 2022, 07:14:06 AM »

FWIW, a properly sized electric set up will be extremely close in weight to a fueled glow one.

Ken Culbertson · « **Reply #18 on:** January 30, 2022, 11:09:15 AM »

Quote from: pmackenzie on January 30, 2022, 07:14:06 AM

FWIW, a properly sized electric set up will be extremely close in weight to a fueled glow one.

Add "and built" to that. What you gain in battery weight is offset by weight distribution AND not needing to make the nose and fuselage bullet proof to absorb the vibration. Converted IC/Electric almost always come out heavier.

Ken

Scientifiction . · « **Reply #19 on:** January 30, 2022, 06:11:56 PM »

Oh Dear ,

Radio thingos were renowned for controls totally inadequate for F2B .

Thus ' Control Systems ' for C/L Aerobatics is from a differant book to the wireless devices !

-------------------------------------------------------------------------------------------------------

CARBON etc for the decadent affluent aeromodellor is invaluable . Particularly if you drop files on your aeroplane .

Strength to weight . But the RIDGIDITY it introduces is perhaps invaluable .
But you might run it ' right through ' on wings & tailplane , prior to fitting the fuselage . Or youll have two ridgid sides with a soft center , if the C F is the covering .

P.S. It'll have to be a Whisper Gazer if its electric ,

Scientifiction . · « **Reply #20 on:** January 30, 2022, 06:25:57 PM »

Quote

That is really what I had in mind when thinking about making changes.

With this electricity , you can fly ' pre - finish ' to evaluate things , like prop / battery location , Et Al . without getting oil all over the bare surfaces .

Theyre quite straightforward . But like Triumphs ( real Ones ) you dont go altering nothing unless your damn sure its an improvement .
that Said , one piece wing & stab arnt a backward step . Like fitting a yamaha engine to a Triumph .

Remarks are induced by pondering a Jose thread on a Electrim P M or suchlike , where they found putting the PROP forward a few inches ,
with everthing else the same , worked wonders .

Might be a step to far , but Electric Powerplants let you try P-39 / P-63 type things , or say a P-51 scoop feeding a chamber , with a dust forward , and exhaust air to the rear .

Bit of a Hi Jack , SORRY .

Air Duct. quack quack . Tho theyred be a ' bleed off - instead of the liquid cooling . unless you liquid cooled it !

MAINLY WHAT INSPIRED THE RANT , is : a One Piece Model - the controls are in for good & maybe ? near inaccessable .
As theyre as important as any other feature , Making Sure the operation is flawless & secure save later major butchery .

If its a kit its likely got satisfactry componentry there .
A dab of locktite etc if theres threads .
Maybe a tiny tube ( like on CRC / WD 40 can ) , aimed at the pivot . if its metal on metal . for later ' lube ' schedule !.

Anyway , The controls are a area where a dose of patiance and repeated double check , before ' closing up ' can avoid later trouble .

Howard Rush · « **Reply #21 on:** February 12, 2022, 02:29:28 PM »

Since the original Impact and Thundergazer designs, airlines have changed baggage size rules. Now you need to remove each wing individually, and it helps to have the fuselage to come apart somewhere amidships.

Trostle · « **Reply #22 on:** February 13, 2022, 10:12:38 AM »

Quote from: Air Ministry . on January 30, 2022, 06:25:57 PM

Might be a step to far , but Electric Powerplants let you try P-39 / P-63 type things , or say a P-51 scoop feeding a chamber , with a dust forward , and exhaust air to the rear .

Taking this Hi-Jack to the next level ----

Why the picture of a set of power plants in the Brabazon? Interesting airplane. It had four sets of paired air cooled Centaurus radial engines imbedded in the wing leading edges.

Keith

Teodorico Terry · « **Reply #23 on:** February 16, 2022, 08:38:14 AM »

That was indeed an interesting airplane but it came too late; the writing was already on the wall. The Comet also made its first flight on the same year. The British actually had some rather interesting aircraft designs. My favorite ugly duckling would have to be the Buccaneer but it did its job well. Down low hardly anything could catch it and the Brits flew it low. There is a story of the planes flying low enough over water to leave a wake in order to entice the opposing force's F-15 CAP to come down to engage them. In their excitement the F-15s failed to see the low flying F111s following the Buccaneers a few miles back. Probably one of the few times a F111 scored a "kill" against a F-15. This was during a joint exercise.

fred cesquim · « **Reply #24 on:** February 17, 2022, 03:38:48 AM »

i have built the TG converted for e-power and put together a manual for RSM ( wich is on RSM page under download section). changed the method of wing construction for a more simple approach.
]here´s the PDF link > https://www.rsmdistribution.com/guides/tgmanual.pdf
hope it helps to build yours

Ted Fancher · « **Reply #25 on:** June 28, 2022, 05:42:16 PM »

Quote from: fred cesquim on February 17, 2022, 03:38:48 AM

i have built the TG converted for e-power and put together a manual for RSM ( wich is on RSM page under download section). changed the method of wing construction for a more simple approach.
]here´s the PDF link > https://www.rsmdistribution.com/guides/tgmanual.pdf
hope it helps to build yours

Beautiful ship Fred!

Somehow I managed to totally miss this thread and just spent I very pleasant half hour or so reminiscing about the development of David's incredibly successful design. Very performance functional while retaining a lot of visual nostalgia of days (OMG! Years!!) gone by.

Ted Fancher

fred cesquim · « **Reply #26 on:** June 30, 2022, 04:01:46 AM »

Quote from: Ted Fancher on June 28, 2022, 05:42:16 PM

Beautiful ship Fred!

Somehow I managed to totally miss this thread and just spent I very pleasant half hour or so reminiscing about the development of David's incredibly successful design. Very performance functional while retaining a lot of visual nostalgia of days (OMG! Years!!) gone by.

Ted Fancher

thanks Ted!
indeed it´s a great looking machine and have lots of interesting ideias on the design and a light structures. This plane was my return to control line building after some 10 years hiatus ( i was a pro r/c jet/scale builder then) and it was an eye opener for me on the build light new methods