Design Drivers?

Homebuilt Aircraft & Kit Plane Forum

Help Support Homebuilt Aircraft & Kit Plane Forum:

TFF

Well-Known Member
Joined
Apr 28, 2010
Messages
15,404
Location
Memphis, TN
Mission is key, but on a homebuilt budget, you pretty much have to follow the engine; you got the Lycoming/ Continental line, you could move up to a Russian radial, maybe a 985. After that it silly season with turbines. We can start at 1/2 VWs and follow what is successful by average success. The line is very narrow. The thing is, someone has already been there before you. No new ground out there. You might be thinking you got a fresh one idea, it’s not. You can hang yourself on a need, but we don’t have the clout. You pretty much have to fall in line if you want an airplane. .005% might pull off something else.
 

TerryM76

Well-Known Member
HBA Supporter
Joined
Sep 8, 2012
Messages
683
Location
Tempe, AZ
No you say I have the most powerful motor available I’ll build the smallest aircraft around it I can.
Or do what they did with P 38 use two of the most powerful engine they cold get.
Sounds like the Messerschmitt Bf-109.
 

mcrae0104

Well-Known Member
HBA Supporter
Log Member
Joined
Oct 27, 2009
Messages
4,034
Location
KBJC
I think P-34 has a bit of a point here. In the world of our little homebuilts, you probably have some idea of the horsepower needs for the desired payload and performance based on planes of similar specifications. Or you might be dead-set on using a particular engine for whatever reason (availability, price, &c.). In that case the powerplant is the fixed point, and performance & payload are the variables.

Even if you don't have a specific engine in mind, you might (for instance) know at the outset that you need roughly 100-120hp. There are a number of engines that could fit the bill, but you're going to do your conceptual design work around a range known engines and their weights, not wait for a super light or super powerful unicorn engine. P-34, have I captured your perspective fairly?
 

Pilot-34

Well-Known Member
Joined
Apr 7, 2020
Messages
1,284
Location
Most of me is in IL but my hearts in Alaska
Lol
This thread has range so far away from my original idea that I’m not sure how to answer the question.

As far as the chicken or egg question I Think I fixated on the military question of build me the best fighter.
I suppose in the end I subscribe to Tim Taylor‘s philosophy of Moe Power.

Now please excuse me while I return to mounting this R2800 on a 152.
 

Saville

Well-Known Member
Joined
Apr 28, 2014
Messages
274
Location
Boston Ma
In the world of our little homebuilts, you probably have some idea of the horsepower needs for the desired payload and performance based on planes of similar specifications. Or you might be dead-set on using a particular engine for whatever reason (availability, price, &c.). In that case the powerplant is the fixed point, and performance & payload are the variables.

Even if you don't have a specific engine in mind, you might (for instance) know at the outset that you need roughly 100-120hp.
Ah but if, as you say, you ".....you might (for instance) know at the outset that you need roughly 100-120hp. " then you've already got your mission in mind. How did you know that 100hp is needed? How did you know that 500hp isn't needed?

If you say: " In the world of our little homebuilts, you probably have some idea of the horsepower needs for the desired payload and performance..." Then you've already specified the mission because you HAVE a "desired payload and performance".
 

Saville

Well-Known Member
Joined
Apr 28, 2014
Messages
274
Location
Boston Ma
No you say I have the most powerful motor available I’ll build the smallest aircraft around it I can.
Or do what they did with P 38 use two of the most powerful engine they cold get.
How did you decide that you needed "the most powerful motor available"?

Why couldn't you use a 300hp engine?

because you had a mission in mind FIRST.

I don't think the P-38 designers said, "Ok so we have Allison V-1710's....now what shall we do with it? I know, let's build a transport. "

Nor do I think they said, "Let's build a bomber"

I think they said, " We have this circular from the Army saying they wanted a high speed interceptor. Let's design one. "

Engine selection comes once you know your mission. The fact that you would defer to the most powerful engine available doesn't mean the engine selection came first. Because if you are designing an AT-6, you don't want nor need the most powerful engine available.

If you really look at your thoughts I think you'll see that the mission came first even in your mind. that's what I'm noticing here in the replies.
 

Heliano

Well-Known Member
Joined
Dec 24, 2015
Messages
156
Location
Campinas, SP, Brazil
My view regarding the 150-200HP range is: the auto conversions are maturing quickly. Not many statistical figures yet, but it is about to change. As the number of aircraft with Viking/Aeromomentum increases, such statistical numbers will appear. Simultaneously those manufacturers will continuously incorporate improvements and correct shortcomings. And the result will be that trust in those engines will grow. To me the technological differences between modern auto engines and Lycoming/Continental engines is huge and can not be ignored. Turbo, fuel injected engines give you not only excellent SFC but also excellent power at high density altitudes, lightest weight for the amount of horsepower and price for horsepower.
Mission is no doubt a key factor. If you want a long distance, cross country economical aircraft you'll want to fly high and with good specific range; if you want to do high speed low passes in air shows then the only thing you need is horsepower, lots of them. And so on.
Looking from a distance my impression (I may be wrong) is that folks in Europe design aircraft for range and efficiency (Europa, Fournier, Pipistrel, etc.) perhaps due to the high cost of avgas, while in the US there seems to be a passion for speed (Velocity, Glasair, Lancair) and for backcountry STOL operations.
 

BJC

Well-Known Member
HBA Supporter
Joined
Oct 7, 2013
Messages
13,584
Location
97FL, Florida, USA
while in the US there seems to be a passion for speed (Velocity, Glasair, Lancair) and for backcountry STOL operations.
... and low and slow paramotors, trikes, gyrocopters, Cub clones, Zenairs of all sorts, Sonnex, aerobatic biplanes, Mini-Max, etc.

The lower-powered RV-9’s, Rutan canards and others are reasonably economical.


BJC
 

Saville

Well-Known Member
Joined
Apr 28, 2014
Messages
274
Location
Boston Ma
My view regarding the 150-200HP range is: the auto conversions are maturing quickly. Not many statistical figures yet, but it is about to change. As the number of aircraft with Viking/Aeromomentum increases, such statistical numbers will appear. Simultaneously those manufacturers will continuously incorporate improvements and correct shortcomings. And the result will be that trust in those engines will grow.
Hello Heliano,

I hope you are right - especially with regard to increasing piles of statistics on auto engines. I think there is a possibility that a mature design with a particular auto engine will emerge as the go-to design.

I also hope - don't know if it will happen - that a solid dependable rugged PSRU will emerge. If that happens then the engine package can be treated as a unit and commodity.

Much like Lycomings are treated now.
 

mcrae0104

Well-Known Member
HBA Supporter
Log Member
Joined
Oct 27, 2009
Messages
4,034
Location
KBJC
Then you've already specified the mission because you HAVE a "desired payload and performance".
Yes.

I’m not suggesting mission doesn’t drive design; that much is obvious. What I am suggesting is that once the design parameters are identified and you go through the preliminary design process with a “rubber” engine, one will very likely find that the available engines force some adjustment of the design parameters. In that sense, the engine drives the design parameters and the design parameters drive the engine. It’s not a one-way street.

I also hope - don't know if it will happen - that a solid dependable rugged PSRU will emerge. If that happens then the engine package can be treated as a unit and commodity.

Much like Lycomings are treated now.
It would be nice to have a plug-and-play universal PSRU but the myriad combinations of engines, propellers, and different resonance characteristics dictate otherwise.
 

TFF

Well-Known Member
Joined
Apr 28, 2010
Messages
15,404
Location
Memphis, TN
Range, efficiency, and speed is what the average US flyer is looking for; that equates cost per time. For a lot of Europeans, they forget the distances in the US.

Because Oshkosh has just happened, that’s a good destination. LA to Oshkosh is about equal to Scotland to the tip of Italy. Double it if you wanted to go to Washington DC from Oshkosh. Extreme of course, but half that distance is common. Someone in the US is usually planning 500-700 mile trips for the weekend.

Sipping fuel is great, but if it means a day lost on your 3 day weekend, the wife will not go with you, and you will be paying for two commercial tickets instead of funneling that money into your own flight time. Speed for distance is not hot dogging in the US. No offense but most a Lancair owners are pretty square; accountants of time and distance. Definitely not a fighter pilot want to be. Airline captain want to be.

Break down for something common like a stater or a Mag, and there will be more help available to get home than a starter on a Kawasaki. You might find anATV store, you will have to go buy some cheap tools to do the work yourself.

The US does have one thing, airports. Some better than others but there are 15,000 in the US and 5000 in the rest of the world. Going somewhere in the US is very easy. A non hyper sensitive engine is very nice where you are visiting instead of fixing. Never fly farther than 100 miles, anything goes for an engine.
 

Pilot-34

Well-Known Member
Joined
Apr 7, 2020
Messages
1,284
Location
Most of me is in IL but my hearts in Alaska
How did you decide that you needed "the most powerful motor available"?

Why couldn't you use a 300hp engine?

because you had a mission in mind FIRST.

I don't think the P-38 designers said, "Ok so we have Allison V-1710's....now what shall we do with it? I know, let's build a transport. "

Nor do I think they said, "Let's build a bomber"

I think they said, " We have this circular from the Army saying they wanted a high speed interceptor. Let's design one. "

Engine selection comes once you know your mission. The fact that you would defer to the most powerful engine available doesn't mean the engine selection came first. Because if you are designing an AT-6, you don't want nor need the most powerful engine available.

If you really look at your thoughts I think you'll see that the mission came first even in your mind. that's what I'm noticing here in the replies.
Lol yes but the mission was to win the war.
That soon became build the most powerful engine and that becomes now put it in something.

Having the best engine makes new missions possible
 

Saville

Well-Known Member
Joined
Apr 28, 2014
Messages
274
Location
Boston Ma
Lol yes but the mission was to win the war.
That soon became build the most powerful engine and that becomes now put it in something.

Having the best engine makes new missions possible
Nooo I don't mean that the mission that "came first" was to win the war.

Engine builders strive to design the most powerful engines.

Airplane designers design an airplane to perform a mission.

As I wrote above if you are designing an AT-6 Texan you do NOT select the most powerful engine.

When the B-17, P-38, P-39 were designed there was no war to win.

When they designed it, they were responding to a Government specification.

P-38 - it was a response to a February 1937 specification from the United States Army Air Corps (USAAC). Circular Proposal X-608

The P-39 was a response to Circular Proposal X-609

The mission of the prospective airplane design must come first. Otherwise you don't know whether to pick an R-2800 (Corsair) or a Pratt & Whitney R-1340-S3H1-G Wasp
(BT-13 Valiant.).

You simply would not say, "Ok I want use an R-2800 so let's design a Basic Trainer which uses it."
 

Pilot-34

Well-Known Member
Joined
Apr 7, 2020
Messages
1,284
Location
Most of me is in IL but my hearts in Alaska
Nope , You say I want to win the war what’s the best thing I got ?
Just because there’s no war going on this minute doesn’t mean you don’t want to win the next one.
Do you believe that the AT-6 was a significant advancement in aircraft design?
 

Pilot-34

Well-Known Member
Joined
Apr 7, 2020
Messages
1,284
Location
Most of me is in IL but my hearts in Alaska
When they designed it, they were responding to a Government specification.

P-38 - it was a response to a February 1937 specification from the United States Army Air Corps (USAAC). Circular Proposal X-608

The P-39 was a response to Circular Proposal X-609

The mission of the prospective airplane design must come first. Otherwise you don't know whether to pick an R-2800 (Corsair) or a Pratt & Whitney R-1340-S3H1-G Wasp
(BT-13 Valiant.).
And how do you suppose somebody got the idea for the parameters a proposal X Dash 609 and X Dash 608?

Why didn’t those proposals come out in 1909?

Why do you suppose there’s a model ABC DEF and G?
Something better came along and we added it onto an existing design. After a while it becomes obvious that existing design isn’t the optimal way to make use of the new equipment .
Yes sometimes we design for a hoped for engine and even though that engine doesn’t exist that’s what’s driving to design.

Believe me nobody would’ve Specked a proposal for a F-16 with the engines available in 1910.

Except buck Rogers buck would’ve done it.
 

TFF

Well-Known Member
Joined
Apr 28, 2010
Messages
15,404
Location
Memphis, TN
The T-6 was. The decade before, everything was fabric covered and two wings. Actually half a decade. 80 years after it actually has a training place still.
You have a war. You have a mission. You can’t hide your stuff in someone else’s. You have to decide if it’s realistic. If it’s not, you change the objective or walk away. You can’t change the physics. That is a constant.
 

Saville

Well-Known Member
Joined
Apr 28, 2014
Messages
274
Location
Boston Ma
And how do you suppose somebody got the idea for the parameters a proposal X Dash 609 and X Dash 608?

Why didn’t those proposals come out in 1909?

Why do you suppose there’s a model ABC DEF and G?
Something better came along and we added it onto an existing design. After a while it becomes obvious that existing design isn’t the optimal way to make use of the new equipment .
Yes sometimes we design for a hoped for engine and even though that engine doesn’t exist that’s what’s driving to design.

Believe me nobody would’ve Specked a proposal for a F-16 with the engines available in 1910.

Except buck Rogers buck would’ve done it.
The title is "Design Drivers". We are talking about the aircraft designs...what the aircraft designers do.

We are NOT talking about Circular composers.

The fact that there are A,B, C versions of a design is utterly irrelevant to the original design. And quite often a new lettered version has nothing to do with the engines.

Aircraft DESIGNERS always - ALWAYS - start with a mission. Not an engine.

For some reason you seem to be fixated on the most powerful engines and ignore that fact that they play no part in,say, a trainer design as I've said several times now.

Boeing, Lockheed, Bell, Curtiss - all started with a mission and designed a plane that they thought would best satisfy the mission. They did NOT start with an engine.

You CANNOT start with an engine unless you first know what the airplane - THAT YOU ARE DESIGNING - must do. I repeat:

You would not start with an R-2800 if you are designing a trainer. So starting with an engine is silly.

I can't help it if you cannot grasp this very simple fact, but I'm done trying to explain it to you.
 
Top