The big engine small plane problem

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Monty

Well-Known Member
Something that seems to be lost on many people.

You can only afford a certain fuel burn in a small airplane, in both dollars and range. Increasing the cruise fuel burn through any means: larger engine, turbo, whatever; impacts the design in a negative way.

You must fix the landing speed at something reasonable. Gust loading limits top speed for a given Q. Therefore additional fuel burn does not help you in any way other than climb rate. This is important for a fighter, not so much for a GA aircraft.

How to best put your additional wing area and fuel burn to use is the issue.

Two solutions emerge:

CS prop, turbo, small engine. (ideal)

Large engine, fixed pitch. (not so ideal...but it works, with a weight penalty)

both cruise at high altitude.

Do the math before you argue....please.

akwrencher

Well-Known Member
HBA Supporter
Fighters, and serious STOL bush planes. Other than than, agree 100 %. Make it as light as reasonably possible for the mission, size the engine acordingly, fly more hours for your hard earned Buck!

TFF

Well-Known Member
Large engine and CS prop. It is more weight for sure but 50 lbs more is not a bad price to pay. Getting to altitude fast is good, it lets you get into cruse mode leaning out the engine. The faster you can do this the longer you can make your economy run to your destination. What about a Questair.

Jay Kempf

Curmudgeon in Training (CIT)
Something that seems to be lost on many people.

You can only afford a certain fuel burn in a small airplane, in both dollars and range. Increasing the cruise fuel burn through any means: larger engine, turbo, whatever; impacts the design in a negative way.

You must fix the landing speed at something reasonable. Gust loading limits top speed for a given Q. Therefore additional fuel burn does not help you in any way other than climb rate. This is important for a fighter, not so much for a GA aircraft.

How to best put your additional wing area and fuel burn to use is the issue.

Two solutions emerge:

CS prop, turbo, small engine. (ideal)

Large engine, fixed pitch. (not so ideal...but it works, with a weight penalty)

both cruise at high altitude.

Do the math before you argue....please.
Agreed. But if you are starting with an assumption that you are sizing the airframe to the engine choice and mission requirements and that airframe makes your weight target then you aren't making a sacrifice. If you are sacrificing CS for DD with a turbo then you have to figure the mission profile out very carefully to find a performance benefit. If one of the goals is to use an existing auto engine then other factors other than performance are givens in the equation like the desire to be different or to not use a certified engine or whatever. Apples to apples is just the swap of prop and engine lb for lb and HP for HP and thrust for thrust at each point on the mission profile. Comparing both engine installations both having turbos probably doesn't work. It would have to be NA CS vs DD with turbo.

Agreed if you are stuffing a heavy engine choice into a small aircraft it makes no sense. But if you are targeting a heavy and large engine installation with an alternate then it "might" makes sense.

Kristoffon

Well-Known Member
You should consider flaps into your problem. Double slotted flaps allied to leading edge high lift devices as used in airliners increase lift by (IIRC) close to 300% where plain or fowler flaps as used in most homebuilts and 2 seater aircraft give an increment of at most 150%.

So if project complexity wasn't a factor I'd stick with the big engine and put some serious flaps in there.

Topaz

Super Moderator
Staff member
Log Member
You should consider flaps into your problem. Double slotted flaps allied to leading edge high lift devices as used in airliners increase lift by (IIRC) close to 300% where plain or fowler flaps as used in most homebuilts and 2 seater aircraft give an increment of at most 150%.

So if project complexity wasn't a factor I'd stick with the big engine and put some serious flaps in there.
Be careful with "overflapping". Yes, it's possible to build some seriously sophisticated flaps for a seriously small wing. But the very high wing loading that results can make for an airplane that is outright dangerous if the powerplant fails. Descent rate may be such that flaring for an emergency landing is well-nigh impossible. The Mitsubishi MU-2 is an example of this. It can be flown by an experienced and competent pilot who has received the proper training, but is not for the "average pilot" by any means.

This also affects the ability of the aircraft to glide in an engine-out situation. Too small a wing and you're coming down like a brick, and may not be able to make a suitable emergency landing area.

A much better way to get more cruise speed than overflapping is to simply pay better attention to drag reduction over the entire airframe - engine cooling drag, interference drag, and optimizing the wing/fuselage junction in particular. In general, a very low drag larger wing is better than a tiny one that requires highly complex, sophisticated flaps. More room for fuel, easier and less expensive to build, etc., and it's a safer airplane when the noisemaker stops.

Monty

Well-Known Member
Jay-YES!

TFF-I totally agree regarding a CS prop. Better all around. Questair is a perfect example. The engine is at least 2 times bigger than it needs to be. So at altitude it still makes good power, and cruise is impressive. Note short fat multi-blade prop. Regarding climbing directly to cruise altitude: Every time I have done mission analysis I have been shocked to realize that you are most often better off doing a slow cruise climb, if you are attempting to maximize range for a given fuel load. The climb schedule varies with atmospheric conditions and aircraft loading. Most of the time, climbing directly to cruise altitude hurts you....very counter intuitive.

akwrencher-Bush planes are an application to themselves, and yes...big engine with a LONG prop and big wing.

Kristoffon-Flaps...not so much. Complex, heavy, and slats really eat up fuel tank volume, plus slats kill laminar flow on forward wing. fuel volume is an acute problem on smaller aircraft. Volume scales with the cube of the aircraft size. Small airplanes just can't carry much fuel. Plus there is the climb issue. Make the wing too small and climb suffers especially above 10Kft, especially on a hot day with an NA engine and fixed pitch propeller.

Topaz-yes!

Toobuilder

Well-Known Member
HBA Supporter
Log Member
Just for a frame of reference, define "big" engine and "small" airframe... Are we talking about an RV4 with 400 HP (16-20GPH)? Or something more radical than that? It seems that you can get enough fuel in a 90 sq ft wing to support 300+ HP easily... At least enough to make some pretty serious cross country trips practical. This assumes you are designing it from the start as a wet wing. I've found the Harmon Rocket to be lacking in fuel, but that's just showing the limitations of the RV lineage.

akwrencher

Well-Known Member
HBA Supporter
Sorry Monty, just had to toss that in there. I live in the Alexander Archipeligo, commonly known as South East Alaska. Lot's of places to go here if you have floats or bush wheels. Otherwise, not so much. I like your thinking though, and I fully subscribe to the KISS principle, and also not building more machine than you nead. Great thread!

TFF

Well-Known Member
How do you plan to operate your engine. The thoughts I know of are one what Mooney recommends which is full power, adjust prop, and lean to 75 rich of peak; then their is the standard of pulling the power back, adjust prop, and leaning rich of peak; and then the lean of peak ops. One thing that they all require is steady state flying to stabilize the load so you can lean; which means getting to altitude. Which way of leaning a turbo has no definitive sides; rich and LOP have equal number of detractors. You gain a weight advantage, but your not going to get a fuel burn advantage HP to HP. Of the turbo aircraft we operate we are rich of peak; we have to give away 1 gal an hour to keep everything from welding its self together.

Jay Kempf

Curmudgeon in Training (CIT)
Of the turbo aircraft we operate we are rich of peak; we have to give away 1 gal an hour to keep everything from welding its self together.
Seems safe. Certified engines don't have things like altitude compensating FI and knock sensors to keep them safe.

Monty

Well-Known Member
Just for a frame of reference, define "big" engine and "small" airframe... Are we talking about an RV4 with 400 HP (16-20GPH)? Or something more radical than that? It seems that you can get enough fuel in a 90 sq ft wing to support 300+ HP easily... At least enough to make some pretty serious cross country trips practical. This assumes you are designing it from the start as a wet wing. I've found the Harmon Rocket to be lacking in fuel, but that's just showing the limitations of the RV lineage.
Good point: I should clarify.

Basically what you are proposing: an RV size aircraft with a giant V8 or even a 540 lyc. like the rockets. They are fun, and I wish I had one, but from an operational standpoint, you only get to use the extra power in takeoff and climb. The rest of the time you are carrying around a lot of extra weight. If you reduce the wing area to the point that you can actually use the power at SL safely, then your landing speed is going to be 100+ mph.

The place you can use the extra engine and wing (needed for safe landing speed) is if you cruise high, and I think it is a viable design choice. The Questair is the best example I can think of.

300 hp is 150 lbs/hr in fuel....at 16kft 75 lb/hr

I agree, it will work nicely, but I would rather have more than 90 ft^2 for wing to carry 2 people, bags, fuel and a big engine around.

The Rocket wing is an abortion of a miscarriage. But it works...More span is definitely desirable, especially with less area, but then the structure gets heavy...everything's a compromise

Well-Known Member
Aren't there any mechanically adjustable (in flight) "big" props? Such a two-speed prop is far simpler as an CS prop and works just as well.

SVSUSteve

Well-Known Member
Therefore additional fuel burn does not help you in any way other than climb rate. This is important for a fighter, not so much for a GA aircraft.
It's important if you want to cruise at a decent altitude or operate out of areas where a climb gradient is important or vital. This goes beyond just "bush" planes and includes almost anyone who operates into or out of airports like Vail/Eagle County, Aspen, Leadville, etc.

Two solutions emerge:

CS prop, turbo, small engine. (ideal)

Large engine, fixed pitch. (not so ideal...but it works, with a weight penalty)
Why are those the "two solutions"? I would think that there is at least a third which is a "large" engine with a constant speed prop as TFF pointed out.

fly more hours for your hard earned Buck!
I would rather fly more miles for my buck. Cruise flight is boring. Also, comfort becomes an issue if one tries to build something you'll be spending any considerable amount of time in.

The Mitsubishi MU-2 is an example of this. It can be flown by an experienced and competent pilot who has received the proper training, but is not for the "average pilot" by any means.
While I agree with the dangers of excessive flap design, most of the problems with MU-2 stem from the issues inherent with relying heavily on roll spoilers. I used to be a medical crew member on an MU-2 air ambulance and this was a frequent topic of discussion. One of the major quirks is that you really can't flare that much because of the design of the gear (our pilot used to describe the MU-2 as a "belly dragger") without a tail strike.

Too small a wing and you're coming down like a brick, and may not be able to make a suitable emergency landing area.
The approach I have taken with regards to this is to target a 50-70 knot "clean" stall speed and then just adjust the flap design to achieve the desired "dirty" stall speed (preferably less than 45 knots)

A much better way to get more cruise speed than overflapping is to simply pay better attention to drag reduction over the entire airframe - engine cooling drag, interference drag, and optimizing the wing/fuselage junction in particular. In general, a very low drag larger wing is better than a tiny one that requires highly complex, sophisticated flaps. More room for fuel, easier and less expensive to build, etc., and it's a safer airplane when the noisemaker stops.
Agreed. Drag reduction is going to be a major part of the detail design work of the Praetorian.

big engine with a LONG prop
Define "long prop". We are planning an 88" M-T six-bladed prop for the Praetorian.

plus slats kill laminar flow on forward wing
Not if you make them retractable. I understand the issues inherent there but if you're trying to maximize slow speed performance....

Also, there's a joke I heard made by someone at Oshkosh that "Nothing, short of ice, kills laminar flow quite like average homebuilt construction".

especially on a hot day with an NA engine and fixed pitch propeller
If you're planning on operating above 10,000 feet, why would you be planning on a normally aspirated engine and fixed pitch prop? To me, that sounds like trying to tow a camper trailer with a Pinto.

Certified engines don't have things like altitude compensating FI and knock sensors to keep them safe.
Hmmmm....

Monty

Well-Known Member
If you're planning on operating above 10,000 feet, why would you be planning on a normally aspirated engine and fixed pitch prop? To me, that sounds like trying to tow a camper trailer with a Pinto.
Because it works, if done properly.

As I stated, there are better solutions to this problem from a technical and theoretical standpoint. They tend towards increasing cost and complexity.

I prefer my aircraft simple, and inexpensive.

You are on a different path. There is nothing wrong with that, we simply choose a different approach.

I think Praetorians are more easily bought. I don't want to have to raise one up and train it from scratch, too afraid it would turn on me in the end. They tend to be rather dangerous.

I'm aiming more for the Plebian. Much easier to get along with.

Monty

Well-Known Member
Aren't there any mechanically adjustable (in flight) "big" props? Such a two-speed prop is far simpler as an CS prop and works just as well.
I don't really think so. With an electric VP prop it is trivial to make it CS. Why not get the advantage of having a CS prop if you've already paid for the complexity?

Monty

Well-Known Member
How do you plan to operate your engine. The thoughts I know of are one what Mooney recommends which is full power, adjust prop, and lean to 75 rich of peak; then their is the standard of pulling the power back, adjust prop, and leaning rich of peak; and then the lean of peak ops. One thing that they all require is steady state flying to stabilize the load so you can lean; which means getting to altitude. Which way of leaning a turbo has no definitive sides; rich and LOP have equal number of detractors. You gain a weight advantage, but your not going to get a fuel burn advantage HP to HP. Of the turbo aircraft we operate we are rich of peak; we have to give away 1 gal an hour to keep everything from welding its self together.
Most of this is operational stuff related to keeping the engine alive.

It really isn't practical to fly the ideal mission profile, you would have to sit down and figure out what the climb schedule would look like for the particular atmospheric conditions and mission. Not really relevant to normal GA flying since ATC is going to tell you what to do anyway, and you are flying with significant reserve fuel.

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SVSUSteve

Well-Known Member
Because it works, if done properly.
There are two problems with that. First, most attempts to do it properly end up not working. Second, even if you do achieve it, it only works in a marginal sense.

You are on a different path. There is nothing wrong with that, we simply choose a different approach.
Actually, I am trying to keep it as simple as possible as well. That's one reason why I am not trying to develop a new engine or some odd configuration. I have enough work ahead of me without all of that.

I think Praetorians are more easily bought. I don't want to have to raise one up and train it from scratch, too afraid it would turn on me in the end. They tend to be rather dangerous.
There's nothing comparable to what I am trying to develop. Some aircraft have the performance, others have the safety and others have the relative comfort. I am trying to find a happy medium between all three.